One persistent puzzle in the life sciences is the asymmetric lipid composition of the cellular plasma membrane: while the exoplasmic leaflet is enriched in lipids carrying predominantly saturated fatty acids, the cytoplasmic leaflet hosts preferentially lipids with (poly‐)unsaturated fatty acids. Given the high energy requirements necessary for cells to maintain this asymmetry, the question naturally arises regarding its inherent benefits. In this paper, we propose asymmetry to represent a potential solution for harmonizing two conflicting requirements for (...) the plasma membrane: first, the need to build a barrier for the uncontrolled influx or efflux of substances; and second, the need to form a fluid and dynamic two‐dimensional substrate for signaling processes. We hence view here the plasma membrane as a composite material, where the exoplasmic leaflet is mainly responsible for the functional integrity of the barrier and the cytoplasmic leaflet for fluidity. We reinforce the validity of the proposed mechanism by presenting quantitative data from the literature, along with multiple examples that bolster our model. (shrink)

The eyespot organelle of the green alga Chlamydomonas allows the cell to phototax toward (or away) from light to maximize the light intensity for photosynthesis and minimize photo‐damage. At cytokinesis, the eyespot is resorbed at the cleavage furrow and two new eyespots form in the daughter cells 180° from each other. The eyespots are positioned asymmetrically with respect to the microtubule cytoskeleton. Eyespots are assembled from all three chloroplast membranes and carotenoid‐filled granules, which form a sandwich structure overlaid by the (...) tightly apposed plasma membrane. This review describes (1) my interest in cellular asymmetry and organelle biology, (2) isolation of mutations that describe four genes governing eyespot placement and assembly, (3) the characterization of the EYE2 gene, which encodes a thioredoxin superfamily member, and (4) the characterization of the MIN1 gene, which is required for the layered organization of granules and membranes in the eyespot. BioEssays 25:410–416, 2003. © 2003 Wiley Periodicals, Inc. (shrink)

Microfilament interactions with the plasma membranes of animal cells appear to vary with cell type and localization. In the erythrocyte, actin oligomers are associated with the membrane via spectrin and ankyrin. The ends of stress fibers in cultured cells, such as fibroblasts, are attached to the plasma membrane at focal adhesion sites and may involve the protein vinculin as a linking protein. In intestinal brush border microvilli a 110,000 dalton protein links the microfilament bundles to sites (...) on the microvillus. A transmembrane complex containing actin stably associated with a cell surface glycoprotein can be isolated from ascites tumor cell microvilli and can be obtained still associated with microfilaments by gentle extraction and gradient centrifugation of the microvilli. These varied interaction mechanisms are believed to be needed to satisfy the different structural and dynamic requirements of the particular systems. (shrink)

All the biological membranes contain oxidoreduction systems actively involved in their bioenergetics. Plasma membrane redox systems seem to be ubiquitous and they have been related to several important functions, including not only their role in cell bioenergetics, but also in cell defense through the generation of reactive oxygen species, in iron uptake, in the control of cell growth and proliferation and in signal transduction. In the last few years, an increasing number of mechanistic and molecular studies have deeply (...) widened our knowledge on the function of these plasma membrane redox systems. The aim of this review is to summarize what is currently known about the components and physiological roles of these systems. (shrink)

Extracellular ATP released from necrotic cells in inflamed tissues activates the P2X7 receptor, stimulates the exposure of phosphatidylserine, and causes cell lysis. Recent findings indicated that XK, a paralogue of XKR8 lipid scramblase, forms a complex with VPS13A at the plasma membrane of T cells. Upon engagement by ATP, an unidentified signal(s) from the P2X7 receptor activates the XK‐VPS13A complex to scramble phospholipids, followed by necrotic cell death. P2X7 is expressed highly in CD25+CD4+ T cells but weakly in (...) CD8+ T cells, suggesting a role of this system in the activation of the immune system to prevent infection. On the other hand, a loss‐of‐function mutation in XK or VPS13A causes neuroacanthocytosis, indicating the crucial involvement of XK‐VPS13A‐mediated phospholipid scrambling at plasma membranes in the maintenance of homeostasis in the nervous and red blood cell systems. (shrink)

Membrane proteins possess certain features that make them susceptible to the electric fields generated at the level of the plasma membrane. A reappraisal of cell signalling, taking into account the protein interactions with the membrane electrostatic profile, suggests that an electrical dimension is deeply involved in this fundamental aspect of cell biology. At least three types of potentials can contribute to this dimension: (1) the potential across the compact layer of water adherent to membrane surfaces; (...) this potential is affected by classical inducers of cell differentiation, like dimethylsulfoxide and hexamethylenebisacetamide; (2) the potential across the Gouy‐Chapman double layer, which accounts for the effects of extracellular cations in the modulation of differentiation; and (3) the resting potential. This last potential and its governing ion currents can be exploited in localised mechanisms of cell signalling centred on the functional association of integrin receptors with ion channels. (shrink)

In nature, cells face a variety of stresses that cause physical damage to the plasma membrane and cell wall. It is well established that evolutionarily conserved cell cycle checkpoints monitor various cellular perturbations, including DNA damage and spindle misalignment. However, the ability of these cell cycle checkpoints to sense a damaged plasma membrane/cell wall is poorly understood. To the best of our knowledge, our recent paper described the first example of such a checkpoint, using budding yeast (...) as a model. In this review, we will discuss this important question as well as provide hypothetical explanations to be tested in the future. (shrink)

Many enveloped viruses are released from infected cells by maturing and budding at the plasma membrane. During this process, viral core components are incorporated into membrane vesicles that contain viral transmembrane proteins, termed ‘spike’ proteins. For many years these spike proteins, which are required for infectivity, were believed to be incorporated into virions via a direct interaction between their cytoplasmic domains and viral core components. More recent evidence shows that, while such direct interactions drive budding of alphaviruses, (...) this may not be the case for negative strand RNA viruses and retroviruses. These viruses can bud particles in the absence of spike proteins, using only viral core components to drive the process. In some cases the spike proteins, without the viral core, can be released as virus‐like particles. Optimal budding and release may, therefore, depend on a ‘push‐and‐pull’ concerted action of core and spike, where oligomerization of both components plays a crucial role. (shrink)

You are what you eat – this well‐known phrase properly describes the phenomenon of the effects of diet on acute and chronic inflammation. Several lipids and lipophilic compounds that are delivered with food or are produced in situ in pathological conditions exert immunomodulatory activity due to their interactions with the plasma membrane. This group of compounds includes cholesterol and its oxidized derivatives, fatty acids, α‐tocopherol, and polyphenols. Despite their structural heterogeneity, all these compounds ultimately induce changes in (...) class='Hi'>plasma membrane architecture and fluidity. By doing this, they modulate the dynamics of plasma membrane receptors, such as TLR4. This receptor is activated by lipopolysaccharide, triggering acute inflammation during bacterial infection, which often leads to sepsis and is linked with diverse chronic inflammatory diseases. In this review, we discuss how the impact on plasma membrane properties contributes to the immunomodulatory activity of dietary compounds, pointing to the therapeutic potential of some of them.Also watch the Video Abstract. (shrink)

Neuronal information processing depends on converting membrane depolarizations into compartmentalized biochemical signals that can modify neuronal activity and structure. However, our understanding of how neurons translate electrical signals into specific biochemical responses remains limited, especially in the soma where gene expression and ion channel function are crucial for neuronal activity. Here, I emphasize the importance of physically compartmentalizing action potential‐triggered biochemical reactions within the soma. Emerging evidence suggests that somatic endoplasmic reticulum–plasma membrane (ER‐PM) junctions are specialized organelles (...) that coordinate electrical and biochemical signaling. The juxtaposition of ion channels and signaling proteins at a prominent subset of these sites enables compartmentalized calcium and cAMP‐dependent protein kinase (PKA) signaling. I explore the hypothesis that these PKA‐containing ER‐PM junctions serve as critical sites for translating membrane depolarizations into PKA signals and identify key gaps in knowledge of the assembly, regulation, and neurobiological functions of this somatic signaling system. (shrink)

The fundamental mechanisms of protein and lipid organization at the plasma membrane have continued to engage researchers for decades. Among proposed models, one idea has been particularly successful which assumes that sterol‐dependent nanoscopic phases of different lipid chain order compartmentalize proteins, thereby modulating protein functionality. This model of membrane rafts has sustainably sparked the fields of membrane biophysics and biology, and shifted membrane lipids into the spotlight of research; by now, rafts have become an integral (...) part of our terminology to describe a variety of cell biological processes. But is the evidence clear enough to continue supporting a theoretical concept which has resisted direct proof by observation for nearly twenty years? In this essay, we revisit findings that gave rise to and substantiated the raft hypothesis, discuss its impact on recent studies, and present alternative mechanisms to account for plasma membrane heterogeneity. (shrink)

Plasma membranes are subject to continuous deformations. Strikingly, some of these transient membrane undulations yield membrane‐associated signaling hubs that differ in composition and function, depending on membrane geometry and the availability of co‐factors. Here, recent advancements on this ubiquitous type of receptor‐independent signaling are reviewed, with a special focus on emerging concepts and technical challenges associated with studying these elusive signaling sites.

Lysosomes are the site of degradation of obsolete intracellular material during autophagy and of extracellular macromolecules following endocytosis and phagocytosis. The membrane of lysosomes and late endosomes is enriched in highly glycosylated transmembrane proteins of largely unknown function. Significant progress has been made in recent years towards elucidating the pathways by which these lysosomal membrane proteins are delivered to late endosomes and lysosomes. While some lysosomal membrane proteins follow the constitutive secretory pathway and reach lysosomes indirectly via (...) the cell surface and endocytosis, others exit the trans‐Golgi network in clathrin‐coated vesicles for direct delivery to endosomes and lysosomes. Sorting from the Golgi or the plasma membrane into the endosomal system is mediated by signals encoded by the short cytosolic domain of these proteins. This review will discuss the role of lysosomal membrane proteins in the biogenesis of the late endosomal and lysosomal membranes, with particular emphasis on the structural features and molecular mechanisms underlying the intracellular trafficking of these proteins. (shrink)

Since the early nineteenth century a membrane or wall has been central to the cell’s identity as the elementary unit of life. Yet the literally and metaphorically marginal status of the cell membrane made it the site of clashes over the definition of life and the proper way to study it. In this article I show how the modern cell membrane was conceived of by analogy to the first “artificial cell,” invented in 1864 by the chemist Moritz (...) Traube (1826–1894), and reimagined by the plant physiologist Wilhelm Pfeffer (1845–1920) as a precision osmometer. Pfeffer’s artificial cell osmometer became the conceptual and empirical basis for the law of dilute solutions in physical chemistry, but his use of an artificial analogue to theorize the existence of the plasma membrane as distinct from the cell wall prompted debate over whether biology ought to be more closely unified with the physical sciences, or whether it must remain independent as the science of life. By examining how the histories of plant physiology and physical chemistry intertwined through the artificial cell, I argue that modern biology relocated vitality from protoplasmic living matter to nonliving chemical substances—or, in broader cultural terms, that the disenchantment of life was accompanied by the (re)enchantment of ordinary matter. (shrink)

It has long been known that the red blood cell contains a membrane skeleton that stabilizes the plasma membrane, determines its shape, and regulates the lateral distribution of the membrane glyco‐proteins to which it is attached. The way in which these functions are regulated in other cells has not been understood. It has now been shown that platelets also contain a membrane skeleton. In contrast to the membrane skeleton of the red blood cell, the (...) platelet membrane skeleton has actin‐binding protein, not spectrin, as a major component. The platelet membrane skeleton regulates the same cellular functions as the red blood cell membrane skeleton. Other cells may contain a membrane skeleton that is critical to their viability and normal functioning. (shrink)

Phosphatidylinositol 5‐phosphate (PtdIns5P), the least characterized among the three phosphatidylinositol monophosphates, is emerging as a bioactive lipid involved in the control of several cellular functions. Similar to PtdIns3P, it is present in low amounts in mammalian cells, and can be detected at the plasma membrane and endomembranes as well as in the nucleus. Changes in PtdIns5P levels are observed in mammalian cells following specific stimuli or stresses, and in human diseases. Recently, the contribution of several enzymes such as (...) PIKfyve, myotubularins, and type II PtdInsP‐kinases to PtdIns5P metabolism has gained a strong experimental support. Here, we provide a picture emerging from recent studies showing how this lipid can be generated and act as a regulator of membrane and cytoskeleton dynamics, and as a modulator of gene expression. We briefly summarize the current methods and tools for studying PtdIns5P, and discuss how PtdIns5P can integrate and coordinate different functions in a spatiotemporal manner. (shrink)

Phosphatidylinositol 3‐phosphate (PtdIns3P) is generated on the cytosolic leaflet of cellular membranes, primarily by phosphorylation of phosphatidylinositol by class II and class III phosphatidylinositol 3‐kinases. The bulk of this lipid is found on the limiting and intraluminal membranes of endosomes, but it can also be detected in domains of phagosomes, autophagosome precursors, cytokinetic bridges, the plasma membrane and the nucleus. PtdIns3P controls cellular functions through recruitment of specific protein effectors, many of which contain FYVE or PX domains. Cellular (...) processes known to be controlled by PtdIns3P and its effectors include endosomal fusion, sorting and motility, autophagy, cytokinesis, regulated exocytosis and signal transduction. Here we discuss how Ptdins3P is generated on specific cellular membranes, how its localizations and functions can be studied, and how its effectors serve to control cellular functions.Editor's suggested further reading in BioEssays:Phosphatidylinositol 4,5‐bisphosphate: Targeted production and signalingAbstractHow does SHIP1/2 balance PtdIns(3,4)P2 and does it signal independently of its phosphatase activity?AbstractPhosphatidylinositol‐3,4,5‐trisphosphate: Tool of choice for class I PI 3‐kinasesAbstractPhosphatidylinositol‐4‐phosphate: The Golgi and beyondAbstract. (shrink)

The lateral mobility of membrane lipids and proteins is presumed to play an important functional role in biomembranes. Photobleaching studies have shown that many proteins in the plasma membrane have diffusion coefficients at least an order of magnitude lower than those obtained when the same proteins are reconstituted in artificial bilayer membranes. Depending on the protein, it has been shown that either the cytoplasmic domain or the ectodomain is the key determinant of its lateral mobility. Single particle (...) tracking microscopy, which allows the motions of single or small groups of membrane molecules to be followed, promises not only to reveal new features of membrane dynamics, but also to help explain longstanding puzzles presented by the photobleaching studies, particularly the so‐called immobile fraction. The combination of the two complementary technologies should measurably enhance our understanding of membrane microstructure. (shrink)

Phosphatidylinositol‐3‐kinases (PI3Ks) are lipid kinases that produce 3‐phosphorylated derivatives of phosphatidylinositol upon activation by various cues. These 3‐phosphorylated lipids bind to various protein effectors to control many cellular functions. Lipid phosphatases such as phosphatase and tensin homolog (PTEN) terminate PI3K‐derived signals and are critical to ensure appropriate signaling outcomes. Many lines of evidence indicate that PI3Ks and PTEN, as well as some specific lipid effectors are highly compartmentalized, either in plasma membrane nanodomains or in endosomal compartments. We examine (...) the evidence for specific recruitment of PI3Ks, PTEN, and other related enzymes to membrane nanodomains and endocytic compartments. We then examine the hypothesis that scaffolding of the sources (PI3Ks), terminators (PTEN), and effectors of these lipid signals with a common plasma membrane nanodomain may achieve highly localized lipid signaling and ensure selective activation of specific effectors. This highlights the importance of spatial regulation of PI3K signaling in various physiological and disease contexts. (shrink)

Cellular differentiation is often accompanied by the expression of specialized plasma membrane proteins which accumulate in discrete regions. The biogenesis of these specialized membrane domains involves the assembly and co‐localisation of a spectrin‐based membrane skeleton. While the constituents of the membrane skeleton in non‐erythroid cells are often immunologically related to erythroid spectrin, ankyrin, and protein 4.1, there are structural and functional differences between the isoforms of these membrane skeleton polypeptides, as well as highly variable (...) patterns of expression during cellular differentiation. We consider this heterogeneity of structure and expression during development in the context of the hypothesis that non‐erythroid spectrin, ankyrin, and protein 4.1 are involved in the formation of specialized membrane domains. (shrink)

Signal transduction via receptors for N‐formylmethionyl peptide chemoattractants (FPR) on human neutrophils is a highly regulated process which involves participation of cytoskeletal elements. Evidence exists suggesting that the cytoskeleton and/or the membrane skeleton controls the distribution of FPR in the plane of the plasma membrane, thus controlling the accessibility of FPR to different proteins in functionally distinct domains. In desensitized cells, FPR are restricted to domains which are depleted of G proteins but enriched in cytoskeletal proteins such (...) as actin and fodrin. Thus, the G protein signal transduction partners of FPR become inaccessible to the agonist‐occupied receptor, preventing cell activation. The mechanism of interaction of FPR with the membrane skeleton is poorly understood but evidence is accumulating that suggests a direct binding of FPR (and other receptors) to cytoskeletal proteins such as actin. (shrink)

Two key questions in the autophagy field are the mechanisms that underlie the signals for autophagy initiation and the source of membrane for expansion of the nascent membrane, the phagophore. In this review, we discuss recent findings highlighting the role of the classical endosomal pathway, from plasma membrane to lysosome, in the formation and expansion of the phagophore and subsequent degradation of the autophagosome contents. We also highlight the striking conservation of regulatory factors between the two (...) pathways, including those regulating membrane budding and fusion, and the role of the lysosome in sensing the nutrient status of the cell, regulating mTORC1 activity, and ultimately the initiation of autophagy.Editor's suggested further reading in BioEssays The evolution of dynamin to regulate clathrin‐mediated endocytosis Abstract. (shrink)

Fertilization triggers cytoplasmic movements in the frog egg that lead in mysterious ways to the stabilization of β‐catenin on the dorsal side of the embryo. The novel Huluwa (Hwa) transmembrane protein, identified in China, is translated specifically in the dorsal side, acting as an egg cytoplasmic determinant essential for β‐catenin stabilization. The Wnt signaling pathway requires macropinocytosis and the sequestration inside multivesicular bodies (MVBs, the precursors of endolysosomes) of Axin1 and Glycogen Synthase Kinase 3 (GSK3) that normally destroy β‐catenin. In (...) Xenopus, the Wnt‐like activity of GSK3 inhibitors and of Hwa mRNA can be blocked by brief treatment with inhibitors of membrane trafficking or lysosomes at the 32‐cell stage. In dorsal blastomeres, lysosomal cathepsin is activated and intriguing MVBs surrounded by electron dense vesicles are formed at the 64‐cell stage. We conclude that membrane trafficking and lysosomal activity are critically important for the earliest asymmetries in vertebrate embryonic development. (shrink)

The plasma membrane of polarized epithelial cells is divided into apical and basolateral surfaces, with different compositions. Proteins can be sent directly from the trans‐Golgi network (TGN) to either surface, or can be sent first to one surface and then transcytosed to the other. The glycosyl phosphatidylinositol anchor is a signal for apical targeting. Signals in the cytoplasmic domain containing a β‐turn determine basolateral targeting and retrieval, and are related to other sorting signals. Transcytosed proteins, such as the (...) polymeric immunoglobulin receptor (plgR), are endocytosed from the basolateral surface and then accumulate in a tubular compartment concentrated underneath the apical surface. This compartment, tentatively termed the apical recycling compartment, may be a central sorting station, as it apparently receives material from both surfaces and sorts them for delivery to the correct surface. Delivery to the apical surface from both the TGN and the apical recycling compartment appears to be regulated by protein kinases A and C, and endocytosis from the apical surface is also regulated by kinases. Transcytosis of the plgR is additionally regulated by phosphorylation of the plgR and by ligand binding to the plgR. Regulation of traffic in polarized epithelial cells plays a central role in cellular homeostasis, response to external signals and differentiation. (shrink)

Intercellular communication is an essential process in all multicellular organisms. During this process, molecules secreted by one cell will bind to a receptor on the cognate cell leading to the subsequent uptake of the receptor‐ligand complex. Once inside, the cell then determines the fate of the receptor‐ligand complex and any other proteins that were endocytosed together. Approximately 80% of endocytosed material is recycled back to the plasma membrane either directly or indirectly via the Golgi apparatus and the remaining (...) 20% is delivered to the lysosome for degradation. Although most pathways have been identified, we still lack understanding on how specificity in sorting of recycling cargos into different pathways is achieved, and how the cell reaches high accuracy of these processes in the absence of clear sorting signals in the bulk of the client proteins. In this review, we will summarize our current understanding of the mechanism behind recycling cargo sorting and propose a model of differential affinities between cargo and cargo receptors/adaptors with regards to iterative sorting in endosomes. (shrink)

Citrate, an organic trivalent anion, is a major substrate for generation of energy in most cells. It is produced in mitochondria and used either in the Krebs' cycle or released into cytoplasm through a specific mitochondrial carriers. Citrate can also be taken up from blood through different plasma membrane transporters. In the cytoplasm, citrate can be used ultimately for fatty acid synthesis, which is increased in cancer cells. Here, we review the ways in which citrate can be transported (...) and discuss the changes in transport and metabolism that occur in cancer cells. The primary focus is on the prostate gland, which is known to produce and release large amounts of citrate during its normal secretory function. The significant changes that occur in citrate‐related metabolism and transport in prostate cancer are the second focus. This review strives to relate these mechanisms to molecular biology on the one hand and to clinical applications on the other. (shrink)

Gap junctions are ubiquitous plasma membrane specializations that allow cells to exchange small molecules and ions directly. The isolation, biochemical characterization and molecular cloning of the major protein of rat liver gap junctions lead to a clearer view of these membrane zones that allow cells to ‘talk’ to each other and co‐ordinate their activities in tissues and organs.

Damage repair is a fundamental requirement of all life as organisms find themselves in challenging and fluctuating environments. In particular, damage to the barrier between an organism and its environment (e.g. skin, plasma membrane, bacterial cell envelope) is frequent because these organs/organelles directly interact with the external world. Here, we discuss the general strategies that bacteria use to cope with damage to their cell envelope and their repair limits. We then describe a novel damage‐coping mechanism used by multicellular (...) myxobacteria. We propose that cell‐cell transfer of membrane material within a population serves as a wound‐healing strategy and provide evidence for its utility. We suggest that – similar to how tissues in eukaryotes have evolved cooperative methods of damage repair – so too have some bacteria that live a multicellular lifestyle. (shrink)

The exocyst is a conserved octameric complex that physically tethers a vesicle to the plasma membrane, prior to membrane fusion. It is important not only for secretion and membrane delivery but also, in mammalian cells, for cytokinesis, ciliogenesis, autophagy, tumorigenesis, and host defense. The combination of genome editing and advanced light microscopy of exocyst subunits in living cells has recently shown the complex to be much more dynamic than previously appreciated, and exposed how little we still (...) know about its function and regulation. (shrink)

Multidrug resistance (MDR) is developed in a population of somatic mammalian cells in vivo or in vitro when they are selected for resistance to each of a large group of drugs (colchicine, adriamycin, actinomycin D, etc.). It is produced by the amplification of some unknown gene(s) whose product(s) evidently change(s) the plasma membrane permeability of a cell to a selective agent and to other unrelated compounds. A large specific genomic region (150–250 kbp) undergoes amplification in MDR cells selected (...) for the resistance to different drugs. Amplified DNA is located in homogeneously staining regions of chromosomes, double minute chromosomes and small chromatin bodies – episome‐like structures that replicate out of the S‐phase of a cell cycle. The selection of MDR cell variants is accompanied by regular karyotypic evolution specific for this type of drug resistance. (shrink)

The origin of the eukaryotic cell nucleus and the selective forces that drove its evolution remain unknown and are a matter of controversy. Autogenous models state that both the nucleus and endoplasmic reticulum (ER) derived from the invagination of the plasma membrane, but most of them do not advance clear selective forces for this process. Alternative models proposing an endosymbiotic origin of the nucleus fail to provide a pathway fully compatible with our knowledge of cell biology. We propose (...) here an evolutionary scenario that reconciles both an ancestral endosymbiotic origin of the eukaryotic nucleus (endosymbiosis of a methanogenic archaeon within a fermentative myxobacterium) with an autogenous generation of the contemporary nuclear membrane and ER from the bacterial membrane. We specifically state two selective forces that operated sequentially during its evolution: (1) metabolic compartmentation to avoid deleterious co‐existence of anabolic (autotrophic synthesis by the methanogen) and catabolic (fermentation by the myxobacterium) pathways in the cell, and (2) avoidance of aberrant protein synthesis due to intron spreading in the ancient archaeal genome following mitochondrial acquisition and loss of methanogenesis. BioEssays 28: 525–533, 2006. © 2006 Wiley Periodicals, Inc. (shrink)

The direction and specificity of endolysosomal membrane trafficking is tightly regulated by various cytosolic and membrane‐bound factors, including soluble NSF attachment protein receptors (SNAREs), Rab GTPases, and phosphoinositides. Another trafficking regulatory factor is juxta‐organellar Ca2+, which is hypothesized to be released from the lumen of endolysosomes and to be present at higher concentrations near fusion/fission sites. The recent identification and characterization of several Ca2+ channel proteins from endolysosomal membranes has provided a unique opportunity to examine the roles of (...) Ca2+ and Ca2+ channels in the membrane trafficking of endolysosomes. SNAREs, Rab GTPases, and phosphoinositides have been reported to regulate plasma membrane ion channels, thereby suggesting that these trafficking regulators may also modulate endolysosomal dynamics by controlling Ca2+ flux across endolysosomal membranes. In this paper, we discuss the roles of phosphoinositides, Ca2+, and potential interactions between endolysosomal Ca2+ channels and phosphoinositides in endolysosomal dynamics. (shrink)

Embryogenesis requires the precise movement and reorganization of many cell and tissue types. Presumably, cell surface receptors allow cells to interact selectively with adjacent cells and with the extracellular environment, as well as initiate differentiative events by transducing appropriate signals across the plasma membrane. One cell surface component that serves as a receptor during a variety of cellular interactions is β1,4‐galactosyltransferase. Cell surface galactosyltransferase participates in diverse cellular interactions by binding its specific glycoconjugate substrate on adjacent cell surfaces (...) or in the extracellular matrix. Biochemical, immunological, and molecular probes are being used to better define cell surface galactosyl‐transferase functional in cellular interactions during fertilization, intercellular adhesion, cell migration, and growth control. (shrink)

Apoptosis proteins have a central role in the development and homeostasis of an organism. These proteins are very important for understanding the mechanism of programmed cell death, and their function is related to their types. According to the classification scheme by Zhou and Doctor (2003), the apoptosis proteins are categorized into the following four types: (1) cytoplasmic protein; (2) plasma membrane-bound protein; (3) mitochondrial inner and outer proteins; (4) other proteins. A powerful learning machine, the Support Vector Machine, (...) is applied for predicting the type of a given apoptosis protein by incorporating the sqrt-amino acid composition effect. High success rates were obtained by the re-substitute test (98/98 = 100 %) and the jackknife test (89/98 = 90.8%). (shrink)

Although the generation of reactive oxygen species is an activity normally associated with phagocytic leucocytes, mammalian spermatozoa were, in fact, the first cell type in which this activity was described. In recent years it has become apparent that spermatozoa are not the only nonphagocytic cells to exhibit a capacity for reactive oxygen species production, because this activity has been detected in a wide variety of different cells including fibroblasts, mesangial cells, oocytes, Leyding cells endothelial cells, thryroid cells, adipocytes, tumour cell (...) and platelets. Since the capacity to generate reactive oxygen species is apparently so widespread, the risk‐benefit equation for these potentially pernicious molecules becomes a matter of intese interest. In the case of human spermatozoa, the risk of manufacturing reactive oxygen metabolites is considerable because these cells are particularly vulnerable to lipid peroxidation. Indeed, there is now good evidence to indicate that oxygen radicals are involved in the initiation of peroxidative damage to the sperm plasma membrane, seen in many cases of male infertility. This risk is off‐set by recent data suggesting that superoxide anions and hydrogen peroxide also participate in the induction of key biological events such as hyperactiavated motility and the acrosome reaction. Thus, human spermatozoa appear to use reactive oxygen species for a physiological purpose and have the difficult task of ensuring the balanced generation of these potentially harmful, but biologically important, modulators of cellular function. (shrink)

The rate‐limiting step in the uptake and metabolism of Dglucose by insulin target cells is thought to be glucose transport mediated by glucose transporters (primarily the GLUT4 isoform) localized to the plasma membrane. However, subcellular fractionation, photolabelling and immunocytochemical studies have shown that the pool of GLUT4 present in the plasma membrane is only one of many subcellular pools of this protein. GLUT4 has been found in occluded vesicles at the plasma membrane, clathrin‐coated pits (...) and vesicles, early endosomes, and tubulo‐vesicular structures; the latter are analogous to known specialized secretory compartments. Tracking the movement of GLUT4 through these compartments, and defining the mechanism and site of action of insulin in stimulating this subcellular trafficking, are major topics of current investigation. Recent evidence focuses attention on the exocytosis of GLUT4 as the major site of insulin action. Increased exocytosis may be due to decreased retention of glucose transporters in an intracellular pool, or possibly to increased assembly of a vesicle docking and fusion complex. Although details are unknown, the presence in GLUT4 vesicles of a synaptobrevin homologue leads us to propose that a process analogous to that occurring in synaptic vesicle trafficking is involved in the assembly of GLUT4 vesicles into a form suitable for fusion with the plasma membrane. Evidence that the pathways of signalling from the insulin receptor and of GLUT4 vesicle exocytosis may converge at the level of the key signalling enzyme, phosphatidylinositol 3‐kinase, is discussed. (shrink)

Recently it has become clear that trafficking of integrins to late endosomes is key to the regulation of integrin expression and function during cell migration. Here we discuss the molecular machinery that dictates whether integrins are sorted to recycling endosomes or are targeted to late endosomes and lysosomes. Integrins and other receptors that are sorted to late endosomes are not necessarily degraded and, under certain circumstances, can be spared destruction and returned to the cell surface to drive cell migration and (...) invasion. We will discuss how the exchange of adhesion receptors and other key regulators of cell migration between late endosomes/lysosomes and the plasma membrane can promote dynamic turnover of adhesions during cell migration. (shrink)

There are many morphologically distinct membrane structures with different functions at the surface of epithelial cells. Among these, adherens junctions (AJ) and tight junctions (TJ) are responsible for the mechanical linkage of epithelial cells and epithelial barrier function, respectively. In the process of new cell–cell adhesion formation between two epithelial cells, such as after wounding, AJ form first and then TJ form on the apical side of AJ. This process is very complicated because AJ formation triggers drastic changes in (...) the organization of actin cytoskeleton, the activity of Rho family of small GTPases, and the lipid composition of the plasma membrane, all of which are required for subsequent TJ formation. In this review, the authors focus on the relationship between AJ and TJ as a representative example of specialization of plasma membrane regions and introduce recent findings on how AJ formation promotes the subsequent formation of TJ. (shrink)

Early in animal development, gradients of secreted morphogenic molecules, such as Sonic hedgehog (Shh), Wnt and TGFβ/Bmp family members, regulate cell proliferation and determine the fate and phenotype of the target cells by activating well‐characterized signalling pathways, which ultimately control gene transcription. Shh, Wnt and TGFβ/Bmp signalling also play an important and evolutionary conserved role in neural circuit assembly. They regulate neuronal polarization, axon and dendrite development and synaptogenesis, processes that require rapid and local changes in cytoskeletal organization and (...) class='Hi'>plasma membrane components. A key question then is whether morphogen signalling at the growth cone uses similar mechanisms and intracellular pathway components to those described for morphogen‐mediated cell specification. This review discusses recent advances towards the understanding of this problem, showing how Shh, Wnt and TGFβ/Bmp have adapted their ‘classical’ signalling pathways or adopted alternative and novel molecular mechanisms to influence different aspects of neuronal circuit formation. (shrink)

Abstractβ1,4‐galactosyltransferase is unusual among the glycosyltransferases in that it is found in two subcellular compartments where it performs two distinct functions. In the trans‐Golgi complex, galactosyltransferase participates in oligosaccharide biosynthesis, as do the other glycosyltransferases. On the cell surface, however, galactosyltransferase associates with the cytoskeleton and functions as a receptor for extracellular oligosaccharide ligands. Although we now know much regarding galactosyltransferase function in these two compartments, little is known about how it is targeted to these different sites. By cloning the (...) galactosyltransferase gene products, certain features of the protein have been identified that may be critical for its expression on the cell surface or retention within the Golgi complex. This article discusses recent studies which suggest that a cytoplasmic sequence unique to one galactosyltransferase isoform is required for targeting a portion of this protein to the plasma membrane, enabling it to function as a cell adhesion molecule. These findings allow one to manipulate surface galactosyltransferase expression, either positively or negatively, and perturb galactosyltransferase‐dependent cellular interactions during fertilization and development. (shrink)

Mammalian outer hair cells generate mechanical forces at acoustic frequencies and can thus amplify the sound stimulus within the inner ear. The mechanism of force generation depends upon the plasma membrane potential but not upon either calcium or ATP. Forces are generated in the lateral cortex along the full length of the cell. The cortex includes a two‐dimensional cytoskeletal lattice composed of circumferential filaments 6–7 nm thick that are cross‐linked by filaments 3–4 nm thick and 40–60 nm long. (...) The two filament types may, respectively, be actin and some form of spectrin. The lattice reinforces the cylindrical shape of the cell and permits limited changes in length. Beneath it lie the lateral cisternae, a regular system of multi‐layered membranes. Force–generation may depend upon voltage‐dependent shape changes in proteins that lie either in the plasma membrane or in the cytoskeletal lattice. (shrink)

Thy‐1 is a small glycoprotein of 110 amino acids which, folded in the characteristic structure of an immunoglobulin variable domain1, are anchored to the plasma membrane via a glycophosphatidylinositol (GPI) tail(2,3) (Fig. 1). It is a major component of the surface of various cell types, including neurons, at certain stages of their development (4). These qualities doubtlessly appeal to certain cognoscenti, but it is not clear why they would raise Thy‐1 to the status of a favourite molecule. Indeed, (...) few scientists readily admit to having a favourite. We study individual molecules because science is rooted in specific observations; but we do so in order to discover mechanisms of general importance. A molecule's appeal is dependent on its ability to reveal novel aspects of how nature works.Thy‐1 has been unusual in this respect. It was the first lymphocyte surface antigen shown to be restricted to a functional subset of lymphocytes (T cells in the mouse), a finding crucial to the development of cellular immunology(5); it was one of the first cell surface molecules to be sequenced and indicated the importance of immunoglobulin domains and GPI anchors as structural motifs(1–3); it has been pivotal in studies demonstrating that GPI‐anchored molecules are able to signal across the membrane they do not span(6, 7). Thy‐1 has revealed this much, however, with the charm of an adroit stripper: it has always promised glimpses of things more exciting than that displayed. In particular, the function of this molecule has never emerged.Our current work on Thy‐1 in the nervous system is, we believe, finally prizing this last secret into the open. It suggests that Thy‐1 on the neuronal surface interacts with one of the main glial types of adult brain, the astrocyte, to restrict axonal growth(8). There is no current consensus that astrocytes do regulate such growth, and this is the wider question we are addressing from the perspective of Thy‐1. At issue also is how to progress from knowing the nature of a molecule to defining its function. This task has already been achieved with molecules which were harder to characterise than Thy‐1. The problem is a classic Catch 22 situation: Thy‐1 has been readily characterised because it is small and abundant it is ten times more abundant than any other surface glycoprotein on rat thymocytes(9); but since there is so much of it, whatever it does it must do rather badly (or there would be no need to have so much of it!). Defining a function based on low activity or affinity is not trivial.Here I describe two aspects of work on Thy‐1. One is long completed ‐ the research which led to Thy‐1 being isolated and characterised ‐ and included because it demonstrates features which are still applicable to much research using a antibodies today. The other is our ongoing work on the nervous system, which is at that exciting early stage where hypotheses are formed, but the answers are still coming in. (shrink)

Recent findings regarding the cellular biology and immunology of BST‐2 (also known as tetherin) indicate that its function could be exploited as a universal replication inhibitor of enveloped respiratory viruses (e.g., influenza, respiratory syncytial virus, etc.). BST‐2 inhibits viral replication by preventing virus budding from the plasma membrane and by inducing an antiviral state in cells adjacent to infection via unique inflammatory signaling mechanisms. This review presents the first comprehensive summary of what is currently known about BST‐2 anti‐viral (...) function against respiratory viruses, how these viruses construct countermeasures to antagonize BST‐2, and how BST‐2 function might be targeted to develop therapies to treat respiratory virus infections. The authors address the current gaps in knowledge, including the need for mechanistic understanding of BST‐2 antagonism by respiratory viruses, that should be bridged to achieve that goal. (shrink)

The advent of simple in vitro fertilisation techniques has provided the reproductive biologist with an invaluable system for assaying sperm fertilising ability. In particular, they provide a useful way of identifying and characterising gamete‐specific proteins that play a role in sperm‐egg interactions, and in recent years, a growing number of sperm surface proteins have been identified that appear to be involved in these processes. Fertilin β was one of the first sperm membrane proteins to be implicated in egg interactions (...) and it has been proposed that this is mediated by means of binding of its disintegrin‐like domain to cognate integrin receptors on the egg plasma membrane. A recent paper in Science by Cho and colleagues [Cho et al. 1998. Fertilisation defects in sperm from mice lacking fertilin β. Science 281:1857–1859 (Ref. 1)] provides preliminary data on a fertilin β knockout mouse. Whilst fertilin βnull males had greatly reduced fertility, somewhat surprisingly, this could be largely attributed to causes other than impaired binding to the egg plasma membrane. BioEssays 21:183–187, 1999. © 1999 John Wiley & Sons, Inc. (shrink)

Protein ubiquitination constitutes a post‐translational modification mediated by ubiquitin ligases whereby ubiquitinated substrates are degraded through the proteasomal or lysosomal pathways, or acquire novel molecular functions according to their “ubiquitin codes.” Dysfunction of the ubiquitination process in cells causes various diseases such as cancers along with neurodegenerative, auto‐immune/inflammatory, and metabolic diseases. KCTD10 functions as a substrate recognition receptor for cullin‐3 (CUL3), a scaffold protein in RING‐type ubiquitin ligase complexes. Recently, studies by ourselves and others have identified new substrates that are (...) ubiquitinated by the CUL3/KCTD10 ubiquitin ligase complex. Moreover, the type of polyubiquitination (e.g., K27‐, K48‐, or K63‐chain) of various substrates (e.g., RhoB, CEP97, EIF3D, and TRIF) mediated by KCTD10 underlies its divergent roles in endothelial barrier formation, primary cilium formation, plasma membrane dynamics, cell proliferation, and immune response. Here, the physiological functions of KCTD10 are summarized and potential mechanisms are proposed. (shrink)

The developing neuromuscular junction has provided an important paradigm for studying synapse formation. An outstanding feature of neuromuscular differentiation is the aggregation of acetylcholine receptors (AChRs) at high density in the postsynaptic membrane. While AChR aggregation is generally believed to be induced by the nerve, the mechanisms underlying aggregation remain to be clarified. A 43‐kD protein (43k) normally associated with the cytoplasmic aspect of AChR clusters has long been suspected of immobilizing AChRs by linking them to the cytoskeleton. In (...) recent studies, the AChR clustering activity of 43k has, at last, been demonstrated by expressing recombinant AChR and 43k in non‐muscle cells. Mutagenesis of 43k has revealed distinct domains within the primary structure which may be responsible for plasma membrane targeting and AChR binding. Other lines of study have provided clues as to how nerve‐derived (extracellular) AChR‐cluster inducing factors such as agrin might activate 43k‐driven postsynaptic membrane specialization. (shrink)