Dyneins are a family of motor proteins that move along the microtubule. Motility is generated in the motor domain, which consists of a ring of six AAA+ (ATPases associated with diverse cellular activities) domains, the linker and the microtubule‐binding domain (MTBD). The cyclic ATP‐hydrolysis in the AAA+ ring causes the remodelling of the linker, which creates the necessary force for movement. The production of force has to be synchronized with cycles of microtubule detachment and rebinding to efficiently create movement along (...) the microtubule. The analysis of four dynein motor domain crystal structures in the essay presented here provides evidence that this crucial coordination is carried out by open/closed AAA+ ring conformations. (shrink)

Cytoplasmic dynein is the major minus‐end‐directed motor protein in eukaryotes, and has functions ranging from organelle and vesicle transport to spindle positioning and orientation. The mode of regulation of dynein in the cell remains elusive, but a tantalising possibility is that dynein is maintained in an inhibited, non‐motile state until bound to cargo. In vivo, stable attachment of dynein to the cell membrane via anchor proteins enables dynein to produce force by pulling on microtubules and (...) serves to organise the nuclear material. Anchor proteins of dynein assume diverse structures and functions and differ in their interaction with the membrane. In yeast, the anchor protein has come to the fore as one of the key mediators of dynein activity. In other systems, much is yet to be discovered about the anchors, but future work in this area will prove invaluable in understanding dynein regulation in the cell. (shrink)

How do cells direct the microtubule motor protein dynein to move cellular components to the right place at the right time? Recent studies in budding yeast shed light on a new mechanism for directing dynein, involving the protein She1. She1 restricts where and when dynein moves the nucleus and mitotic spindle. Experiments with purified proteins show that She1 binds to microtubules and inhibits dynein by stalling the motor on its track. Here I describe what we have (...) learned so far about She1, based on a combination of genetic, cell biology, and biophysical approaches. These findings set the stage for further interrogation of the She1 mechanism, and raise the question of whether similar mechanisms exist in other species. (shrink)

Most accounts of mechanism discovery have focused on mechanisms that perform the work required to produce a phenomenon. These mechanisms are often subject to regulation by control mechanisms. Using the example of the molecular motor dynein, this paper examines one process by which such control mechanisms are discovered—the process by which researchers, after identifying additional components required to produce the phenomenon but not directly involved in the work of producing that phenomenon, investigate both how these components act on the (...) original mechanism and how they do so in response to measurements of conditions relevant to the operation of the controlled mechanism. (shrink)

Asymmetric RNA localization is required for many developmental processes in a wide range of organisms. For example, wingless and pair‐rule transcripts are localized to the apical membrane of polarized cells. It has been unclear, however, if this localization is important for biological activity and, in addition, how the transcripts are transported. Two recent studies(1,2) have identified cis‐elements and trans‐acting factors that are required for the asymmetric localization of mRNAs. Correct localization is shown to be required for biological activity, and a (...) mechanism of RNA transport involving the microtubule motor dynein has been revealed. BioEssays 23:869–872, 2001. © 2001 John Wiley & Sons, Inc. (shrink)

In this paper, I review a collection of recently published papers that have provided significant new information about the biogenesis and functions of motile cilia. In vertebrates, the activity of motile cilia has been associated with a fascinating diversity of developmental and physiological processes. Despite the importance, much remains to be learned about the genetic control and cellular events that are involved in the differentiation of motile cilia. We also need to better understand the mechanisms by which cilia‐driven fluid flow (...) is able to influence such a variety of developmental and physiological processes. The Foxj1 family of proteins has now been definitively established as master regulators of motile ciliogenesis.1,2 Identification of the Kintoun/PF13 protein has shed light on the assembly of dynein arms,3 whereas live imaging of ciliary motility has led to the discovery of an intriguing new role for motile cilia in otolith formation in the ear.4. (shrink)

Autoinhibition is a design principle realized in many molecular mechanisms in biology. After explicating the notion of a design principle and showing that autoinhibition is such a principle, we focus on how researchers discovered instances of autoinhibition, using research establishing the autoinhibition of the molecular motors kinesin and dynein as our case study. Research on kinesin and dynein began in the fashion described in accounts of mechanistic explanation but, once the mechanisms had been discovered, researchers discovered that they (...) exhibited a second phenomenon, autoinhibition. The discovery of autoinhibition not only reverses the pattern in terms of which philosophers have understood mechanism discovery but runs counter to the one phenomenon-one mechanism principle assumed to relate mechanisms and the phenomena they explain. The ubiquity of autoinhibition as a design principle, therefore, necessitates a philosophical understanding of mechanisms that recognizes how they can participate in more than one phenomenon. Since mechanisms with this design are released from autoinhibition only when they are acted on by control mechanisms, we advance a revised account of mechanisms that accommodates attribution of multiple phenomena to the same mechanism and distinguishes them from other processes that control them. (shrink)

This bibliographical review of the modelling of the mitotic apparatus covers a period of one hundred and twenty years, from the discovery of the bipolar mitotic spindle up to the present day. Without attempting to be fully comprehensive, it will describe the evolution of the main ideas that have left their mark on a century of experimental and theoretical research. Fol and Bütschli's first writings date back to 1873, at a time when Schleiden and Schwann's cell theory was rapidly gaining (...) ground throughout Germany. Both mitosis and chromosomes were to be discovered within the space of thirty years, along with the two key events in the animal and plant reproductive cycle, namely fecondation and meiosis. The mitotic pole, a term still in use to this day, was employed to describe a morphological fact which was noted as early as 1876, namely that the lines and the dots of the karyokinetic figure, with its spindle and asters, looks remarkably like the lines of force around a bar magnet. This was to lead to models designed to explain the movements of chromosomes which take place when the cell nucleus appears to cease to exist as an organelle during mitosis. The nature of those mechanisms and the origin of the forces behind the chromosomes' ordered movements were central to the debate. Auguste Prenant, in a remarkable bibliographical synthesis published in 1910, summed up the opposing viewpoints of the vitalists, on the one hand, who favoured the theory of contractility or extensility in spindle fibres, and of those who believed in models based on physical phenomena, on the other. The latter subdivided into two groups: some, like Bütschli, Rhumbler or Leduc, referred to diffusion, osmosis and superficial tension, whilst the others, led by Gallardo and Hartog, focussed on the laws of electromagnetism. Lillie, Kuwada and Darlington followed up this line of research. The mid-20th century was a major turning point. Most of the modelling mentioned above was criticized and fell into disuse after disappearing from research publications and textbooks.This marked the onset of a new era, as electron microscopes made possible the materialization and detailed study of the macromolecular elements of the fibres, filaments and microtubules of the cytoskeleton. The successive phases of (a) de Harven and Bernhard's 1956 discovery of the centriole's ultrastructure, (b) its identification with the basal body of the cilia and flagella, confirming the theory set out by Henneguy and von Lenhossek (1898–99), (c) the universal presence of microtubules in animal, vegetal and eukaryotic protist cells, (d) the polymerization-depolymerization induced reversible transformations of the tubulin pool in mitosing cells (Inoue, 1960), (e) ultrastructural comparative studies of the mitotic apparatus of eukaryotes illustrating the Pickett-Heaps integrating concept of the MTOC (microtubule-organizing centre), (f) the possibility ofin vitro experiments on mtocs or on microtubules, brings us upon the present day, which has seen the focus placed on the concept of motor-proteins (kinesin, dynein) and on cell cycle models. The latter are based on a close coincidence between the observable modifications of the mitotic apparatus and the periodic variations in intracellular concentrations of calcium or of certain enzymes (cyclins, Cdc2) during the main transitions of the cell cycle. (shrink)

Understanding how chemical energy is converted into directed movement is a fundamental problem in biology. In higher organisms this is accomplished through the hydrolysis of ATP by three families of motor proteins: myosin, dynein and kinesin. The most abundant of these is myosin, which operates against actin and plays a central role in muscle contraction. As summarized here, great progress has been made towards understanding the molecular basis of movement through the determination of the three‐dimensional structures of myosin and (...) actin and through the establishment of systems for site‐directed mutagenesis of this motor protein. It now appears that the generation of movement is coupled to ATP hydrolysis by a series of domain movements within myosin. (shrink)

How do cells order their cytoplasm? While microtubule organizing centers have long been considered essential to conferring order by virtue of their microtubule nucleating activity, attention has currently refocused on the role that microtubule motors play in organizing microtubules. An intriguing set of recent findings(1) reveals that cell fragments, lacking microtubule organizing centers, rapidly organize microtubules into a radial array during organelle transport driven by the microtubule motor, cytoplasmic dynein. Further, interaction of radial microtubules with the cell surface centers (...) the array, revealing that centering information resides not with centrosomes but with organized microtubules. (shrink)

Tissues are shaped and patterned by mechanical and chemical processes. A key mechanical process is the positioning of the mitotic spindle, which determines the size and location of the daughter cells within the tissue. Recent force and position-fluctuation measurements indicate that pushing forces, mediated by the polymerization of astral microtubules against­ the cell cortex, maintain the mitotic spindle at the cell center in Caenorhabditis elegans embryos. The magnitude of the centering forces suggests that the physical limit on the accuracy and (...) precision of this centering mechanism is determined by the number of pushing microtubules rather than by thermally driven fluctuations. In cells that divide asymmetrically, anti-centering, pulling forces generated by cortically located dyneins, in conjunction with microtubule depolymerization, oppose the pushing forces to drive spindle displacements away from the center. Thus, a balance of centering pushing forces and anti-centering pulling forces localize the mitotic spindles within dividing C. elegans cells. We discuss microtubule-based mechanisms of the mitotic spindle positioning, a key mechanical process that shapes and patterns tissues. The magnitude of the centering forces indicates that the physical limit on the accuracy and precision of centering is set by the number of contributing microtubules rather than by thermally driven fluctuations. (shrink)

Beta‐catenin is a multifunctional protein with critical roles in cell‐cell adhesion, Wnt‐signaling and the centrosome cycle. Whereas the roles of β‐catenin in cell‐cell adhesion and Wnt‐signaling have been studied extensively, the mechanism(s) involving β‐catenin in centrosome functions are poorly understood. β‐Catenin localizes to centrosomes and promotes mitotic progression. NIMA‐related protein kinase 2 (Nek2), which stimulates centrosome separation, binds to and phosphorylates β‐catenin. β‐Catenin interacting proteins involved in Wnt signaling such as adenomatous polyposis coli, Axin, and GSK3β, are also localized at (...) centrosomes and play roles in promoting mitotic progression. Additionally, proteins associated with cell‐cell adhesion sites, such as dynein, regulate mitotic spindle positioning. These roles of proteins at the cell cortex and Wnt signaling that involve β‐catenin indicate a cross‐talk between different sub‐cellular sites in the cell at mitosis, and that different pools of β‐catenin may co‐ordinate centrosome functions and cell cycle progression. (shrink)

One major milestone in the development of the sea urchin embryo is the assembly of a single cilium on each blastomere just before hatching. These cilia are constructed both from pre‐existing protein building blocks, such as tubulin and dynein, and from a number of 9+2 architectural elements that are synthesized de novo at ciliogenesis. The finite or quantal synthesis of certain key architectural proteins is coincident with ciliary elongation and proportional to ciliary length. Upon deciliation, the synthesis of architectural (...) proteins occurs anew, a new cilium grows, and the stores of various building blocks are replenished. This routine of coordinated ciliary gene expression may be replayed experimentally many times without delaying normal development. The ability to regenerate cilia has allowed elucidation of these various protein synthetic relationships and has led to the discovery of the pathways by which membrane‐associated tubulin and axoneme‐associated architectural proteins are conveyed into the highly compartmentalized growing cilium. The sea urchin embryo thus provides a very convenient model system for studies of ciliary assembly and maintenance, coordinate gene expression and membrane dynamics. (shrink)

The cytoskeleton has a central role in eukaryotic biology, enabling cells to organize internally, polarize, and translocate. Studying cytoskeletal machinery across the tree of life can identify common elements, illuminate fundamental mechanisms, and provide insight into processes specific to less‐characterized organisms. Red algae represent an ancient lineage that is diverse, ecologically significant, and biomedically relevant. Recent genomic analysis shows that red algae have a surprising paucity of cytoskeletal elements, particularly molecular motors. Here, we review the genomic and cell biological evidence (...) and propose testable models of how red algal cells might perform processes including cell motility, cytokinesis, intracellular transport, and secretion, given their reduced cytoskeletons. In addition to enhancing understanding of red algae and lineages that evolved from red algal endosymbioses (e.g., apicomplexan parasites), these ideas may also provide insight into cytoskeletal processes in animal cells. (shrink)