In eukaryotic cells, messenger RNAs are formed by extensive posttranscriptional processing of primary transcripts, assembled with a large number of proteins and processing factors in ribonucleoprotein complexes. The protein moiety of these complexes mainly constitutes a class of about 20 major polypeptides called heterogeneous nuclear ribonucleoproteins or hnRNPs. The function and the mechanism of action of hnRNPs is still not fully understood, but the identification of RNA binding domains and RNA binding specificities, and the development of new (...) functional assays, has stimulated interest in them. In contrast to previous models that hypothesised a mere structural (histone‐like) function, a more diversified and dynamic role for these proteins is now emerging. In fact, they can be viewed as a subset of the trans‐acting pre‐mRNA maturation factors. They might actively participate in post‐transcriptional events such as regulated splicing and mRNA export. Moreover, recent data suggest an involvement of some of these proteins in molecular diseases. Here we present an overview of the most relevant properties of hnRNPs and discuss some emerging ideas on theiir roles. (shrink)

Since its original identification as a component of the heterogeneous nuclear ribonucleoprotein (hnRNP) complex, K protein has been found not only in the nucleus but also in the cytoplasm and mitochondria and is implicated in chromatin remodeling, transcription, splicing and translation processes. K protein contains multiple modules that, on one hand, bind kinases while, on the other hand, recruit chromatin, transcription, splicing and translation factors. Moreover, the K‐ protein‐mediated interactions are regulated by signaling cascades. These observations (...) are consistent with K protein acting as a docking platform to integrate signaling cascades by facilitating cross‐talk between kinases and factors that mediate nucleic‐acid‐directed processes. Comparison of K across species reveals that it is an essential factor in metazoans, but not in yeast. Although some of the K protein interactions and functions are conserved in eukaryotes from yeast to man, the mammalian protein seems to play a wider role. The greater diversity of mammalian K protein interactions and function may reflect gain of novel docking sites and expansion evolutionary of gene expression networks. BioEssays 26:629–638, 2004. © 2004 Wiley Periodicals, Inc. (shrink)

The noncoding RNA 7SK is a critical regulator of transcription by adjusting the activity of the kinase complex P‐TEFb. Release of P‐TEFb from 7SK stimulates transcription at many genes by promoting productive elongation. Conversely, P‐TEFb sequestration by 7SK inhibits transcription. Recent studies have shown that 7SK functions are particularly important for neuron development and maintenance and it can thus be hypothesized that 7SK is at the center of many signaling pathways contributing to neuron function. 7SK activates neuronal gene expression programs (...) that are key for terminal differentiation of neurons. Proteomics studies revealed a complex protein interactome of 7SK that includes several RNA‐binding proteins. Some of these novel 7SK subcomplexes exert non‐canonical cytosolic functions in neurons by regulating axonal mRNA transport and fine‐tuning spliceosome production in response to transcription alterations. Thus, a picture emerges according to which 7SK acts as a multi‐functional RNA scaffold that is integral for neuron homeostasis. (shrink)