Structure and dynamics of the post-transcriptional mRNA-processing machinery
© GenomeBiology.com 2000
Published: 6 December 2000
A report on the 'RNA processsing' and 'Nuclear Import and Export' minisymposia at the first meeting of the European Life Scientist Organization (ELSO), Geneva, 2-6 September, 2000.
Eukaryotic gene expression depends on the synthesis, processing, export and translation of mRNA. These minisymposia tackled multiple aspects of the post-transcriptional pathway of mRNAs including the structure and dynamics of proteins implicated in splicing, polyadenylation and nuclear export.
Dynamics of the cell nucleus
It has been known for some time that the mammalian cell nucleus contains numerous subcompartments, or bodies, which have been implicated in essential processes such as transcription and RNA processing. Cytological studies in which researchers observed the localization of various nuclear proteins have led to the definition of distinct nuclear subcompartments: the nucleolus, the nuclear speckles and several small nuclear bodies including the Cajal bodies (also called coiled bodies). An emerging theme of the meeting was the realization that these nuclear subcompartments are extremely dynamic and that proteins that are usually localized within these compartments are in fact moving rapidly throughout the entire nucleus. Highlights of these minisymposia included reports on the dynamics of proteins localized in these nuclear compartments from Tom Misteli (National Cancer Institute, National Institutes of Health, Bethesda, USA) and the biochemical purification of nucleoli and Cajal bodies from Angus Lamond (University of Dundee, UK).
This report was complemented by Lamond's work on the biochemical purification and dynamics of Cajal bodies and nuclei. Strikingly, Lamond's studies revealed that the nucleolus, the most extensively studied nuclear subcompartment, is largely composed of 'unknown' proteins. Further information on the sequence, structural domains and function of these novel proteins is eagerly awaited by many.
Nuclear export of messenger RNA
Both in the nucleus and in the cytoplasm, mRNAs form complexes (messenger ribonucleoproteins or mRNPs) with multiple RNA-binding proteins that affect RNA processing, transport, translation, stability and localization. An excellent example of this is the Balbiani ring mRNPs from the insect Chironomus tentans, which are composed of an RNA molecule of 35-40 kilobases associated with about 500 protein molecules. Published work from the laboratory of Bertil Daneholt (Karolinska Institute, Stockholm, Sweden), describes the identification of many of the proteins associated with this mRNA, including the splicing factor Hrp45 (a homolog of SF2/ASF) and several RNA-binding proteins. Daneholt reported at this meeting that monomeric actin is also associated with the Balbiani ring mRNP in the nucleoplasm. Although the role of actin in this context is unknown, the data raise the intriguing possibility that actin may be involved in some aspects of mRNA export, perhaps facilitating the movement of large mRNPs in the nucleoplasm or across the NPC.
From function to structure
Removal of introns from pre-mRNA is mediated by the spliceosome, a large complex consisting of four small ribonucleoprotein particles (snRNPs) and numerous proteins. Although the determination of the structure of the spliceosome is lagging behind that of the recently reported structures of ribosomal subunits, remarkable progress has been made in determining the structure of some of its individual components. The determination of the structure of the U1 snRNP using cryoelectron microscopy was reported by Holger Stark (Max Planck Institute for Biophysical Chemistry, Göttingen, Germany).
The RNA processing and export field are undoubtedly benefiting from the use of structural biology, proteomics and imaging techniques, which has allowed important contributions to our understanding of gene expression events in vivo.