Active transport between the nucleus and cytoplasm involves primarily three classes of macromolecules: substrates, adaptors, and receptors. Some transport substrates bind directly to an import or an export receptor while others require one or more adaptors to mediate formation of a receptor-substrate complex. Once assembled, these transport complexes are transferred in one direction across the nuclear envelope through aqueous channels that are part of the nuclear pore complexes (NPCs). Dissociation of the transport complex must then take place, and both adaptors and receptors must be recycled through the NPC to allow another round of transport to occur. Directionality of either import or export therefore depends on association between a substrate and its receptor on one side of the nuclear envelope and dissociation on the other. The Ran GTPase is critical in generating this asymmetry. Regulation of nucleocytoplasmic transport generally involves specific inhibition of the formation of a transport complex; however, more global forms of regulation also occur.

Nucleocytoplasmic Transport: The Soluble Phase

Current evidence suggests that the nucleus has a distinct substructure, albeit one that is dynamic rather than a rigid framework. Viral infection, oncogene expression, and inherited human disorders can each cause profound and specific changes in nuclear organization. This review summarizes recent progress in understanding nuclear organization, highlighting in particular the dynamic aspects of nuclear structure.

/pdf/structure-and-function-in-the-nucleus-3vxruca62h.pdf

Structure and Function in the Nucleus

The nucleolus of eukaryotic cells was first described in the early 19th century and was discovered in the 1960s to be the seat of ribosome synthesis. Although rRNA transcription, rRNA processing and ribosome assembly have been clearly established as major functions of the nucleolus, recent studies suggest that the nucleolus participates in many other aspects of gene expression as well. Thus, the nucleolus has been implicated in the processing or nuclear export of certain mRNAs. In addition, new results indicate that biosyntheses of signal recognition particle RNA and telomerase RNA involve a nucleolar stage and that the nucleolus is also involved in processing of U6 RNA, one of the spliceosomal small nuclear RNAs. Interestingly, these three nucleolus-associated small nuclear RNAs (signal recognition particle RNA, telomerase RNA and U6 RNA) are components of catalytic ribonucleoprotein machines. Finally, recent work has also suggested that some transfer RNA precursors are processed in the nucleolus. The nucleolus may have evolutionarily descended from a proto-eukaryotic minimal genome that was spatially linked to vicinal RNA processing and ribonucleoprotein assembly events involved in gene read-out. The nucleolus of today's eukaryotes, now surrounded by the chromatin of over 2 billion years of genome expansion, may still perform these ancient functions, in addition to ribosome biosynthesis. The plurifunctional nucleolus concept has a strong footing in contemporary data and adds a new perspective to our current picture of the spatial-functional design of the cell nucleus.

/pdf/the-plurifunctional-nucleolus-n80fklrlv3.pdf

The plurifunctional nucleolus

Nucleus-vacuole (NV) junctions in Saccharomyces cerevisiae are formed through specific interactions between Vac8p on the vacuole membrane and Nvj1p in the nuclear envelope. Herein, we report that N...

/pdf/piecemeal-microautophagy-of-nucleus-in-saccharomyces-hbik5qdfky.pdf

Piecemeal Microautophagy of Nucleus in Saccharomyces cerevisiae

Summary of NPL3 mutations and phenotypes Nuclear accumulation Npl3p mislocalization of poly(A) + RNA ("C) Allele Mutationa Domainb 2 5 30 3 7 25 30 37 Class 1 np13-1 np13-26 np13-38 np13-48 Class 2 np13-3 np13-17 Class 3 np13 -2 7 RRMl - - - + + + + RRM 1 + + - - + + + RRM2 N-term - - - + + + + RRMl RRM 1 RGGY + + + + + + - - - - - -- - - - -- - - Phenotypes are described as approximate number of cells displaying the defect either Npl3p mislocalization to the cytoplasm or accumulation of RNA in the nucleus. ( + ) 0%-50% ; ( + + ) 50%-100%. "Amino acid changes are described using the one-letter amino acid code with the first letter indicating the wild-type amino acid followed by its number and the amino acid to which it is changed in the mutant. b~he domain in which the mutation is present. for all of the mutants tested, the level of the mutant Np13 protein is unchanged after temperature shift, as determined by Western blotting of total protein from cell extracts (not shown). A new assay for Npl3p nuclear export To study the relationship between Npl3p and RNA nu- clear export, we needed to devise a new assay that effec- tively uncoupled the two processes from nuclear protein import. A previous assay employed the formation of yeast heterokaryons, where the readout is the re-uptake of a shuttling protein into a second nucleus (Flach et al.

/pdf/a-protein-that-shuttles-between-the-nucleus-and-the-3amjp542ol.pdf

A protein that shuttles between the nucleus and the cytoplasm is an important mediator of RNA export.

Transport of mRNA from the nucleus to the cytoplasm plays an important role in gene expression in eukaryotic cells. In wild-type Schizosaccharomyces pombe cells poly(A)+ RNA is uniformly distributed throughout the nucleoplasm and cytoplasm. However, we found that a severe heat shock blocks mRNA transport in S. pombe, resulting in the accumulation of bulk poly(A)+ RNA, as well as a specific intron-less transcript, in the nucleoli. Pretreatment of cells with a mild heat shock, which induces heat shock proteins, before a severe heat shock protects the mRNA transport machinery and allows mRNA transport to proceed unimpeded. In heat-shocked S. pombe cells, the nucleolar region condensed into a few compact structures. Interestingly, poly(A)+ RNA accumulated predominantly in the condensed nucleolar regions of the heat-shocked cells. These data suggest that the yeast nucleolus may play a role in mRNA transport in addition to its roles in rRNA synthesis and preribosome assembly.

/pdf/nucleolar-accumulation-of-poly-a-rna-in-heat-shocked-yeast-3z04fd8m37.pdf

T Tani

Papers

Nucleolar accumulation of poly (A)+ RNA in heat-shocked yeast cells: implication of nucleolar involvement in mRNA transport.