Nuclear shuttling of She2p couples ASH1 mRNA localization to its translational repression by recruiting Loc1p and Puf6p

Cotranscriptional recruitment of She2p by RNA pol II elongation factor Spt4-Spt5/DSIF promotes mRNA localization to the yeast bud

Pre-mRNA processing is coupled with transcription. It is still unclear if the transcription machinery can also directly affect the cytoplasmic fate of a transcript, such as its intracellular localization. In yeast, the RNA-binding protein She2p binds several mRNAs and targets them for localization at the bud. Here we report that She2p is recruited cotranscriptionally to the nascent bud-localized ASH1, IST2, and EAR1 mRNA. She2p interacts in vivo with the elongating forms of RNA polymerase II (pol II) via the transcription elongation factor Spt4-Spt5. Mutations in either SPT4 or SPT5 reduce the cotranscriptional recruitment of She2p on the ASH1 gene, disrupt the proper localization of ASH1 mRNA at the bud tip, and affect Ash1p sorting to the daughter cell nucleus. We propose that She2p is recruited by the RNA pol II machinery prior to its transfer to nascent bud-localized mRNAs. Indeed, She2p is present with RNA pol II on genes coding for localized or nonlocalized transcripts, but is associated with nascent mRNA only on genes coding for bud-localized transcripts. Moreover, a She2p mutant defective in RNA binding still associates with RNA pol II transcribed genes. This study uncovers a novel mechanism for the cotranscriptional assembly of mRNP complexes primed for localization in the cytoplasm.

The nuclear experience of CPEB: implications for RNA processing and translational control

CPEB is a sequence-specific RNA binding protein that promotes polyadenylation-induced translation in early development, during cell cycle progression and cellular senescence, and following neuronal synapse stimulation. It controls polyadenylation and translation through other interacting molecules, most notably the poly(A) polymerase Gld2, the deadenylating enzyme PARN, and the eIF4E-binding protein Maskin. Here, we report that CPEB shuttles between the nucleus and cytoplasm and that its export occurs via the CRM1-dependent pathway. In the nucleus of Xenopus oocytes, CPEB associates with lampbrush chromosomes and several proteins involved in nuclear RNA processing. CPEB also interacts with Maskin in the nucleus as well as with CPE-containing mRNAs. Although the CPE does not regulate mRNA export, it influences the degree to which mRNAs are translationally repressed in the cytoplasm. Moreover, CPEB directly or indirectly mediates the alternative splicing of at least one pre-mRNA in mouse embryo fibroblasts as well as certain mouse tissues. We propose that CPEB, together with Maskin, binds mRNA in the nucleus to ensure tight translational repression upon export to the cytoplasm. In addition, we propose that nuclear CPEB regulates specific pre-mRNA alternative splicing.

Formation of She2p tetramers is required for mRNA binding, mRNP assembly, and localization

In eukaryotic cells, dozens to hundreds of different mRNAs are localized by specialized motor-dependent transport complexes. One of the best-studied examples for directional mRNA transport is the localization of ASH1 mRNA in Saccharomyces cerevisiae. For transport, ASH1 mRNA is bound by the unusual RNA-binding protein She2p. Although previous results indicated that She2p forms dimers required for RNA binding and transcript localization, it remained unclear if the dimer constitutes the minimal RNA-binding unit assembling in vivo. By using analytical ultracentrifugation we found that She2p forms larger oligomeric complexes in solution. We also identified a point mutant that shows impaired oligomer formation. Size-exclusion chromatography suggests that She2p forms defined tetramers at physiological concentrations. Subsequent structural studies by small-angle X-ray scattering confirmed this finding and demonstrated that the previously observed She2p dimers interact in a head-to-head conformation to form an elongated tetrameric complex. This She2p tetramer suggests the generation of large continuous RNA-binding surfaces at both sides of the complex. Biochemical studies and immunostaining of cells confirmed that She2p tetramer formation is required for RNA binding, efficient mRNP assembly, and mRNA localization in vivo. Our finding on She2p tetramerization resolves previously raised questions on complex formation and mRNP function.

She3p possesses a novel activity required for ASH1 mRNA localization in Saccharomyces cerevisiae

Intracellular and intercellular polarity requires that specific proteins be sorted to discreet locations within and between cells. One mechanism for sorting proteins is through RNA localization. In Saccharomyces cerevisiae, ASH1 mRNA localizes to the distal tip of the bud, resulting in the asymmetric sorting of the transcriptional repressor Ash1p. ASH1 mRNA localization requires four cis-acting localization elements and the trans-acting factors Myo4p, She3p, and She2p. Myo4p is a type V myosin motor that functions to directly transport ASH1 mRNA to the bud. She2p is an RNA-binding protein that directly interacts with the ASH1 mRNA cis-acting elements. Currently, the role for She3p in ASH1 mRNA localization is as an adaptor protein, since it can simultaneously associate with Myo4p and She2p. Here, we present data for two novel mutants of She3p, S348E and the double mutant S343E S361E, that are defective for ASH1 mRNA localization, and yet both of these mutants retain the ability to associate with Myo4p and She2p. These observations suggest that She3p possesses a novel activity required for ASH1 mRNA localization, and our data imply that this function is related to the ability of She3p to associate with ASH1 mRNA. Interestingly, we determined that She3p is phosphorylated, and global mass spectrometry approaches have determined that Ser 343, 348, and 361 are sites of phosphorylation, suggesting that the novel function for She3p could be negatively regulated by phosphorylation. The present study reveals that the current accepted model for ASH1 mRNA localization does not fully account for the function of She3p in ASH1 mRNA localization.