Assembly of export-competent mRNP: it's all about being connected

The export factor Yra1 modulates mRNA 3' end processing

The Saccharomyces cerevisiae mRNA export adaptor Yra1 binds the Pcf11 subunit of cleavage-polyadenylation factor CF1A that links export to 3' end formation. We found that an unexpected consequence of this interaction is that Yra1 influences cleavage-polyadenylation. Yra1 competes with the CF1A subunit Clp1 for binding to Pcf11, and excess Yra1 inhibits 3' processing in vitro. Release of Yra1 at the 3' ends of genes coincides with recruitment of Clp1, and depletion of Yra1 enhances Clp1 recruitment within some genes. These results suggest that CF1A is not necessarily recruited as a complete unit; instead, Clp1 can be incorporated co-transcriptionally in a process regulated by Yra1. Yra1 depletion causes widespread changes in poly(A) site choice, particularly at sites where the efficiency element is divergently positioned. We propose that one way Yra1 modulates cleavage-polyadenylation is by influencing co-transcriptional assembly of the CF1A 3' processing factor.

Structure and function of mRNA export adaptors

The mRNA export adaptors provide an important link between multiple nuclear mRNA processing events and the mRNA export receptor TAP/NXF1/Mex67p. They are recruited to mRNA through transcriptional and post-transcriptional events, integrating this information to licence mRNA for export. Subsequently they hand mRNA over to TAP and switch TAP to a higher-affinity RNA-binding state, ensuring its stable association with mRNA destined for export. Here we discuss the structure and function of adaptors and how they are recruited to mRNA.

"Cotranscriptionality": the transcription elongation complex as a nexus for nuclear transactions

Much of the complex process of RNP biogenesis takes place at the gene cotranscriptionally. The target for RNA binding and processing factors is, therefore, not a solitary RNA molecule but, rather, a transcription elongation complex (TEC) comprising the growing nascent RNA and RNA polymerase traversing a chromatin template with associated passenger proteins. RNA maturation factors are not the only nuclear machines whose work is organized cotranscriptionally around the TEC scaffold. Additionally, DNA repair, covalent chromatin modification, "gene gating" at the nuclear pore, Ig gene hypermutation, and sister chromosome cohesion have all been demonstrated or suggested to involve a cotranscriptional component. From this perspective, TECs can be viewed as potent "community organizers" within the nucleus.

Assembly of an export-competent mRNP is needed for efficient release of the 3'-end processing complex after polyadenylation

Before polyadenylated mRNA is exported from the nucleus, the 3'-end processing complex is removed by a poorly described mechanism. In this study, we asked whether factors involved in mRNP maturation and export are also required for disassembly of the cleavage and polyadenylation complex. An RNA immunoprecipitation assay monitoring the amount of the cleavage factor (CF) IA component Rna15p associated with poly(A)(+) RNA reveals defective removal of Rna15p in mutants of the nuclear export receptor Mex67p as well as other factors important for assembly of an export-competent mRNP. In contrast, Rna15p is not retained in mutants of export factors that function primarily on the cytoplasmic side of the nuclear pore. Consistent with a functional interaction between Mex67p and the 3'-end processing complex, a mex67 mutant accumulates unprocessed SSA4 transcripts and exhibits a severe growth defect when this mutation is combined with mutation of Rna15p or another CF IA subunit, Rna14p. RNAs that become processed in a mex67 mutant have longer poly(A) tails both in vivo and in vitro. This influence of Mex67p on 3'-end processing is conserved, as depletion of its human homolog, TAP/NXF1, triggers mRNA hyperadenylation. Our results indicate a function for nuclear mRNP assembly factors in releasing the 3'-end processing complex once polyadenylation is complete.