Systems and targeted analyses of mRNA export in metazoans

The process of mRNA nuclear export is essential to all eukaryotic gene expression. Despite its universality, however, much remains unknown about the factors involved in metazoan mRNA export and how export is coupled to other nuclear mRNA processing events. Using a genome-wide RNAi screen, we defined the complete complement of factors required for bulk mRNA nuclear export in the metazoan organism Drosophila melanogaster . In addition to identifying factors that had been previously implicated in the export pathway, we isolated components of functional categories that had yet to be linked to the export process, namely cell cycle and ribosomal proteins, as well as proteins that had no prior annotated function. By comparing our fly export network to that of yeast, we revealed both the conservation and the divergence between the two pathways. We additionally demonstrated that particular members of the fly export pathway are differentially required for the export of two endogenous messages, the intronless heat shock protein (HSP70) and the intron-containing HSP83, suggesting that an mRNA's export pathway is dictated by its processing requirements. We next investigated the role that dZC3H3, a novel export factor possessing similarity to a component of the mRNA 3'-end processing machinery, has in the export process. Consistent with a role in coupling mRNA adenylation with export, we demonstrated that dZC3H3 interacts with core components of the nuclear export and polyadenylation machineries and that it is required for proper transcript adenylation. Furthermore, we show that the export function of dZC3H3 is conserved as depletion of its human homolog, ZC3H3, results in abnormal nuclear accumulations of poly(A) RNA in human cells. As the nuclear poly(A) foci resultant upon ZC3H3 depletion are redistributed to regions distinct from those in control cells, we propose that they are representative of transcripts stalled at a stage in processing post-adenylation and pre-export. This work has furthered our understanding of the metazoan export network both at a global level, via identification of its constituents, and at a targeted level, via characterization of the specific roles that factors play in the coupling of mRNA processing with the export process.