Splicing promotes rapid and efficient mRNA export in mammalian cells

The numerous steps in protein gene expression are extensively coupled to one another through complex networks of physical and functional interactions. Indeed, > 25 coupled reactions, often reciprocal, have been documented among such steps as transcription, capping, splicing, and polyadenylation. Coupling is usually not essential for gene expression, but instead enhances the rate and/or efficiency of reactions and, physiologically, may serve to increase the fidelity of gene expression. Despite numerous examples of coupling in gene expression, whether splicing enhances mRNA export still remains controversial. Although splicing was originally reported to promote export in both mammalian cells and Xenopus oocytes, it was subsequently concluded that this was not the case. These newer conclusions were surprising in light of the observations that the mRNA export machinery colocalizes with splicing factors in the nucleus and that splicing promotes recruitment of the export machinery to mRNA. We therefore reexamined the relationship between splicing and mRNA export in mammalian cells by using FISH, in combination with either transfection or nuclear microinjection of plasmid DNA. Together, these analyses indicate that both the kinetics and efficiency of mRNA export are enhanced 6- to 10-fold (depending on the construct) for spliced mRNAs relative to their cDNA counterparts. We conclude that splicing promotes mRNA export in mammalian cells and that the functional coupling between splicing and mRNA export is a conserved and general feature of gene expression in higher eukaryotes.