The Molecular Mechanisms of mRNA Export

A general paradigm for nuclear transport was established primarily through studies of protein import and export. Until recently, this paradigm was generally presumed also to apply to the process of RNA export from the nucleus. In particular, it was assumed that general mRNA export was mediated by one or more transport receptors of the importin-β family and that the RanGTP/GDP gradient was required to impart directionality to the process. The highly abundant class of nuclear RNA-binding proteins—the hnRNP proteins— were regarded as primary candidates for mRNA export adapter proteins that could link mRNAs to importin-β family export factors. Within the past few years, however, an explosion of data has largely disproven prior assumptions about the mechanisms of mRNA export, permanently changing the face of the field. The dust is still settling, but what we now see, albeit incompletely, is the outline of a probable major route of mRNA export that is independent of the importin-β family and the Ran GTPase system.