Key Concepts of Section 10.3

• Most mRNPs are exported from the nucleus by a heterodimeric mRNP exporter that interacts with unstructured protein domains that fill the central channel of the nuclear pore complex (NPC). The direction of transport (nucleus to cytoplasm) results from dissociation of the mRNP exporter–mRNP complex in the cytoplasm due to the phosphorylation of mRNP adapter proteins by cytoplasmic kinases and the action of an RNA helicase associated with cytoplasmic filaments of the nuclear pore complexes. As a result, mRNP exporter–mRNP complexes diffuse down a concentration gradient across the NPC from the nucleus to the cytoplasm.

• The mRNP exporter binds to most mRNAs cooperatively with SR proteins bound to exonic splicing enhancers and with REF associated with exon-junction complexes as well as with additional mRNP proteins.

• Pre-mRNAs bound by a spliceosome normally are not exported from the nucleus, ensuring that only fully processed, functional mRNAs reach the cytoplasm for translation.