It is critical that only fully processed mature mRNAs be exported from the nucleus because translation of incompletely processed pre-mRNAs containing introns would produce defective proteins that might interfere with the functioning of the cell. To prevent this, pre-mRNAs associated with snRNPs in spliceosomes are usually prevented from being transported to the cytoplasm.
In one type of experiment demonstrating this restriction, a gene encoding a pre-mRNA with a single intron that is normally spliced out was mutated to introduce deviations from the consensus splice-site sequences. Mutation of either the 5′ or the 3′ invariant splice-site bases at the ends of the intron resulted in pre-mRNAs that were bound by snRNPs to form spliceosomes; however, RNA splicing was blocked, and the pre-mRNA was retained in the nucleus. In contrast, mutation of both the 5′ and 3′ splice sites in the same pre-mRNA resulted in export of the unspliced pre-mRNA, although less efficiently than for the spliced mRNA, probably because of the absence of an exon-junction complex. When both splice sites were mutated, the pre-mRNAs were not efficiently bound by snRNPs, and consequently, their export was not blocked.
Studies in yeast have shown that a protein component of the NPC nuclear basket is required to retain pre-mRNAs associated with snRNPs in the nucleus. If either this protein or the nuclear basket protein to which it binds is deleted, unspliced pre-mRNAs are exported. Consequently, these proteins prevent hnRNPs associated with snRNPs from traversing the NPC.