A Role for the M9 Transport Signal of hnRNP A1 in mRNA Nuclear Export

Among the nuclear proteins associated with mRNAs before their export to the cytoplasm are the abundant heterogeneous nuclear (hn) RNPs. Several of these contain the M9 signal that, in the case of hnRNP A1, has been shown to be sufficient to signal both nu-clear export and nuclear import in cultured somatic cells. Kinetic competition experiments are used here to demonstrate that M9-directed nuclear import in Xeno-pus oocytes is a saturable process. Saturating levels of M9 have, however, no effect on the import of either U snRNPs or proteins carrying a classical basic NLS. Pre-vious work demonstrated the existence of nuclear ex-port factors specific for particular classes of RNA. In-jection of hnRNP A1 but not of a mutant protein lacking the M9 domain inhibited export of mRNA but not of other classes of RNA. This suggests that hnRNP A1 or other proteins containing an M9 domain play a role in mRNA export from the nucleus. However, the requirement for M9 function in mRNA export is not identical to that in hnRNP A1 protein transport. T he transport of macromolecules between the nu-cleus and cytoplasm is a bi-directional process. The best understood aspect is the import of nuclear pro-teins that carry a basic nuclear localization signal (NLS)