MTR10P FUNCTIONS AS A NUCLEAR IMPORT RECEPTOR FOR THE MESSENGER-RNA-BINDING PROTEIN NP13P

MTR10, previously shown to be involved in mRNA export, was found in a synthetic lethal relationship with nucleoporin NUP85, Green fluorescen t protein (GFP)-tagged Mtr10p localizes preferentially inside the nucl eus, but a nuclear pore and cytoplasmic distribution is also evident, Purified Mtr10p forms a complex with Np13p, an RNA-binding protein tha t shuttles in and out of the nucleus, In mtr10 mutants, nuclear uptake of Np13p is strongly-impaired at the restrictive temperature, while i mport of a classic nuclear localization signal (NLS)-containing protei n is not, Accordingly, the NLS within Np13p is extended and consists o f the RGG box plus a short and non-repetitive C-terminal tail, Mtr10p interacts in vitro with Gsp1p-GTP, but with low affinity. Interestingl y, Np13p dissociates from Mtr10p only by incubation with Ran-GTP plus RNA. This suggests that Np13p follows a distinct nuclear import pathwa y and that intranuclear release from its specific import receptor Mtr1 0p requires the cooperative action of both Ran-GTP and newly synthesiz ed mRNA.