A DISTINCT AND PARALLEL PATHWAY FOR THE NUCLEAR IMPORT OF AN MESSENGER-RNA-BINDING PROTEIN

Three independent pathways of nuclear import have so far been identifi ed in yeast, each mediated by cognate nuclear transport factors, or ka ryopherins. Here we have characterized a new pathway to the nucleus, m ediated by Mtr10p, a protein first identified in a screen for strains defective in polyadenylated RNA export. Mtr10p is shown to be responsi ble for the nuclear import of the shuttling mRNA-binding protein Np13p . A complex of Mtr10p and Np13p was detected in cytosol, and deletion of Mtr10p was shown to lead to the mislocalization of nuclear Np13p to the cytoplasm, correlating with a block in import. Mtr10p bound pepti de repeat-containing nucleoporins and Ran, suggesting that this import pathway involves a docking step at the nuclear pore complex and is Ra n dependent. This pathway of Np13p import is distinct and does not app ear to overlap with another known import pathway for an mRNA-binding p rotein. Thus, at least two parallel pathways function in the import of mRNA-binding proteins, suggesting the need for the coordination of th ese pathways.