FUNCTIONAL CONSERVATION OF THE TRANSPORTIN NUCLEAR IMPORT PATHWAY IN DIVERGENT ORGANISMS

Human transportin1 (hTRN1) is the nuclear import receptor for a group of pre-mRNA/mRNA-binding proteins (heterogeneous nuclear ribonucleopro teins [hnRNP]) represented by hnRNP A1, which shuttle continuously bet ween the nucleus and the cytoplasm. hTRN1. interacts with the M9 regio n of hnRNP A1, a 38-amino-acid domain rich in Gly, Ser, and Asn, and m ediates the nuclear import of M9-bearing proteins in vitro. Saccharomy ces cerevisiae transportin (yTRN; also known as YBR017c or Kap104p) ha s been identified and cloned. To understanding the nuclear import medi ated by yTRN, we searched with a yeast two-hybrid system for proteins that interact with it. In an exhaustive screen of the S. cerevisiae ge nome, the most frequently selected open reading frame was the nuclear mRNA-binding protein, Nab2p. We delineated a ca.50-amino-acid region i n Nab2p, termed NAB35, which specifically binds yTRN and is similar to the M9 motif. NAB35 also interacts with hTRN1 and functions as a nucl ear localization signal in mammalian cells. Interestingly, yTRN can al so mediate the import of NAB35-bearing proteins into mammalian nuclei in vitro. We also report on additional substrates for TRN as well as s equences of Drosophila melanogaster, Xenopus laevis, and Schizosacchar omyces pombe TRNs. Together, these findings demonstrate that both the M9 signal and the nuclear import machinery utilized by the transportin pathway are conserved in evolution.