Heterogeneous nuclear ribonucleoprotein A1 contains a sequence, termed M9, that functions as a potent nuclear localization signal (NLS) yet bears no similarity to the well-defined basic class of NLSs. Here, we report the identification of a novel human protein, termed MIP, that binds M9 specifically both in vivo and in vitro yet fails to interact with non-functional M9 point mutants. Of note, the 101 kDa MIP protein bears significant homology to human karyopherin/importin-beta, a protein known to mediate the function of basic NLSs. The in vitro nuclear import of a protein substrate containing the M9 NLS was found to be dependent on provision of the MIP protein in trans. Cytoplasmic microinjection of a truncated form of MIP that retains the M9 binding site blocked the in vivo nuclear import of a substrate containing the M9 NLS yet failed to affect the import of a similar substrate bearing a basic NLS. These data indicate that nuclear import of hnRNP A1 is mediated by a novel cellular import pathway that is distinct from, yet evolutionarily related to, the pathway utilized by basic NLS sequences.

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