IDENTIFICATION AND FUNCTIONAL-CHARACTERIZATION OF A NOVEL NUCLEAR-LOCALIZATION SIGNAL PRESENT IN THE YEAST NAB2 POLY(A)(-BINDING PROTEIN() RNA)

The nuclear import of proteins bearing a basic nuclear localization si gnal (NLS) is dependent on karyopherin alpha/importin alpha, which act s as the NLS receptor, and karyopherin beta 1/importin beta, which bin ds karyopherin alpha and mediates the nuclear import of the resultant ternary complex. Recently, a second nuclear import pathway that allows the rapid reentry into the nucleus of proteins that participate in th e nuclear export of mature mRNAs has been identified. In mammalian cel ls, a single NLS specific for this alternate pathway, the M9 NLS of he terogeneous nuclear ribonucleoprotein A1 (hnRNPA1), has been described . The M9 NLS binds a transport factor related to karyopherin beta 1, t ermed karyopherin beta 2 or transportin, and does not require a karyop herin alpha-like adapter protein. A yeast homolog of karyopherin beta 2, termed Kap104p, has also been described and proposed to play a role in the nuclear import of a yeast hnRNP-like protein termed Nab2p. Her e, we define a Nab2p sequence that binds to Kap104p and that functions as an NLS in both human and yeast cells despite lacking any evident s imilarity to basic or M9 NLSs. Using an in vitro nuclear import assay, we demonstrate that Kap104p can direct the import into isolated human cell nuclei of a substrate containing a wild-type, but not a defectiv e mutant, Nab2p NLS. In contrast, other NLSs, including the M9 NLS, co uld not function as substrates for Kap104p. Surprisingly, this in vitr o assay also revealed that human karyopherin beta 1, but not the Kap10 4p homolog karyopherin beta 2, could direct the efficient nuclear impo rt of a Nab2p NLS substrate in vitro in the absence of karyopherin alp ha. These data therefore identify a novel NLS sequence, active in both yeast and mammalian cells, that is functionally distinct from both ba sic and M9 NLS sequences.