Cloning and characterization of hSRP1γ, a tissue-specific nuclear transport factor

Nuclear import of proteins containing a nuclear localization signal (NLS) is dependent on the presence of a cytoplasmic NLS receptor, the GTPase Ran, and p10/NTF2. The NLS receptor is a heterodimeric protein consisting of subunits of approximately 60 and 97 kDa, which have been termed importin alpha/beta, karyopherin alpha/beta, or PTAC 58/97. Members of the 60-kDa/importin alpha subunit family directly bind to the NLS motif and have been shown to function as adaptors that tether NLS-containing proteins to the p97/importin beta subunit and to the downstream transport machinery, Herein we report the identification and characterization of hSRP1 gamma, a human importin alpha homologue. The hSRP1 gamma protein is around 45% identical to the two previously identified human importin a homologues hSRP1 alpha/Rch1 and NPI/hSRP1. hSRP1 gamma can form a complex with importin beta and is able to mediate import of a BSA-NLS substrate in an in vitro nuclear import system, Interestingly, hSRP1 gamma shows a very selective expression pattern and is most abundantly expressed in skeletal muscle, representing more than 1% of the total protein in this tissue. A potential role for hSRP1 gamma in tissue-specific transport events is discussed.