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 α/β, karyopherin α/β, or PTAC 58/97. Members of the 60-kDa/importin α 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 β subunit and to the downstream transport machinery. Herein we report the identification and characterization of hSRP1γ, a human importin α homologue. The hSRP1γ protein is around 45% identical to the two previously identified human importin α homologues hSRP1α/Rch1 and NPI/hSRP1. hSRP1γ can form a complex with importin β and is able to mediate import of a BSA-NLS substrate in an in vitro nuclear import system. Interestingly, hSRP1γ 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 hSRP1γ in tissue-specific transport events is discussed.