Identification of novel homologues of mouse importin α, the α subunit of the nuclear pore‐targeting complex, and their tissue‐specific expression

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Abstract
Transport of karyophilic proteins into the nucleus is mediated by nuclear localization signals (NLSs) via a multistep process. The karyophiles are recognized by the importin α subunit in the cytoplasm to form a stable complex, termed the nuclear pore-targeting complex (PTAC). To date, three different mammalian α subunits (mSRP1/NPI-1, PTAC58/mPendulin/Rch1 and Qip1) have been identified. In this study, we report the identification of three additional mouse genes homologous to the known α subunits using RT-PCR methodology and show that the mouse α subunits can be classified into at least three subfamilies, α-P, α-Q and α-S families, each composed of closely related members (more than 80% amino acid sequence identity). These three subfamilies, however, have ∼50% amino acid identity to one another. Northern blot analysis showed that all were differentially expressed in various mouse tissues. These results suggest that the function of these proteins may be controlled in a tissue-specific manner and that their combinatorial expression may play a role in differentiation and organogenesis.