Three Distinct Classes of the α-Subunit of the Nuclear Pore-targeting Complex (Importin-α) Are Differentially Expressed in Adult Mouse Tissues

SUMMARY The process of active nuclear protein transport is mediated by the nuclear localization signal (NLS). An NLS-containing karyophile forms a stable complex, termed the nuclear pore-targeting complex, to target nuclear pores. The � -subunit of the complex (importin- � ) binds to the NLS and the � -subunit (importin- � ) carries the � -subunit, bound to the NLS substrate, into the nucleus. To date, five mouse � -subunits have been identified and classified into three subfamilies ( � -P, � -Q, and � -S). The expression of these � -subunits and the � -subunit in various adult mouse tissues was examined by immunoblotting and immunohistochemistry using antibodies specific for each subfamily of the � -subunit or the � -subunit. The � -subunit was found to be ubiquitously expressed, whereas each subfamily of the � -subunit showed a unique expression pattern in various tissues, especially in brain and testis. In brain, the expression of � -P was not observed, whereas � -S was significantly expressed in Purkinje cells, and pyramidal cells of the hippocampus and cerebral cortex. In testis, � -P was expressed predominantly in primary spermatocytes, whereas � -Q was found mainly in Leydig cells. Expression of � -S was detected in almost all cells in convoluted seminiferous tubules and Leydig cells to a similar extent. These results suggest that nuclear protein import may be controlled in a tissue-specific manner by � -subunit family proteins. (J Histochem Cytochem 47:363–372, 1999)