Functions in nuclear protein import. Binds specifically and directly to substrates containing either a simple or bipartite NLS motif. Promotes docking of import substrates to the nuclear envelope. In conjunction with importin beta-1, mediates the nuclear envelope docking, and the subsequent translocation into the nucleus of the constitutive morphogenetic 1 (COP1) protein containing bipartite NLS motif. Forms a complex with importin subunit beta-1. The whole complex, most stable and composed of importin alpha and importin beta, is referred to as PTAC or pore targeting complex. Highly expressed in root and weakly in callus, etiolated leaf and green leaf. Slightly increased by dark treatment. Belongs to the importin alpha family.