Binds specifically and directly to substrates containing either a simple or bipartite NLS motif. Promotes docking of import substrates to the nuclear envelope. Forms a complex with importin subunit beta-1. The whole complex, most stable and composed of importin alpha, importin beta and NLS substrate, is referred to as PTAC or pore targeting complex. Expressed in root, callus, and etiolated leaf. Low expression in green leaf. By dark in green leaf. Down-regulated by light. Belongs to the importin alpha family.