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. 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. Interacts with mungbean yellow mosaic virus capsid protein. Highly expressed in callus, followed by root and etiolated leaf. Low expression in green leaf. In both etiolated and green leaves, down-regulated by light. In green leaf, increased by dark treatment. Belongs to the importin alpha family.