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. {Experimental EvidencePubMed:11124253}.