Import of proteins into the nucleus is a two-step process, involving nuclear localization sequence (NLS)-dependent docking of the substrate at the nuclear envelope followed by translocation through the nuclear pore. A recombinant human protein, hSRP1 alpha, bound in vitro specifically and directly to substrates containing either a simple or bipartite NLS motif. hSRP1 alpha promoted docking of import substrates to the nuclear envelope and together with recombinant human Ran reconstituted complete nuclear protein import. Thus, hSRP1 alpha has the properties of a cytosolic receptor for both simple and bipartite NLS motifs.