Mouse Monoclonal antibody to Importin beta-1The accumulation of proteins in the nucleus is mediated by short sequences of basic amino acids called nuclear localization sequences (NLSs). These sequences are necessary and sufficient to direct a protein or inert carrier to the nuclear interior. Nuclear protein import is accomplished by two sequential, energy dependent events. The first step, docking at the nuclear pore complex, requires a 54/56 kDa nuclear localization signal receptor (a-karyopherin, importin-a, SRP-a) and the nuclear transport factor p97 (NTF97, b-karyopherin, importin-b). The second step, translocation across the nuclear envelope (NE), requires the GTP-binding protein, Ran/TC4.