Structural basis of recognition of monopartite and bipartite nuclear localization sequences by mammalian importin-alpha

Importin-alpha is the nuclear import receptor that recognizes cargo protein s which contain classical monopartite and bipartite nuclear localization se quences (NLSs), and facilitates their transport into the nucleus. To determ ine the structural basis of the recognition of the two classes of NLSs by m ammalian importin-alpha, we co-crystallized an N-terminally truncated mouse receptor protein with peptides corresponding to the monopartite NLS from t he simian virus 40 (SV40) large T-antigen, and the bipartite NLS from nucle oplasmin. We show that the monopartite SV40 large T-antigen NLS binds to tw o binding sites on the receptor, similar to what was observed in yeast impo rtin-alpha. The nucleoplasmin NLS-importin-alpha complex shows, for the fir st time, the mode of binding of bipartite NLSs to the receptor. The two bas ic clusters in the NLS occupy the two binding sites used by the monopartite NLS, while the sequence linking the two basic clusters is poorly ordered, consistent with its tolerance to mutations. The structures explain the stru ctural basis for binding of diverse NLSs to the sole receptor protein. (C) 2000 Academic Press.