Nuclear localization signal recognition causes release of importin-alpha from aggregates in the cytosol

Importin-alpha is a cytosolic receptor that recognizes classical Nuclear Lo calization Signals (NLSs) and mediates import into the nucleus. We have use d a number of methods to investigate the aggregation state of Xenopus impor tin-alpha both Els a recombinant, purified protein and in cytosolic extract s. We have found that recombinant importin-alpha aggregates at a protein co ncentration similar to that estimated to be present in the Xenopus cytoplas m,and that the importin-alpha aggregation is relieved by NLS peptide bindin g, with the importin-alpha then binding the NLS as a monomer. We have also found that in HeLa cytosolic extracts, importin-alpha is present in an aggr egated form. Similarly to the purified importin-alpha aggregation, NLS pept ides relieve the aggregation of importin-alpha in the cytosol. These observ ations indicate that aggregation of importin-alpha in the cytosol may be an intrinsic property of the import receptor and may be functionally related to NLS binding. Our results suggest a novel mechanism for NLS recognition, whereby NLSs med iate disassembly of importin-alpha aggregates in the cytosol. (C) 1999 Acad emic Press.