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

Importin-α is a cytosolic receptor that recognizes classical Nuclear Localization Signals (NLSs) and mediates import into the nucleus. We have used a number of methods to investigate the aggregation state of Xenopus importin-α both as a recombinant, purified protein and in cytosolic extracts. We have found that recombinant importin-α aggregates at a protein concentration similar to that estimated to be present in the Xenopus cytoplasm, and that the importin-α aggregation is relieved by NLS peptide binding, with the importin-α then binding the NLS as a monomer. We have also found that in HeLa cytosolic extracts, importin-α is present in an aggregated form. Similarly to the purified importin-α aggregation, NLS peptides relieve the aggregation of importin-α in the cytosol. These observations indicate that aggregation of importin-α 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 mediate disassembly of importin-α aggregates in the cytosol.