Nuclear import factors importin alpha and importin beta undergo mutually induced conformational changes upon association

A heterodimer of importin alpha and importin beta accomplishes the nuclear import of proteins carrying classical nuclear localization signals (NLS). T he interaction between the two import factors is mediated by the IBB domain of importin alpha and involves an extended recognition surface as shown by X-ray crystallography. Using a combination of biochemical and biophysical techniques we have investigated the formation of the importin beta :IBB dom ain complex in solution. Our data suggest that upon binding to the IBB doma in, importin beta adopts a compact, proteolytically resistant conformation, while simultaneously the IBB domain folds into an alpha helix. We suggest a model to describe how these dual mutually induced conformational changes may orchestrate the nuclear import of NLS cargo in vivo. (C) 2000 Federatio n of European Biochemical Societies. Published by Elsevier Science B,V. All rights reserved.