Determination of the Functional Domain Organization of the Importin α Nuclear Import Factor

Although importin alpha (Imp alpha) has been shown to act as the receptor for basic nuclear localization signals (NLSs) and to mediate their recruitment to the importin beta nuclear import factor, little is known about the functional domains present in Imp alpha, with the exception that importin beta binding is known to map close to the Imp alpha NH2 terminus. Here, we demonstrate that sequences essential for binding to the CAS nuclear export factor are located near the Imp alpha COOH terminus and include a critical acidic motif. Although point mutations introduced into this acidic motif inactivated both CAS binding and Imp alpha nuclear export, a putative leucine-rich nuclear export signal proved to be neither necessary nor sufficient for Imp alpha nuclear export. Analysis of sequences within Imp alpha that bind to the SV-40 T antigen NLS or to the similar LEF-1 NLS revealed that both NLSs interact with a subset of the eight degenerate armadillo (Arm) repeats that form the central part of Imp alpha. However, these two NLS-binding sites showed only minimal overlap, thus suggesting that the degeneracy of the Arm repeat region of Imp alpha may serve to facilitate binding to similar but nonidentical basic NLSs. Importantly, the SV-40 T NLS proved able to specifically inhibit the interaction of Imp alpha with CAS in vitro, thus explaining why the SV-40 T NLS is unable to also function as a nuclear export signal.