A gradient of RanGTP typically regulates traffic through the nuclear pore by modulating association of receptors with cargo. However, we have demonstrated that the yeast high mobility group box (HMGB) transcription factor Nhp6Ap enters the nucleus via a novel nuclear localization signal (NLS) recognized by calcium calmodulin in a process that does not require Ran. Calmodulin is strictly required for the non-diffusional nuclear entry of Nhp6Ap. Calmodulin and DNA exhibit mutually exclusive binding to NHP6A, indicating that the directionality of Nhp6Ap nuclear accumulation may be driven by DNA-dependent dissociation of calmodulin. Our findings demonstrate that calmodulin can serve as a molecular switch triggering nuclear entry with subsequent dissociation of calmodulin binding upon interaction of cargo with chromatin. This pathway appears to be evolutionarly conserved; mammalian HMGB transcription factors often have two NLSs, one a classical Ran-dependent signal and a second that binds calmodulin. The finding that Nhp6Ap nuclear entry requires calmodulin but not Ran indicates that Nhp6Ap is a good model for studying this poorly understood but evolutionarily conserved calmodlin-dependent nuclear import pathway. This work is described in a paper that has been submitted for publication.
