An actin-regulated importin α/β-dependent extended bipartite NLS directs nuclear import of MRTF-A

Myocardin-related transcription factors (MRTFs) are actin-regulated transcriptional coactivators, which bind G-actin through their N-terminal RPEL domains. In response to signal-induced actin polymerisation and concomitant G-actin depletion, MRTFs accumulate in the nucleus and activate target gene transcription through their partner protein SRF. Nuclear accumulation of MRTFs in response to signal is inhibited by increased G-actin level. Here, we study the mechanism by which MRTF-A enters the nucleus. We show that MRTF-A contains an unusually long bipartite nuclear localisation signal (NLS), comprising two basic elements separated by 30 residues, embedded within the RPEL domain. Using siRNA-mediated protein depletion in vivo, and nuclear import assays in vitro, we show that the MRTF-A extended bipartite NLS uses the importin (Imp)α/β-dependent import pathway, and that import is inhibited by G-actin. Interaction of the NLS with the Impα–Impβ heterodimer requires both NLS basic elements, and is dependent on the Impα major and minor binding pockets. Binding of the Impα–Impβ heterodimer to the intact MRTF-A RPEL domain occurs competitively with G-actin. Thus, MRTF-A contains an actin-sensitive nuclear import signal.