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.