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

ABSTRACT: 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)?/?-dependent import pathway, and that import is inhibited by G-actin. Interaction of the NLS with the Imp?-Imp? heterodimer requires both NLS basic elements, and is dependent on the Imp? major and minor binding pockets. Binding of the Imp?-Imp? heterodimer to the intact MRTF-A RPEL domain occurs competitively with G-actin. Thus, MRTF-A contains an actin-sensitive nuclear import signal.

SUBMITTER: Pawlowski R 

PROVIDER: S-EPMC2964165 | biostudies-literature | 2010 Oct

REPOSITORIES: biostudies-literature