Structure of a Pentavalent G-Actin•MRTF-A Complex Reveals How G-Actin Controls Nucleocytoplasmic Shuttling of a Transcriptional Coactivator

Abstract

Subcellular localization of the actin-binding transcriptional coactivator MRTF-A is controlled by its interaction with monomeric actin (G-actin). Signal-induced decreases in G-actin concentration reduce MRTF-A nuclear export, leading to its nuclear accumulation, whereas artificial increases in G-actin concentration in resting cells block MRTF-A nuclear import, retaining it in the cytoplasm. This regulation is dependent on three actin-binding RPEL motifs in the regulatory domain of MRTF-A. We describe the structures of pentavalent and trivalent G-actin•RPEL domain complexes. In the pentavalent complex, each RPEL motif and the two intervening spacer sequences bound an actin monomer, forming a compact assembly. In contrast, the trivalent complex lacked the C-terminal spacer- and RPEL-actins, both of which bound only weakly in the pentavalent complex. Cytoplasmic localization of MRTF-A in unstimulated fibroblasts also required binding of G-actin to the spacer sequences. The bipartite MRTF-A nuclear localization sequence was buried in the pentameric assembly, explaining how increases in G-actin concentration prevent nuclear import of MRTF-A. Analyses of the pentavalent and trivalent complexes show how actin loads onto the RPEL domain and reveal a molecular mechanism by which actin can control the activity of one of its binding partners.

Actin-regulated feedback loop based on Phactr4, PP1 and cofilin maintains the actin monomer pool

“…The RPEL domain of MRTF-A consists of three RPEL motifs of 22 amino acids organized around the core consensus sequence RPxxxEL (arginine R, proline P, any amino acid x, glutamate E and leucine L). Each RPEL motif functions as an actin-binding element but also the linker sequences separating the motifs contribute to the binding (Mouilleron et al, 2011). The RPEL domain is both necessary and sufficient for controlling the nucleo-cytoplasmic shuttling of MRTF-A in response to actin dynamics.…”

MRTF-A controls vessel growth and maturation by increasing the expression of CCN1 and CCN2

“…SRF is co-activated by myocardin 15 or MRTFs A and B 16 . These SRF coactivators are interacting with G-actin and translocate to the nucleus 17 , when G-actin levels decrease, for example, due to sequestration by the G-actinbinding peptide thymosin 4 (T 4) [18][19][20] . Since T 4 is a potent pro-angiogenic factor 21 , an involvement of the T 4-MRTF-SRF axis in vascular growth would require transcriptional activation beyond well-known myogenic proteins 13,14 .…”

Myocardin-like protein 2 regulates TGFβ signaling in embryonic stem cells and the developing vasculature

“…As shown in Fig. 7, in the embryonic vasculature, developmental cues and extracellular signals transduced via RhoA and cytoskeletal actin promote the translocation of G-actin/MKL2 complexes from the cytoplasm to the SMC nucleus (Mouilleron et al, 2011). In the nucleus, MKL2 physically associates with SRF, which promotes the binding of the MKL2/SRF complex to the TGF2 promoter (and related TGF family members), activating TGF signaling in the arterial wall.…”