Transcription coactivator that associates with the serum response factor (SRF) transcription factor to control expression of genes regulating the cytoskeleton during development, morphogenesis and cell migration (PubMed:12019265, PubMed:12732141, PubMed:17588931, PubMed:19350017, PubMed:24732378). The SRF-MRTFA complex activity responds to Rho GTPase-induced changes in cellular globular actin (G-actin) concentration, thereby coupling cytoskeletal gene expression to cytoskeletal dynamics (PubMed:24732378). MRTFA binds G-actin via its RPEL repeats, regulating activity of the MRTFA-SRF complex (PubMed:12732141, PubMed:17588931). Activity is also regulated by filamentous actin (F-actin) in the nucleus (PubMed:23558171, PubMed:25759381). Interacts with SRF, forming the SRF-MRTFA nuclear complex which binds the 5'-CArG-3' consensus motif (CArG box) on DNA via SRF (PubMed:12732141, PubMed:19350017). Interacts (via RPEL repeats) with globular actin (G-actin), thereby regulating its subcellular location and activity of the complex formed with SRF (PubMed:12732141, PubMed:17588931, PubMed:19350017, PubMed:19008859, PubMed:27304076, PubMed:21673315). Either forms a trivalent (by binding three G-actin monomers) or pentavalent (by binding five G-actin monomers) complex with G-actin (PubMed:21673315). Forms a nuclear ternary complex with SCAI and SRF, leading to suppress MRTFA-induced SRF transcriptional activity (PubMed:19350017). Interacts with beta-actin (ACTB); interaction with ACTB prevents interaction with SCAI (PubMed:19350017). Interacts with MRTFB (By similarity). Subcellular location is tightly regulated by actin both in cytoplasm and nucleus: high levels of G-actin in the nucleus observed during serum deprivation lead to low levels of nuclear MRTFA, while reduced levels of nuclear G-actin result in accumulation of MRTFA in the nucleus (PubMed:17588931, PubMed:21673315). G-actin-binding in the cytoplasm inhibits nuclear import by masking the nuclear localization signal (NLS) (PubMed:17588931, PubMed:21673315). In contrast, binding to nuclear globular actin (G-actin) promotes nuclear export to the cytoplasm (PubMed:17588931). Nuclear localization is regulated by MICAL2, which mediates depolymerization of nuclear actin, which decreases nuclear G-actin pool, thereby promoting retention of MRTFA in the nucleus and subsequent formation of an active complex with SRF (By similarity). Expressed in heart, brain, spleen, lung, liver, muscle, kidney and testis. Detected throughout the embryo at 10.5 dpc; higher expression is found at 13.5 dpc in neural mesenchymal cells, skeletal muscle of the tongue, and epithelial cells of the colon and small intestine; at 15.5 dpc, expression in epithelial cells of lung, kidney, bladder, and colon is also detected. The N-terminal region is required for nuclear localization and the C-terminal region mediates transcriptional activity. The RPEL repeats mediate binding to globular actin (G-actin); each RPEL repeat-binding to one G-actin monomer (PubMed:19008859, PubMed:21673315). In addition, each intervening spacer sequence region can bind one G-actin monomer, to reach a pentavalent complex (PubMed:21673315). Phosphorylation at Ser-41 by Erk inhibits binding of globular actin (G-actin), unmasking the nuclear localization signal (NLS) and promoting nuclear import. Some publications use a protein sequence that is longer at the N-terminus and is based on an artificial construct (PubMed:12732141, PubMed:27304076). The sequence used in these publications modifies a non-canonical CTG leucine codon upstream of the initiator codon into ATG, generating a protein of 1021 residues (PubMed:12732141, PubMed:27304076). The existence of this form has not been confirmed in vivo and is therefore unsure (PubMed:12732141, PubMed:27304076). Similarly, the existence of the S33 ('Ser-33') phosphorylation site described in Panayiotou et al. is unsure (PubMed:27304076).