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. 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. Mrtfa binds G-actin via its RPEL repeats, regulating activity of the mrtfa-srf complex. Activity is also regulated by filamentous actin (F-actin) in the nucleus. Interacts with srf, forming the srf-mrtfa nuclear complex which binds the 5'-CArG-3' consensus motif (CArG box) on DNA via srf. Interacts (via RPEL repeats) with globular actin (G-actin), thereby regulating its subcellular location and activity of the complex formed with srf. 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. 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. In addition, each intervening spacer sequence region can bind one G-actin monomer, to reach a pentavalent complex.