MAL: an actin-regulated SRF transcriptional coactivatorRecent years have seen a revitalised interest in the role of actin in nuclear processes, but the molecular mechanisms involved remain largely unexplored. We will elucidate the molecular basis for the actin-based control of the SRF transcriptional coactivator, MAL. SRF controls transcription through two families of coactivators, the actin-binding MRTFs (MAL, Mkl2), which couple its activity to cytoskeletal dynamics, and the ERK-regulated TCFs (Elk-1, SAP-1, Net). MAL subcellular localisation and transcriptional activity responds to signal-induced changes in G-actin concentration, which are sensed by its actin-binding N-terminal RPEL domain. Members of a second family of RPEL proteins, the Phactrs, also exhibit actin-regulated nucleocytoplasmic shuttling. The proposal addresses the following novel features of actin biology:¿ Actin as a transcriptional regulator¿ Actin as a signalling molecule¿ Actin-binding proteins as targets for regulation by actin, rather than regulators of actin functionWe will analyse the sequences and proteins involved in actin-regulated nucleocytoplasmic shuttling, using structural biology and biochemistry to analyse its control by changes in actin-RPEL domain interactions. We will characterise the dynamics of shuttling, and develop reporters for changes in actin-MAL interaction for analysis of pathway activation in vivo. We will identify genes controlling MAL itself, and the balance between the nuclear and cytoplasmic actin pools. The mechanism by which actin represses transcriptional activation by MAL in the nucleus, and its relation to MAL phosphorylation, will be elucidated. Finally, we will map MRTF and TCF cofactor recruitment to SRF targets on a genome-wide scale, and identify the steps in transcription controlled by actin-MAL interaction.