Stress-dependent phosphorylation of myocardin-related transcription factor A (MRTF-A) by the p38(MAPK)/MK2 axis

Myocardin-related transcription factor A (MRTF-A) is a known actin-regulated transcriptional coactivator of serum response factor (SRF). Stimulation of actin polymerization activates MRTF-A by releasing it from G-actin and thus allowing it to bind to and activate SRF. Here, we compared protein phosphorylation in MK2/3-deficient cells rescued or not by ectopic expression of MK2 in two independent phosphoproteomic approaches using anisomycin-treated MEF cells and LPS-stimulated mouse macrophages, respectively. Two MRTF-A sites, Ser(351) (corresponding to Ser(312) in human) and Ser(371) (Ser(333) in human), showed significantly stronger phosphorylation (12-fold and 6-fold increase) in the cells expressing MK2. MRTF-A is phosphorylated at these sites in a stress-, but not in a mitogen-induced manner, and p38(MAPK)/MK2 catalytic activities are indispensable for this phosphorylation. MK2-mediated phosphorylation of MRTF-A at Ser(312) and Ser(333) was further confirmed in an in vitro kinase assay and using the phospho-protein kinase-D (PKD)-consensus motif antibody (anti-LXRXXpS/pT), the p38(MAPK) inhibitor BIRB-796, MK2/3-deficient cells and MRTF-A phospho-site mutants. Unexpectedly, dimerization, subcellular localization and translocation, interaction with actin, SRF or SMAD3 and transactivating potential of MRTF-A seem to be unaffected by manipulating the p38(MAPK)/MK2-dependent phosphorylations. Hence, MRTF-A is stress-dependently phosphorylated by MK2 at Ser(312) and Ser(333) with so far undetected functional and physiological consequences.