Protein Phosphatase 1, Regulatory Subunit 12C (PPP1R12C)

MRCK-alpha (CDC42BPA) phosphorylated thr560 within the MBS85 inhibitory motif. This phosphorylation was required for MBS85-mediated inhibition of PP1-delta (PPP1CB) phosphorylation of the myosin substrate MLC2 (MYL9). Immunoprecipitation analysis showed that MBS85 specifically bound PP1-delta, but not other PP1 subunits. Mutation analysis revealed that the intact MBS85 N terminus was required for interaction with PP1-delta, and deletion of a single ankyrin repeat reduced the interaction. The N-terminal domain of MBS85 also directly interacted with MLC2, and when associated with active PP1-delta, caused dissociation of actin stress fibers in HeLa cells. The inhibitory motif and leucine zipper domain in the MBS85 C-terminal half independently antagonized this function and caused stress fiber formation.