Phosphorylation of a novel myosin binding subunit of protein phosphatase 1 reveals a conserved mechanism in the regulation of actin cytoskeleton

The myotonic dystrophy kinase-related kinases RhoA binding kinase and myotonic dystrophy kinase-related Cdc42 binding kinase (MRCK) are effecters of RhoA and Cdc42, respectively, for actin reorganization, Using substrate screening in various tissues, we uncovered two major substrates, p130 and p85, for MRCK alpha -kinase, p130 is identified as myosin binding subunit p130, whereas p85 is a novel related protein. p85 contains N-terminal ankyrin repeats, an a-helical C terminus with leucine repeats, and a centrally located conserved motif with the MRCK alpha -kinase phosphorylation site. Like MBS130, p85 is specifically associated with protein phosphatase 1 delta (PP1 delta), and this requires the N terminus, including the ankyrin repeats. This association is required for the regulation of both the catalytic activities and the assembly of actin cytoskeleton, The N terminus, in association with PP1 delta, is essential for actin depolymerization, whereas the C terminus antagonizes this action. The C-terminal effects consist of two independent events that involved both the conserved phosphorylation inhibitory motif and the a-helical leucine repeats. The former was able to interact with PP1 delta only in the phosphorylated state and result in inactivation of PP1 delta activity, This provides further evidence that phosphorylation of a myosin binding subunit protein by specific kinases confers conformational changes in a highly conserved region that plays an essential role in the regulation of its catalytic subunit activities.