Abstract:  

Previous studies have shown that cGMPdependent protein kinase (PKG) act on several targets in the contractile pathway to reduce intracellular Ca2? and/or augment RhoA-regulated myosin light chain phosphatase (MLCP) activity and cause muscle relaxation. Recent studies have identified a novel protein M-RIP that associates with MYPT1, the regulatory subunit of MLCP. Herein, we examine whether PKG enhance MLCP activity downstream of Ca2? and RhoA via phosphorylation of M-RIP in gastric smooth muscle cells. Treatment of permeabilized muscle cells with 10 lM Ca2 ? caused an increase in MLC20 phosphorylation and muscle contraction, but had no effect on Rho kinase activity. Activators of PKG (GSNO or cGMP) decreased MLC20 phosphorylation and contraction in response to 10 lM Ca2?, implying existence of inhibitory mechanism independent of Ca2? and RhoA. The effect of PKG on Ca2?-induced MLC20 phosphorylation was attenuated by M-RIP siRNA. Both GSNO and 8-pCPTcGMP induced phosphorylation of M-RIP; phosphorylation was accompanied by an increase in the association of M-RIP with MYPT1 and MLCP activity. Taken together, these results provide evidence that PKG induces phosphorylation of M-RIP and enhances its association with MYPT1 to augment MLCP activity and MLC20 dephosphorylation and inhibits muscle contraction, downstream of Ca2?- or RhoA-dependent pathways.