Regulation of myosin phosphatase by a specific interaction with cGMP- dependent protein kinase Ialpha.

Contraction and relaxation of smooth muscle are regulated by myosin light-chain kinase and myosin phosphatase through phosphorylation and dephosphorylation of myosin light chains. Cyclic guanosine monophosphate (cGMP)-dependent protein kinase Ialpha (cGKIalpha) mediates physiologic relaxation of vascular smooth muscle in response to nitric oxide and cGMP. It is shown here that cGKIalpha is targeted to the smooth muscle cell contractile apparatus by a leucine zipper interaction with the myosin-binding subunit (MBS) of myosin phosphatase. Uncoupling of the cGKIalpha-MBS interaction prevents cGMP-dependent dephosphorylation of myosin light chain, demonstrating that this interaction is essential to the regulation of vascular smooth muscle cell tone.