SMOOTH-MUSCLE PHOSPHATASES - STRUCTURE, REGULATION, AND FUNCTION

Smooth muscle contraction is regulated primarily by the reversible pho sphorylation of myosin by myosin light chain kinase. Secondary mechani sms that might modulate contractility are phosphorylation-dephosphoryl ation of myosin light chain kinase and thin-filament proteins, caldesm on and calponin. Purification of several protein phosphatases that are active toward myosin light chains and (or) myosin and heavy meromyosi n from smooth muscles has been reported. All the cytosolic turkey gizz ard smooth muscle phosphatases, termed SMP-I, -II, -III, and -IV, deph osphorylate myosin light chains rapidly, but only SMP-III and -IV are active toward myosin and heavy meromyosin, suggesting that SMP-III and -IV might be directly involved in the relaxation of smooth muscle. SM P-III and -IV exhibit properties typical of type 1 protein phosphatase s following tryptic digestion. These enzymes appear to share structura l similarity with myofibrillar phosphatase PP1M. Purified calponin pho sphatase and caldesmon phosphatase from chicken gizzards are structura lly and immunologically identical with SMP-I, a type 2A protein phosph atase. SMP-I dephosphorylates calponin faster than it does caldesmon, and has much higher activity toward these substrates than SMP-II, -III , and -IV. Thus, one role for SMP-I might be to regulate the activitie s of caldesmon and calponin. Since SMP-I is active toward myosin light chain kinase, it might also modulate this enzyme.