Purification and characterization of the myofibrillar form of myosin light-chain phosphatase from turkey gizzard smooth muscle.

The myofibrillar form of smooth-muscle myosin light-chain phosphatase (MLCP) was isolated from turkey gizzards. The enzyme was extracted from washed myofibrils and purified by affinity chromatography on a column of thiophosphorylated myosin 20 kDa light-chain (LC20). The purified enzyme was a monomeric protein of 35 kDa and bound to unphosphorylated myosin with a binding constant of 5.45 x 10(4) M-1. It dephosphorylated both isolated phosphorylated light-chain (PLC20) and intact myosin as well as myosin light-chain kinase. The enzyme activity was stimulated by Mn2+, Mg2+, Ca2+, and inhibited by Co2+ ions. Okadaic acid inhibited the phosphatase activity with an IC50 (concentration required for 50% inhibition) value of 250 nM, that is around 25-times higher than required for type 1 protein phosphatase; however, the heat stable inhibitor-2 had no effect. The unique properties of the myofibrillar phosphatase as compared to smooth-muscle phosphatases so far described, suggest that the myofibrillar MLCP is a novel protein of this class. It has been also suggested that in vivo the myofibrillar MLCP exists in a complex with myosin light-chain kinase (MLCK) and a 63 kDa protein.