CALPONIN PHOSPHATASE FROM SMOOTH-MUSCLE - A POSSIBLE ROLE OF TYPE-1 PROTEIN PHOSPHATASE IN SMOOTH-MUSCLE RELAXATION

Smooth muscle myosin-bound phosphatase (MBP) purified from chicken gizzard, which is a holoenzyme of type 1δ protein phosphatase and dephosphorylated intact myosin, catalyzed the dephosphorylation of calponin phosphorylated by protein kinase C (PK-C). The Km of MBP for calponin was 0.6 μM and the Vmax was 350 nmol/min/mg. All of the multiple sites of phosphorylation by PK-C of calponin were completely dephosphorylated by MBP. Functionally, calponin dephosphorylated by MBP recovered its inhibitory effect on the actinactivated Mg2+-ATPase activity of myosin. Therefore, these results suggest that a type lδ protein phosphatase causes relaxation of smooth muscle by the dephosphorylation not only of myosin but also of calponin. © 1993 Academic Press, Inc.