The contractile activity of leukocyte contractile protein

Abstract

Contractile activity of myosin-B from porcine leukocytes was regulated by Ca2+, which was associated with the level of phosphorylation of its myosin light chain. In another series of experiment, it was found that the myosin-B lost its contractility by washing with 1 mM NaHCO3. However, the washed myosin-B restored its Ca2+-sensitive contractility by the addition of not only a fraction prepared from the NaHCO3-wash of the myosin-B, but also “native tropomyosin (NT) ” which was the regulatory protein fraction from arterial smooth muscle. From the above results, its was considered that the mechanism of Ca2+-sensitive contractility of leukocyte-myosin-B was analogous to that of arterial contractile protein. Some preparations of leukocyte-myosin-B showed the same properties as the washed myosin-B in the Ca2+-sensitive contractile activity. In the myosin B, the Mg2+-ATPase activity was regulated by Ca2+ through “NT” fraction from arterial smooth muscle. On the other hand, leukocyte-myosin and arterial tropomyosin (TM) were purified from leukocyte-myosin B and arterial “NT”, respectively. In a synthetic actomyosin consisting of skeletal actin and leukocyte-myosin, arterial NT enhanced the actinactivated Mg2+-ATPase activity of leukocyte-myosin. The enhancement was much higher in the presence of Ca2+ and associated with an increase in the level of phosphorylation of myosin light chain. The arterial TM enhanced also the Mg2+-ATPase activity of the synthetic actomyosin, but its enhancement was independent of Ca2+. The enhancement was lower than that of NT and was not associated with an increase in the phosphorylation of myosin light chain. These results indicate that leukocyte contractile protein is essentially regulated by the level of Ca2+-dependent myosin light chain phosphorylation, but its contractility is augumented by TM associated with actin, independent of Ca2+