Thymus myosin. Isolation and characterization of myosin from calf thymus and thymic lymphocytes, and studies on the effect of phosphorylation of its Mr = 20,000 light chain.

Calf thymus has been used as a source of vertebrate non-muscle myosin.Starting from 100 g of fresh tissue, between 5 and 15 mg of myosin were isolated to a level of purity greater than 90%.Electron microscopy showed that the purified protein comprises two globular heads attached to a fibrous rod.The myosin contains approximately 2 mol of heavy chain (Mr -200,-000), 2 mol of M, = 20,000 light chain, and 2 mol of M, = 16,000 light chain.In 0.6 M KCl, its ATPase activity is stimulated by EDTA or Ca2+ and inhibited by M&*.Evidence was obtained which suggests that the myosin is not contaminated by significant levels of vascular smooth muscle myosin, and it is probably derived predominantly from thymic lymphocytes.The M, = 20,000 light chain of thymus myosin can substitute for the regulatory light chain of scallop myosin and thereby resensitize desensitized scallop myofibrils, suggesting that it may function as a regulatory subunit in its parent myosin.Data are presented which demonstrate that phosphorylation of the Mr = 20,000 light chain regulates the interaction between thymus myosin and actin.A positive correlation between the level of light chain phosphorylation and the actin-activated MgATPase activity of thymus myosin was consistently observed.In the presence of MgATP, at -0.15 M NaCl and pH 7.0, phosphorylation of the M, = 20,000 light chain also controls the stability of thymus myosin filaments.These data suggest that phosphorylation of the M, = 20,000 "regulatory" myosin light chain may play an important role in controlling actomyosin-based motility in thymus cells.During the past decade, considerable progress has been made in characterizing myosins isolated from a variety of nonmuscle cells (1-4).This paper describes the isolation and characterization of myosin from calf thymus, which we selected as a source of vertebrate non-muscle myosin because: (i) large quantities of fresh material can be used to obtain sufficient myosin for detailed biochemical studies and (ii)