SUBUNIT FUNCTION IN CARDIAC MYOSIN - EFFECT OF REMOVAL OF LC2 (18000 MOLECULAR-WEIGHT) ON ENZYMATIC PROPERTIES

The 18 000-dalton subunit of dog cardiac myosin (Lc2) was selectively removed by the treatment of myosin with a cardiac myofibrillar protease under mild experimental conditions. Removal of Lc2 did not affect the Ca2+ ATPase activity of myosin. The basic Mg2+ ATPase and actin-ac-tivated Mg2+ ATPase activities (in 0.1 M KC1) were significantly altered as a consequence of removal of the Lc2. A comparison of native myosin and Lc2 deficient myosin showed (a) the actin-activated ATPase of Lc2 deficient myosin was increased threefold over that of native myosin; identical results were obtained with respect to this measurement with pure actin or regulated actin as a cofactor; (b) the actin-activated ATPase of Lc2 deficient myosin was relatively insensitive to increase in KC1 concentration; (c) Mg2+ ATPase of actomyosin, reconstituted with Lc2 deficient myosin, showed a fivefold increase in the optimum substrate (Mg2+ ATP) concentration, indicating a resistance of the rigor complexes to dissociation by ATP; (d) Ca2+ sensitivity of the actin-activated ATPase (in the presence of troponin-tropomyosin) was identical for both the Lc2 deficient and native myosins; (e) 18 000 dalton light chain from canine cardiac and rabbit skeletal myosin was found to reassociate with the Lc2 deficient myosin with the lowering of both Mg2+ ATPase and actin-activated ATPase activities to values close to those for native myosin. The results of our study suggest that removal of Lc2 from cardiac myosin changes the conformation of myosin to a state that enhances its interaction with actin. Evidence is also presented for functional homology between the cardiac Lc2 and 18 000-dalton light chain of skeletal muscle myosin. © 1979, American Chemical Society. All rights reserved.