SKELETAL-MUSCLE MYOSIN REGULATORY LIGHT-CHAINS CONFORMATION AFFECTS THE PAPAIN CLEAVAGE OF A1 LIGHT-CHAINS

In the present study, the influence of magnesium-for-calcium exchange and phosphorylation of regulatory light chain (RLC) on accessibility o f myosin and heavy meromyosin alkali light chains (A1) for papain dige stion was investigated. The properties of native and papain treated my osin and heavy meromyosin were compared. Exchange of magnesium ions bo und to RLCs for calcium ions accelerates the digestion of A1 in the pr esence of ATP in dephosphorylated myosin, heavy meromyosin, acto-myosi n and the acto-heavy meromyosin complex. In the absence of ATP the exc hange of magnesium ions bound to RLCs for calcium ions delays the dige stion of A1 in the acto-myosin complex. Myosin and heavy meromyosin ha ving shortened A1 by papain cleavage shows decreased K+-ATPase and inc reased actin binding ability in the presence and absence of ATP. The c ooperation of RLC and A1 with heavy chains in the changes of structura l organization of myosin head during muscle contraction is discussed.