Different phosphorylation patterns of cardiac myosin light chains using ATP and ATP gamma S as substrates

Controversial views have been reported regarding the role of myosin light chain phosphorylation in the regulation of cardiac contractility (for review see. In the past, adenosine 5'-(-thio)triphosphate) (ATP gamma S) instead of ATP has frequently been used to study mechanical and biochemical consequences of myosin P-light chain (P-LC, LC-2) phosphorylation since thiophosphorylated sites are not significantly attacked by phosphatases. Unlike thiophosphorylation phosphorylation of myosin by myosin light chain kinase did neither decrease maximal (unloaded) shortening velocity of cardiac skinned fibres nor ATPase activity of cardiac myofibrils. We have accordingly investigated the phosphorylation pattern of purified cardiac myosin light chains using radioactive labeled ATP gamma S and ATP. We found that both the 28 kDa myosin light chain (LC-1) and the 18 kDa myosin light chain (LC-2, P-LC) were phosphorylated when ATP gamma S was present. In the presence of ATP, however, only LC-2 was found to be phosphorylated.