Phosphorylation of an actin · tropomyosin · troponin complex from human skeletal muscle

Abstract

A human skeletal actin · tropomyosin · troponin complex was phosphorylated in the presence of [γ- 32P]ATP, Mg 2+, adenosine 3′:5′-monophosphate (cyclic AMP) and cyclic AMP-dependent protein kinase (protein kinase). Phosphorylation was not observed when the actin complex was incubated in the absence of protein kinase or 1 μM cyclic AMP. In the presence of 10 −7 M Ca 2+ and protein kinase 0.1 mole of [ 32P]phosphate per 196 000 g of protein was incorporated. This was two-fold higher than the [ 32P]phosphate content of a rabbit skeletal actin complex but two-fold lower than that of a bovine cardiac actin complex. At high Ca 2+, 5 · 10 −5 M, little change in the phosphorylation of a human skeletal actin complex occurred. Phosphoserine and phosphothreonine were identified in the [ 32P]phosphorylated actin complex. Polyacrylamide gel electrophoresis in sodium dodecyl sulfate showed that 60% of the label was associated with the tropomyosin binding component of troponin. The inhibitory component of troponin contained 16% of the bound [ 32P]phosphate. Increasing the Ca 2+ concentration did not significantly decrease the [ 32P]phosphate content of the phosphorylated proteins in the actin complex. No change in the distribution of phosphoserine or phosphothreonine was observed. Half maximal calcium activation of the ATPase activity of reconstituted human skeletal actomyosin made with the [ 32P]phosphorylated human skeletal actin complex was the same as a reconstituted actomyosin made with an actin complex incubated in the absence of protein kinase at low or high Ca 2+.