Phosphorylation of the 18,000-dalton light chain of myosin during a single tetanus of frog muscle.

Changes in the 32P content of proteins due to muscle contraction were investigated, using muscles dissected from liver frogs injected with [32P]orthophosphate. The only significant change found was in the radioactivity of the 18,000-dalton light chain of myosin; during a single tetanus, an increase of 85 to 90% occurred as compared to the resting muscle. This increase corresponded to about 0.4 mol of 32P per mol of light chain. The same increase in radioactivity of this light chain was also found upon caffeine-induced contracture of the intact muscle. It is postulated that the increased Ca2+ concentration in the sarcoplasm resulting from electrical stimulus or caffeine treatment activates the myosin light chain kinase which phosphorylates the 18,000-dalton light chain.