Epidermal growth factor stimulates tyrosine phosphorylation of the myosin regulatory light chain from smooth muscle

Epidermal growth factor (EGF) stimulates the phosphorylation by A431 membranes of tyrosine residues in the myosin regulatory light chain (LC₂₀) from chicken gizzard smooth muscle. In the presence of EGF, the K(m) of the EGF receptor kinase for LC₂₀ is 73 μM and the V(max) is 17 nmol/min/mg. Two moles of phosphate are incorporated into tyrosine per mol of LC₂₀. Trypsin digestion of the phosphorylated LC₂₀ produces two phosphopeptides which are phosphorylated to approximately equal extents. Sequential Edman degradation of the separated phosphopeptides shows that tyrosines 142 and 155 of the protein are phosphorylated. Tyrosine 142 is located within a sequence similar to that of autophosphorylated tyrosine kinases in that five of the seven amino acids on the NH₂-terminal side are acidic. Tyrosine 155 has no acidic amino acids near its NH₂-terminal side. A comparison of the initial rates of phosphorylation of the two tyrosines shows that tyrosine 142 is phosphorylated at three times the rate of tyrosine 155.