Role of basic residues in the phosphorylation of synthetic peptides by myosin light chain kinase.

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Abstract
The substrate specificity of the chicken gizzard myosin L chain kinase has been studied by using a series of synthetic peptide analogs of the NH2-terminal sequences of the chicken gizzard myosin L chain (MW = 20, 000). An 18-residue synthetic peptide, .**GRAPHIC**. corresponding to the sequence reported by Maita et al., was phosphorylated with a 22-fold higher Km and a Vmax that was decreased to 1% of the native protein substrate. This peptide was also an inferior substrate when compared with an 18-residue synthetic peptide with an alternative sequence, .**GRAPHIC**. which was phosphorylated with an apparent Km of 6.9 .mu.M, comparable to the native protein substrate of 8.6 .mu.M, and a Vmax of 3.9 .mu.mol .cntdot. min-1 .cntdot. mg-1, 11% of that for the protein substrate. The kinetics of phosphorylation of shortened peptides corresponding to both sequences, together with peptides with appropriate substitutions, indicated that basic residues were the primary determinants of specificity for the smooth muscle myosin L chain kinase. In the latter peptide sequence, lysine residues 11 and 12 and the arginine at position 13 had a major influence on the kinetics of peptide phosphorylation.

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