Structural requirements for phosphorylation of myosin regulatory light chain from smooth muscle

Site-directed and chimeric mutations of myosin regulatory light chains were used to identify residues important for phosphorylation of Ser¹⁹ by smooth muscle myosin light chain kinase. Arg¹⁶ and hydrophobic residues C- terminal of Ser¹⁹ in smooth muscle light chain were important substrate determinants in the intact protein. However, changes in the kinetic properties with mutations in the light chain were substantially smaller than results reported with structurally similar synthetic peptide substrates. These results together with the low V(max) value for short peptide substrates containing the consensus phosphorylation sequence suggest that there may be additional sites of interactions between the kinase and protein substrate. Chimeras of skeletal and smooth muscle light chains were constructed with exchanges at the N terminus and subdomains I, II, III, and IV. Analysis of results obtained on the kinetic properties for phosphorylation showed that subdomains I and II contribute to high V(max) values. Thus, a region distant from the consensus phosphorylation sequence in smooth muscle light chain is also an important substrate determinant for myosin light chain kinase.