The kinase activity and CaM activation of recombinant wild-type (WT) and mutant MLCK were assessed, and the results are shown along with previously published data on a variant shown to decrease MLCK activity, p.Ser1759Pro.4 The rate of 32P incorporation into the regulatory light chair (RLC) was measured. The maximal activities of WT MLCK (circle), variant MLCK (square), and Ser1759Pro MLCK (diamond) were obtained at different RLC concentrations. The three variants which were assessed: Tyr1575His (A), Gly1317Cys (B), and Cys1384Gly (C). The relative percentage of maximal kinase activity was plotted as a function of CaM concentration. The data points represented the mean ± standard error of three or more determinations. The data were fit to the Michaelis-Menten equation for estimation of the Vmax values and KCaM value.