Calmodulin is labeled at lysine 148 by a chemically reactive phenothiazine

10-(3-Propionyloxysuccinimide)-2-(trifluoromethyl)phenothiazine (POS-TP) is a chemically reactive calmodulin antagonist: 2 mol are incorporated per mol of calmodulin when excess reagent is used, and only lysyl side chains are modified. Tryptic peptide mapping demonstrated that a single unique site on calmodulin reacts at low molar ratios of POS-TP. Labeled peptides were isolated and analyzed by amino acid composition and sequence analysis. The unique site was identified as Lys148 of calmodulin, the carboxyl-terminal residue. At higher molar ratios of the reagent Lys21, Lys75, and Lys77 are labeled as are several minor peptides that were not characterized.