Spinach thylakoids contain at least 8 proteins (8.3–58 kDa) whose phosphorylation is strongly inhibited by the plastoquinone antagonist 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone (DBMIB). Reduction of DBMIB with ascorbate completely or partially relieves inhibition for all proteins other than the 27 kDa light-harvesting complex (LHC) II and a synthetic dodecapeptide (MRKSATTKKAVC). The peptide, an analog of the phosphorylation site of pea 27 kDa LHC II, is phosphorylated with the same site specificity, kinetics, redox control and sensitivity to DBMIB/ascorbate as the protein itself. The data indicate that synthetic peptides can be used to study the number, substrate specificity and redox regulation of thylakoid protein kinases.
