Further enzymatic characteristics of a thylakoid protein kinase

The enzymatic characteristics of a protein kinase purified from thylakoids are further described. ATP (KM approximately 30 microM) and Mg2+ ion (greater than 1.0 mM) were required for activity, while ADP was a competitive inhibitor (Ki = 100 microM). Activity was 55% inhibited by the sulfhydryl inhibitor p-chloromercuribenzoate (1 mM) and was less sensitive to substituted maleimides. Lysine-rich histones (H1) were utilized as an exogenous phosphorylation substrate both by thylakoid-bound kinase and by isolated enzyme; threonine was predominantly phosphorylated by the in situ enzyme, whereas the isolated enzyme phosphorylated closely related serine residues as determined by peptide mapping. Detergents that proved useful in extracting the kinase from thylakoids markedly inhibited activity of the isolated enzyme, whereas Triton X-100 and 3-[(3-cholamidopropyl)dimethylammonio]-1-propane-sulfonic acid had little effect. The enzyme could be freed from detergent and behaved as an active monomer on size-exclusion chromatography. The phosphate contents of the light-harvesting chlorophyll a/b protein complex of photosystem II isolated from maximally phosphorylated thylakoid membranes of spinach and pea were equivalent to approximately 6% and approximately 19% phosphorylation, respectively. Corresponding values for nonphosphorylated membranes were approximately 3% and approximately 14.5%.