A CK2 SITE IS REVERSIBLY PHOSPHORYLATED IN THE PHOTOSYSTEM-II SUBUNITCP29

Protein phosphorylation is a major mechanism in the regulation of prot ein function, In chloroplast thylakoids several photosystem II subunit s, including the major antenna light-harvesting complex II and several core complex components, are reversibly phosphorylated depending on t he redox state of the electron carriers, A previously unknown reversib le phosphorylation event has recently been described on the CP29 subun it which leads to conformational changes and protection from cold stre ss (Bergantino, E., Dainese, P., Cerovic, Z. Sechi, S. and Bassi, R. ( 1995) J, Biol Chem. 270, 8474-8481), In this study, we have identified the phosphorylation site on the N-terminal, stroma-exposed domain, sh owing that it is located in a sequence not homologous to the other mem bers of the Lhc family. The phosphorylated sequence is unique in chlor oplast membranes since it meets the requirements for CK2 (casein kinas e II) kinases, The possibility that this phosphorylation is involved i n a signal transduction pathway is discussed.