Phosphorylated Polypeptides of the Photosystem 2 Core Complex

Abstract

Several proteins of higher plant thylakoids can be phosphorylated through the catalytic action of endogenous, membrane-associated kinase (1). Most prominently phosphorylated are the polypeptide components of the chlorophyll a/b-containing light-harvesting complex (2–4). Reversible phosphorylation of these 23–27 kDa polypeptides is thought to provide a molecular mechanism whereby the distribution of quanta between the two photosystems can be regulated (4–7). The identities of other proteins which are phosphorylated by thylakoid-bound kinase have not yet been established, though it has been suggested that at least some of them are associated with photosystem 2 (PS2), in particular those polypeptides with apparent molecular masses of 40–45, 30–35 and 10 kDa (8). Indeed, the use of neutral detergents such as Triton X-100, has permitted the preparation of membrane subfractions highly enriched in PS2 (9,10) and allowed us, as reported in a previous paper (11), to show that the major [³² P] labelled polypeptides were present in the PS2-LHC-2 supramolecular complex. Recently there has been increased interest in isolating more purified PS2 preparations along lines pioneered by Satoh (12). Density gradient fractionation allows the removal of LHC-2 and other polypeptides from Triton X-1.00-solubilized PS2-enriched membranes, yielding PS2 ‘core’ particles (13), here called RCC2, which possess a small number of polypeptides.