The 64 kDa polypeptide of spinach may not be the LHCII kinase, but a lumen-located polyphenol oxidase.
Phosphorylation of chlorophyll alb-binding proteins of the of photosystem II light-harvesting assembly controls the energy distribution between the two photosystems as well as the turnover of thylakoid membrane proteins. The LHCII kinase, suggested to be a 64 kDa protein, is light-regulated by a mechanism involving reduction of plastoquinone and the participation of the cytochrome b6lf complex. A cDNA encoding that protein has been isolated from a lambda gt11-based library made from spinach polyadenylated RNA using a two-step strategy involving screening by polyclonal monospecific antisera and plaque hybridization. The protein of 73.1 kDa molecular mass represents a precursor which contains a bipartite transit peptide of 101 amino acid residues (11.0 kDa) that directs the protein into the thylakoid lumen. It can be phosphorylated in vitro, and exhibits significant homology to plant polyphenol oxidases not to kinases. The gene was therefore designated PpoA. Reinvestigation of components in the molecular mass range of 50-70 kDa disclosed five additional proteins which can accompany kinase-active cytochrome b6lf, photosystem II and AMS [1] preparations. Four of them can be phosphorylated in vitro; two with apparent molecular masses of 53 and 66 kDa are capable of phosphorylation and represent new, yet unidentified proteins.