PHOSPHORYLATION OF SERINE RESIDUES IN ENDOGENOUS PROTEINS OF THYLAKOIDS AND SUBTHYLAKOID PARTICLES IN THE DARK UNDER NONREDUCING CONDITIONS

Isolated thylakoids, Photosystem I and Photosystem II particles were phosphorylated with [γ-32P]ATP at high specific radioactivity in the dark under nonreducing conditions and in the light in the absence of electron acceptor. The resulting phosphoproteins were compared by gel electrophoresis and autoradiography. Phosphorylation of thylakoids in the dark and in the light rendered distinct patterns of phosphoproteins. Some of the dark-phosphorylated proteins in thylakoids were diminished or not detected in the light-phosphorylated membranes. Phosphorylation of subthylakoid particles was insensitive to light and most of the phosphoproteins in these membranes were also observed in the dark-phosphorylated thylakoids. Dark phosphorylation rendered mostly phosphoserine in individual proteins of thylakoids, subthylakoid particles and lysine-rich histone phosphorylated by the particles. Conversely, phosphothreonine was prevalent in light-phosphorylated thylakoids. The results are consistent with the presence of a protein serine kinase activity that is distributed homogeneously within the thylakoid regions, is more active in the dark, does not require reducing conditions for activity and phosphorylates a number of endogenous substrates most of which belong to the stroma membranes. © 1990.