Correlations between fluorescence and phosphorylation changes in thylakoid membranes of Chlamydomonas reinhardtii in vivo: A kinetic analysis

Abstract

The reorganization of the light-harvesting antenna in the thylakoid membranes upon phosphorylation of some of its apoproteins was further characterized in vivo using the green algae Chlamydomonas reinhardtii. To this end we have studied light-to-dark transitions on intact cells placed in the anaerobic state using the F34 mutant strain which lacks PS II centers. We show that the 50% decrease in fluorescence yield in such transitions is accompanied by a 50% increase in PS I antenna size. The half-times of the kinetics of the fluorescence changes in the dark-to-light and light-to-dark transitions are of 320 and 120 s, respectively. The rate-limiting steps in these transitions are attributed to the dephosphorylation and phosphorylation processes themselves rather than to the activation of the kinase or to the diffusion of the phosphorylated complexes in the thylakoid membrane. Accordingly, the changes in phosphorylation of three of the main phosphopolypeptides occur with the same kinetics as those of the fluorescence changes. Different phosphorylation kinetics are observed for two phosphopolypeptides which are, however, also part of the light-harvesting complexes. Possible heterogeneities in the kinase enzymatic activities are discussed. The peculiar status of the phosphopolypeptide D2, associated with the PS II center, is described.