Modification of the interaction between Photosystem II and the light-harvesting chlorophyll a b-protein complex by protein phosphorylation in developing wheat thylakoids exhibiting different degrees of lateral heterogeneity

The effects of phosphorylation of thylakoid polypeptides at 2 and 5 mM Mg²⁺ on the chlorophyll fluorescence induction characteristics of developing wheat thylakoids, exhibiting differing degrees of lateral heterogeneity of intrinsic protein complexes, were studied. The relative contributions of the fast and slow components of F_v, defined by β_max, were the same at all stages of thylakoid development. On phosphorylation at 5 mM Mg²⁺, β_max increased similarly at all stages of thylakoid development. Such changes in β_max indicate that the magnitude of phosphorylation-induced disconnection of the light-harvesting chlorophyll a b protein complex from Photosystem II was independent of the degree of lateral heterogeneity of the thylakoid membranes. At 2 mM Mg²⁺ phosphorylation also increased β_max at all developmental stages, but to a lesser extent than was observed at 5 mM Mg²⁺. Discussion is made of how changes in the lateral heterogeneity of the pigment-protein complexes within the thylakoid that occur during membrane development and on reduction of Mg²⁺ concentration can modify the effects of protein phosphorylation on the energetic interactions between the complexes.