Biochimica et Biophysica Acta 848 (1986) 317-323 317 Elsevier BBA 41957 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 Michael P. Percival a, Andrew N. Webber a,,, John P. Markwell b and Neil R. Baker a,** Department of Biology, University of Essex, Colchester, C04 3SQ, Essex (U.K.) and h Department of Agricultural Biochemistry, University of Nebraska, Lincoln, NE 68583-0718 (U.S.A.) (Received August 21st, 1985) (Revised manuscript received October 18th, 1985) Key words: Chlorophyll fluorescence kinetics; Chlorophyll-protein complex; Chloroplast development; Light-harvesting complex; Protein phosphorylation; Wheat The effects of phosphorylation of thylakoid polypeptides at 2 and 5 mM Mg 2+ 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 Fv, defined by/~max, were the same at all stages of thylakoid development. On phosphorylation at 5 mM Mg 2+, /3ma x increased similarly at all stages of thylakoid development. Such changes in /~m,~ indicate that the magnitude of phosphorylation-induced disconnection of the light-harvesting chlorophyll a/b protein complex from Photosystemll was independent of the degree of lateral heterogeneity of the thylakoid membranes. At 2 mM Mg 2+ phosphorylation also increased/~max at all developmental stages, but to a lesser extent than was observed at 5 mM Mg 2+. 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 Mg 2+ concentration can modify the effects of protein phosphorylation on the energetic interactions between the complexes. Introduction The distribution of excitation energy within the photochemical apparatus of thylakoid membranes can be regulated by a process often termed the * Current address: Department of Botany and Plant Sciences, University of California, Riverside, CA 92521, U.S.A. ** To whom all correspondence should be addressed. Abbreviations: DCMU, 3-(3,4-dichlorophenyl)-l,l-dimethyl- urea; Fro,maximal fluorescence level; Fo, minimal fluorescence level; Fv, fluorescence of variable yield (Fv = F m- F0); Hepes, 4-(2-hydroxyethyl)-l-piperazineethanesulphonic acid; LHC II, light-harvesting chlorophyll a/b protein complex of Photosys- tem II; PS, Photosystem. State 1-State 2 transition [1-5]. An important feature of the regulation of the state transition is the dissociation of phosphorylated LHC II from PS II [5-11]. However, it is often difficult to differentiate which changes in membrane char- acteristics are attributable to the removal of phos- phorylated LHC II from PS II and which are due to increased interaction between the phosphory- lated LHC II and PS I. One approach to overcome this problem is to induce a desegregation of LHC II-PS II complexes in the membrane prior to LHC II phosphorylation by reducing the Mg 2÷ con- centration around the membranes [12-14]. This produces a decrease in the degree of thylakoid 0005-2728/86/$03.50 © 1986 Elsevier Science Publishers B.V. (Biomedical Division)