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The effect of protein phosphorylation on the distribution of chlorophyll-protein complexes between appressed and non-appressed thylakoid regions of spinach chloroplasts has been investigated. Stroma lamellae vesicles and inside-out vesicles, representative of non-appressed and appressed thylakoids, respectively, were isolated from thylakoid membranes before and after phosphorylation. The fractions were analyzed with respect to the yield of vesicles, incorporation of [32P]phosphate into the light-harvesting chlorophyll-a/b -protein complex, chlorophyll composition, and the relative content of the main chlorophyll-protein complexes. The yield of inside-out vesicles from phosphorylated thylakoids was 20 % lower than that from control thylakoids, indicating a partial destacking.The specific incorporation of [32P]phosphate into the light-harvesting chlorphyll-a/b -protein complex was at least four times higher in stroma lamellae vesicles than in inside-out vesicles. The proportion of the light-harvesting chlorophyll-a/b-protein complex in stroma lamellae vesicles increased from 13 % to 21 % of their total chlorphyll after phosphorylation of the thylakoids. There was also a corresponding increase in the level of chlorophyll 6. These observations strongly suggest that phosphorylated light-harvesting complexes migrate from appressed thylakoids rich in photosystem 2 to non-appressed thylakoids rich in photosystem 1. In contrast, there was no evidence for a lateral migration of the chlorophyll-a -protein complex of photosystem 2 after phosphorylation. Our results indicate that a lateral migration of phosphorylated light-harvesting complexes in combination with a lateral separation of photosystem 1 and 2 to the different thylakoid regions, is a basis for the regulation of excitation energy between the two photosystems.The chloroplast thylakoid membrane shows a pronounced lateral asymmetry as judged by ultrastructural, labelling and fractionation studies [l, 21. The photosystem 1 complex is mainly excluded from the appressed thylakoid regions and concentrated in the non-appressed ones. On the other hand, most of the photosystem 2 complex and the light-harvesting chlorophyll-a/b -protein complex are concentrated in the appressed thylakoid regions [3 -51. Such a spatial segregation of complexes in the plane of the membrane raises questions both concerning electron shuttling and energy distribution between the two photosystems [l, 21. This communication deals with the latter problem.It has been shown that chloroplasts have a mechanism that prevents imbalance in the excitation of photosystem 1 and 2 [6 -81. Thus, excess of photosystem 2 light induces changes which allow some of the energy to become available for photosystem 1 (state 2), while excess of photosystem 1 light reverses this effect (state 1) [9]. The mechanism behind the state l/state 2 transitions has recently been shown to involve the phosphorylation and dephosphorylation of the lightharvesting chlorophyll-a/b -protein complex [ l o -131. Upon phosphorylation of th...

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