The cation-dependence of the degree of protein phosphorylation-induced unstacking of pea thylakoids

Abstract

Experiments are presented to show that the phosphorylation of the light-harvesting chlorophyll a b - protein complex (LHC) induces structural reorganisation within the thylakoid membrane in response to the introduction of additional negative surface charges. The effect of cations of different valency on chlorophyll fluorescence measurements indicates that LHC-phosphorylation-induced reorganisation involves a change in the electrostatic screening capability of the added cation. At intermediate levels of cations (e.g., 1 or 2 mM Mg 2+), which substantially stack non-phosphorylated membranes, it was found that membrane phosphorylation caused considerable unstacking as monitored by light scattering and electron microscopy. Concomitant with this was a large decrease in chlorophyll fluorescence indicative of randomisation of chlorophyll protein complexes which would result in an increase in energy transfer between the photosystems as well as an absorption cross-section change. At higher concentrations (e.g., above 5 mM Mg 2+) a persistent ATP-induced decrease in chlorophyll fluorescence has been attributed to the displacement of charged phosphorylated LHC from the appressed granal to the non-appressed stromal lamellae, thus decreasing the absorption cross-section of Photosystem II. Under these circumstances only a small degree of unstacking was detected by light scattering and measurements of the percentage of thylakoid length which is stacked to form grana. However, when considered on a surface area basis, the structural changes observed can qualitatively account for the magnitude of the chlorophyll fluorescence quenching due to the lateral diffusion of LHC.