Significance of Protein Ordering in Grana Thylakoids for Light-Harvesting by Photosystem II and Protein Mobility

Controlled by environmental factors, proteins in the grana thylakoid subcompartment in chloroplasts can rearrange into highly ordered semicrystalline arrays. The functional implications of these arrays are analyzed by using an Arabidopsis fatty acid desaturase (fad5) mutant as a model system, which constitutively forms these crystalline structures in thylakoid membranes. Stoichiometric analysis of the fad5 thylakoid membranes reveal the existence of two membrane domains in grana (domains with PSII crystals and LHCII-enriched domains). Probing the light-harvesting of PSII by chlorophyll fluorescence induction indicates a very efficient energy transfer between these domains in the mutant probably by transversal energy transfer across the aqueous stromal partition gap between adjacent grana discs. Furthermore, the protein mobility in fad5 measured by fluorescence recovery after photobleaching is higher compared to WT plants. This gives evidence for a high protein mobility in LHCII-enriched grana regions.