Nanodomains of Cytochrome b6f and Photosystem II Complexes in Spinach Grana Thylakoid Membranes

The cytochrome b(6)f (cytb(6)f) complex plays a central role in photosynthesis, coupling electron transport between photosystem II (PSII) and photosystem I to the generation of a transmembrane proton gradient used for the biosynthesis of ATP. Photosynthesis relies on rapid shuttling of electrons by plastoquinone (PQ) molecules between PSII and cytb(6)f complexes in the lipid phase of the thylakoid membrane. Thus, the relative membrane location of these complexes is crucial, yet remains unknown. Here, we exploit the selective binding of the electron transfer protein plastocyanin (Pc) to the lumenal membrane surface of the cytb(6)f complex using a Pc-functionalized atomic force microscope (AFM) probe to identify the position of cytb(6)f complexes in grana thylakoid membranes from spinach (Spinacia oleracea). This affinity-mapping AFM method directly correlates membrane surface topography with Pc-cytb(6)f interactions, allowing us to construct a map of the grana thylakoid membrane that reveals nanodomains of colocalized PSII and cytb(6)f complexes. We suggest that the close proximity between PSII and cytb(6)f complexes integrates solar energy conversion and electron transfer by fostering short-range diffusion of PQ in the protein-crowded thylakoid membrane, thereby optimizing photosynthetic efficiency.