Structure of a large photosystem II supercomplex from Acaryochloris marina

Acaryochloris marina is a prochlorophyte-like cyanobacterium containing both phycobilins and chlorophyll d as light harvesting pigments. We show that the chlorophyll d light harvesting system, composed of Pcb proteins, functionally associates with the photosystem II (PSII) reaction center (RC) core to form a giant supercomplex. This supercomplex has a molecular mass of about 2300 kDa and dimensions of 385 Å × 240 Å. It is composed of two PSII-RC core dimers arranged end-to-end, flanked by eight symmetrically related Pcb proteins on each side. Thus each PSII-RC monomer has four Pcb subunits acting as a light harvesting system which increases the absorption cross section of the PSII-RC core by almost 200%.