The Structure of Plant PSI - LHCI at 2.8 Angstrom Resolution, a Membrane Super-Complex

Photosynthesis drives both oxygen evolution and carbon fixation in the biosphere. For over 3 billion years, two membrane complexes, photosystem I and photosystem II, evolved into efficient light capturing machines. Photosystem I converts light into reduced chemical species with near perfect quantum efficiency and this efficiency is maintained over large variations in its antennae size. On the other hand, in the face of changing light intensity and quality plants can regulate the distribution and efficiency of their antennae. Plant Photosystem I (PSI) is one of the most intricate membrane complexes in nature. It is comprised of two distinct membrane complexes, a reaction center core (PSI) and the light-harvesting antenna (LHCI). High resolution data and experimental phase estimates were obtained from a new crystal form of plant PSI. The structure includes 16 subunits and more than 200 prosthetic groups, the majority of which are light harvesting pigments. The new structure reveals the details of the connections between PSI and LHCI antenna and suggest mechanisms that regulate both excitation energy transfer between the complexes and the efficiency of LHCI antenna.