Figure1. Schematic of the PSII life cycle based on previous findings and current results, focusing on the proposed positioning of the NRC in the RC. Red arrows, repair cycle steps; red D1, damaged version of D1; light red D2, slightly less frequently damaged version of D2. In a given RC*complex, either D1, D2, or both may be damaged. Several intermediary stages (steps 1 to 3 in Discussion) in the life cycle are omitted for clarity: After damage, PsbO, PsbU, PsbV, and PsbQ dissociate, Psb27 rebinds, and the complex monomerizes before the proposed NRC formation step. Black text and red/green coloring represent steps common to both de novo synthesis and repair. From Weisz and Johnson et al., PNAS, 2019.
Photosystem II (PSII) is a large membrane-protein complex that initiates photosynthesis by capturing sunlight and converting it to chemical energy, in the process splitting water into protons, electrons, and oxygen. This reaction supplies the energy to power nearly all life on Earth, and provides the oxygen that we breathe. The water-splitting reaction, the most energetically demanding in all of biology, is carried out at the active site of PSII, a Mn4CaO5 cluster (“Mn cluster”) buried deep within the complex. Active PSII is damaged frequently in vivo, particularly on the core D1 protein, as a result of the highly reactive oxidizing and reducing species that are generated near the Mn cluster and at other redox-active sites throughout the complex. Damaged PSII must be partially disassembled, a new copy of the damaged protein(s) must be inserted, and the complex must be re-assembled and photoactivated. This intricate sequence is known as the PSII “life-cycle,” and it is becoming increasingly clear that it is a complicated, tightly regulated process consisting of numerous stages, in which many external protein cofactors bind transiently to optimize the process and protect the vulnerable intermediate PSII sub-complexes. Our lab studies various stages of the PSII life-cycle. We have identified key steps during PSII assembly and have characterized the role and structural binding location of proteins that protect PSII during this process. We use a combination of genetics, biochemistry, spectroscopy, and mass spectrometry techniques to study the PSII life-cycle.
