In addition to de novo biogenesis, PSII undergoes a frequent damage and repair cycle. The D1 protein in the core of PSII complex coordinates the majority of the redox active cofactors through which electrons are transferred. Because several reactive intermediates are formed during the normal course of this reaction, the D1 protein is the target of constant and irreversible damage. Consequently, the damaged D1 protein must be proteolytically removed from the complex and replaced with a newly synthesized copy to regenerate PSII function.
D1 protein damage occurs all light intensities, but the rate of D1 damage increases with light intensity. This damage results in loss of PSII function, a process termed photoinhibition. The PSII weak link is the crux of the ‘light problem’ and why photosynthetic organisms have elaborate mechanisms for getting rid of excess light energy. The D1 protein is rapidly turned over in a damage-repair cycle to maintain steady state levels of active PSII. This is a multi-step process that must proceed efficiently so that the photosynthetic light reactions continue to run optimally.
Photosynthetic organisms have elaborate systems to detect damage to PSII (primarily on the D1 protein) and trigger the removal of non-functional subunits. Phosphorylation of PSII subunits is one way damaged subunits may be marked to trigger the repair cycle. Two different proteases (FtsH and Deg) are required for cleavage and degradation of the damaged D1 protein.
Due to the central location of the D1 protein within the PSII complex, its constant damage requires frequent and rapid PSII assembly from partially disassembled complexes. Because the D1 protein contributes the majority of the ligands to the Mn4Ca1Clx cluster, this catalytic center must be taken apart and reassembled with each turnover of the D1 protein. Presumably, the lumenal extrinsic proteins must also be released from the complex upon removal of the D1 protein. Little else is known about the consequences of D1 protein removal on the association of the membrane proteins within the complex. At the very least, major structural rearrangements are necessary on the lumenal side of the complex to accommodate the turnover of the D1 protein.
Assembly factors also facilitate the efficiency of the repair pathway. Some of these proteins have the same function whether the assembly occurs from scratch or via the repair cycle. Other assembly factors may be required for the repair pathway only (like those specializing in disassembly of the complex). The activities of some assembly factors are absolutely required for PSII assembly to proceed, but the trend is that many are for efficiency only. In other words, assembly or damage-repair can occur in the absence of these assembly factors, but the process is just slower. Under normal laboratory growth conditions in which the photosynthetic organisms aren’t particularly stressed, there may be little difference between wild type and a mutant lacking one of these proteins. However, under stress conditions like high light when PSII must more frequently undergo the repair cycle, the absence of one of these assembly factors would be more noticeable.
Recent advances in understanding the assembly and repair of photosystem II
Assembling and maintaining the Photosystem II complex in chloroplasts and cyanobacteria
Towards a critical understanding of the photosystem II repair mechanism and its regulation during stress conditions
Protein quality control in chloroplasts: a current model of D1 protein degradation in the photosystem II repair cycle
Thylakoid protein phosphorylation in dynamic regulation of photosystem II in higher plants
Photoprotection in plants: a new light on photosystem II damage
Dark-adapted spinach thylakoid protein heterogeneity offers insights into the photosystem II repair cycle
The stromal chloroplast Deg7 protease participates in the repair of photosystem II after photoinhibition in Arabidopsis
Quality control of photosystem II: FtsH hexamers are localized near photosystem II at grana for the swift repair of damage
Role of novel dimeric Photosystem II (PSII)-Psb27 protein complex in PSII repair