While the current structural models have provided new information on the function of PSII, they are a static view of the complex. The sheer number of components within the models provokes questions as to the intricacy of its biogenesis, but these structural snapshots cannot provide direct information on this process. Many biochemical and genetic studies have contributed to our understanding of PSII biogenesis. In fact, PSII has a dynamic life cycle within the thylakoid membranes and undergoes constant assembly and disassembly.
The structural complexity of PSII requires accurate and regulated assembly. All of the proteins and associated cofactors must come together in the appropriate redox environment for efficient electron transfer and water oxidation. The PSII assembly process involves an ordered accumulation of subunits and cofactors within the membrane. Studies have shown that the D2, cytochrome b559 and PsbI subunits form a receptor for the newly translated D1 protein, which is co-translationally integrated into the membrane. This is followed by association of the CP47 protein. The next defined steps involve the association of the low molecular weight proteins PsbH, PsbM and PsbTC. Next, the CP43 protein along with PsbK associates with these components. At the incorporation of the CP43 protein, all of the ligands for the Mn4Ca1Clx cluster are present within the complex and presumably integration of this catalytic center occurs. At this point the lumenal extrinsic proteins (PsbO, PsbU, PsbV and PsbQ in cyanobacteria; PsbO, PsbP and PsbQ in plants) can also associate with the complex. Finally, this assembly process culminates in the formation of a dimeric complex.
In addition to the stepwise accumulation of PSII subunits, there are a number of accessory factors that facilitate the biogenesis of functional PSII complexes. These factors only transiently associate with PSII assembly intermediates, but are not components of the final enzyme. One example is the D1 processing protease CtpA, which is critical for functional PSII assembly in cyanobacteria and plants. Another example is the Psb27 protein, a protein that interacts with the lumenal side of PSII during assembly. Other PSII assembly factors include PratA, Slr2013, HCF136, Lpa1, Lpa2, Lpa3, PAM68, and 2pac. The identification and functional characterization of additional assembly factors is an active area of photosynthesis research.
The life of a PSII enzyme is not exactly linear when it comes to assembly. PSII constantly undergoes a cycle of assembly and damage-repair, in which the complex must be disassembled to some degree and reassembled to its functional form. Some of the steps involved in generating the fully functional PSII are the same whether PSII is being assembled from scratch or whether it is being repaired from recycled components. Other steps are unique to the damage-repair cycle. From a research perspective, it can be quite difficult to tease apart the roles of assembly factors and timing of steps between these two processes because they may be shared or distinct.
Photosystem II Assembly from Cyanobacteria to Plants
Assembling and maintaining the PSII complex in chloroplasts and cyanobacteria
Recent advances in understanding the assembly and repair of photosystem II
Auxiliary proteins involved in the assembly and sustenance of photosystem II
The Psb27 protein facilitates manganese cluster assembly in photosystem II
Evidence that D1 processing is required for manganese binding and extrinsic protein assembly into photosystem II
The HCF136 protein is essential for assembly of the photosystem II reaction center in Arabidopsis thaliana
PratA, a periplasmic tetratricopeptide repeat protein involved in biogenesis of photosystem II in Synechocystis sp. PCC 6803
Cooperation of LPA3 and LPA2 is essential for photosystem II assembly in Arabidopsis
The Arabidopsis thylakoid protein PAM68 is required for efficient D1 biogenesis and photosystem II assembly
A conserved rubredoxin is necessary for photosystem II accumulation in diverse oxygenic photoautotrophs