Abstract
Assembly of Photosystem (PS) II in plants has turned out to be a highly complex process which, at least in part, occurs in a sequential order and requires many more auxiliary proteins than subunits present in the complex. Owing to the high evolutionary conservation of the subunit composition and the three-dimensional structure of the PSII complex, most plant factors involved in the biogenesis of PSII originated from cyanobacteria and only rarely evolved de novo. Furthermore, in chloroplasts the initial assembly steps occur in the non-appressed stroma lamellae, whereas the final assembly including the attachment of the major LHCII antenna proteins takes place in the grana regions. The stroma lamellae are also the place where part of PSII repair occurs, which very likely also involves assembly factors. In cyanobacteria initial PSII assembly also occurs in the thylakoid membrane, in so-called thylakoid centers, which are in contact with the plasma membrane. Here, we provide an update on the structures, localisations, topologies, functions, expression and interactions of the low molecular mass PSII subunits PsbY, PsbW and the auxiliary factors HCF136, PsbN, TerC and ALB3, assisting in PSII complex assembly and protein insertion into the thylakoid membrane.
Highlights
Photosynthesis converts sunlight energy into chemical energy and takes place in chloroplasts and cyanobacteria
For optimal flexibility the photosystem II (PSII) complex requires a set of auxiliary proteins which assist in quenching of excited states, electron flow, assembly, repair and stability
PsbN and HCF136 are required for heterodimerization of PSII reaction center in the stroma lamellae, whereas TerC and ALB3 act on translation and/or incorporation of proteins into PSII and other thylakoid membrane complexes
Summary
Photosynthesis converts sunlight energy into chemical energy and takes place in chloroplasts and cyanobacteria. This process is fundamental for life on our planet and starts with the excitation of electrons in the multi-subunit and pigment-containing photosystem (PS) II complex. PSII has a molecular mass of more than 600 kDa and comprises more than 30 protein subunits, pigments and cofactors, such as Ca2+, Cl− and different oxidation states of Fe and Mn. Our current knowledge about the function, structure and components of PSII is quite comprehensive, yet still little is known about how the complex assembles or disassembles for repair and even less regarding how its function is fine-tuned for optimal performance under fast and ever changing light and temperature conditions as well as varying water availability. For optimal flexibility the PSII complex requires a set of auxiliary proteins which assist in quenching of excited states, electron flow, assembly, repair and stability.
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