Structure of Psb27 in solution: implications for transient binding to photosystem II during biogenesis and repair

Psb27 is a membrane-extrinsic subunit of photosystem II (PSII) where it is involved in the assembly and maintenance of this large membrane protein complex that catalyzes one of the key reactions in the biosphere, the light-induced oxidation of water. Here, we report for the first time the structure of Psb27 that was not observed in the previous crystal structures of PSII due to its transient binding mode. The Psb27 structure shows that the core of the protein is a right-handed four-helix bundle with an up-down-up-down topology. The electrostatic potential of the surface generated by the amphipathic helices shows a dipolar distribution which fits perfectly to the major PsbO binding site on the PSII complex. Moreover, the presented docking model could explain the function of Psb27, which prevents the binding of PsbO to facilitate the assembly of the Mn(4)Ca cluster.