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The photosynthetic oxidation of water is catalyzed by photosystem II (PSII), a multisubunit pigment-protein embedded in the thylakoid membranes of cyanobacteria, algae, and plants. This remarkable photoenzyme is capable of generating the highly oxidizing chemical species required to extract four tightly bound electrons of two substrate water-molecules, yielding biosynthetically useful reductant and by-product O2. PSII is a large homodimeric complex in vivo, with a combined mass of ∼700 kDa. Each PSII monomer comprises more than 20 different proteins collectively coordinating ∼60 cofactors, including 35 chlorophylls, 2 pheophytins, 2 plastoquinone molecules, and the Mn4Ca cluster, responsible for H2O-oxidation (1, 2). Despite much progress, including the 1.9 Å crystal structure of cyanobacterial PSII (1, 3), crucial structural information is missing. This lack includes extrinsic proteins affecting the Mn4Ca cluster, not observed in the PSII crystal structure because they are lost during the purification and crystallization process. In PNAS, Liu et al. use complementary technical approaches to construct a model for the binding of one of the lost extrinsic proteins, PsbQ, to the PSII, complex, and in doing so provide an example for the solution of the more general structural problem of defining the 3D structure of separately crystallized proteins that assemble into larger macromolecular complexes (4). Besides its prodigious catalytic capacities, the PSII complex provides a study in contrasts: its core exhibits an extremely high level of evolutionary conservation, considering the ∼2.7 billion y of evolutionary divergence, whereas the peripheral components display a dizzying degree of variation (Fig. 1). Although much of the peripheral variation is associated with the different light-harvesting complexes conferring adaptation to different light environments, there is also a …

Mass spectroscopy locates the extrinsic proteins of photosystem II