The cyanobacterial cytochrome b6f subunit PetP adopts an SH3 fold in solution

•Solution structure of the cyanobacterial cyt. b6f subunit PetP has been determined.•PetP adopts an SH3 domain fold similar to subunits of PS1, NDH-1 and FTR.•Heteronuclear steady-state {1H}–15N NOE and RCI parameters have been determined.•The potential interaction surface for b6f binding is described.•A model of the roles of SH3 domains in oxygenic photosynthesis is presented.

PetP is a peripheral subunit of the cytochrome b6f complex (b6f) present in both, cyanobacteria and red algae. It is bound to the cytoplasmic surface of this membrane protein complex where it greatly affects the efficiency of the linear photosynthetic electron flow although it is not directly involved in the electron transfer reactions.Despite the crystal structures of the b6f core complex, structural information for the transient regulatory b6f subunits is still missing. Here we present the first structure of PetP at atomic resolution as determined by solution NMR. The protein adopts an SH3 fold, which is a common protein motif in eukaryotes but comparatively rare in prokaryotes. The structure of PetP enabled the identification of the potential interaction site for b6f binding by conservation mapping. The interaction surface is mainly formed by two large loop regions and one short 310 helix which also exhibit an increased flexibility as indicated by heteronuclear steady-state {1H}–15N NOE and random coil index parameters. The properties of this potential b6f binding site greatly differ from the canonical peptide binding site which is highly conserved in eukaryotic SH3 domains. Interestingly, three other proteins of the photosynthetic electron transport chain share this SH3 fold with PetP: NdhS of the photosynthetic NADH dehydrogenase-like complex (NDH-1), PsaE of the photosystem 1 and subunit α of the ferredoxin–thioredoxin reductase have, similar to PetP, a great impact on the photosynthetic electron transport. Finally, a model is presented to illustrate how SH3 domains modulate the photosynthetic electron transport processes in cyanobacteria.