This family of proteins is predominantly found in cyanobacteria and red algae, including Tsr0524 from Thermosynechococcus vestitus. Tsr0524 (also known as PetP) is a component of the cyanobacterial cytochrome b6f that may play an essential role in the regulation of photosynthetic electron transport. It shows an SH3 domain fold with five antiparallel beta-sheets [1].

1: The cyanobacterial cytochrome b6f subunit PetP adopts an SH3 fold in solution. Veit S, Nagadoi A, Rogner M, Rexroth S, Stoll R, Ikegami T; Biochim Biophys Acta. 2016;1857:705-714. PMID:27033306