Photosystem I (PSI) is one of two large reaction centres responsible for converting light photons into the chemical energy needed to sustain life. PSI is an integral membrane protein complex that uses light energy to mediate electron transfer from plastocyanin (Pc) to ferredoxin (Fd) found in photosynthetic organisms such as plants, algae and cyanobacteria.

This superfamily entry consists of a helical domain representative in subunit PsaF (subunit III or PSI-F) of PSI. PsaF is one of the transmembrane alpha-helical subunits and consists of two hydrophilic alpha-helices F-c and F-d at the N terminus of transmembrane helix F-f, facing the luminal side of PSI.

In eukaryotes, the PsaF subunit is important for electron transfer between Pc and the PSI complex. Within this domain, the eukaryotic PsaF contains several additional positively charged residues that stabilise the interaction between PSI and Pc. Despite the structural similarities between cyanobacterial and plant PsaF, the role of PsaF is less well defined in cyanobacteria. In the viral PSI, the N-terminus of PsaF is deleted and as a result the luminal face of PSI is expected to be relatively flat. PFAM:PF02507, INTERPRO:IPR003666,DOI:10.1038/nplants.2017.14,DOI:10.7554/eLife.01496PMID:24473073