This family consists of the PsaX family of photosystem I (PSI) protein subunits. PSI is a large multi-subunit pigment protein complex embedded in the thylakoid membranes of green plants and cyanobacteria. PsaX is one of the 12 protein subunits found in PSI and these subunits are arranged as monomers or trimers within the membrane as shown by the structure of the trimeric complex from Synechococcus elongatus [1].

1: Photosystem I trimers from Synechocystis PCC 6803 lacking the PsaF and PsaJ subunits bind an IsiA ring of 17 units. Kouril R, Yeremenko N, D'Haene S, Yakushevska AE, Keegstra W, Matthijs HC, Dekker JP, Boekema EJ; Biochim Biophys Acta 2003;1607:1-4. PMID:14556907