This family consists of PsaD from plants and cyanobacteria. PsaD is an extrinsic polypeptide of photosystem I (PSI) and is required for native assembly of PSI reaction clusters and is implicated in the electrostatic binding of ferredoxin within the reaction centre [1]. PsaD forms a dimer in solution which is bound by PsaE however PsaD is monomeric in its native complexed PSI environment [1].

1: Structure and properties in solution of PsaD, an extrinsic polypeptide of photosystem I. Xia Z, Broadhurst RW, Laue ED, Bryant DA, Golbeck JH, Bendall DS; Eur J Biochem 1998;255:309-316. PMID:9692933