InterPro: IPR003359 Photosystem I assembly protein Ycf4

Photosystem I (PSI) is a large protein complex embedded within the photosynthetic thylakoid membrane. It consists of 11 subunits, ~100 chlorophyll a molecules, 2 phylloquinones, and 3 Fe4S4-clusters. The three dimensional structure of the PSI complex has been resolved at 2.5 A [1], which allows the precise localisation of each cofactor. PSI together with photosystem II (PSII) catalyses the light-induced steps in oxygenic photosynthesis - a process found in cyanobacteria, eukaryotic algae (e.g. red algae, green algae) and higher plants.

To date, three thylakoid proteins involved in the stable accumulation of PSI have been identified: BtpA (IPR005137) [2], Ycf3 [3, 4], and Ycf4 [3]. Because translation of the psaA and psaB mRNAs encoding the two reaction centre polypeptides, of PSI and PSII respectively, is not affected in mutant strains lacking functional ycf3 and ycf4, the products of these two genes appear to act at a post-translational step of PSI biosynthesis. These gene products are therefore involved either in the stabilisation or in the assembly of the PSI complex. However, their exact roles remain unknown. The BtpA protein appears to act at the level of PSI stabilisation [5]. It is an extrinsic membrane protein located on the cytoplasmic side of the thylakoid membrane [6, 5]. Homologs of BtpA are found in the crenarchaeota and euryarchaeota, where their function remains unknown. The Ycf4 protein is firmly associated with the thylakoid membrane, presumably through a transmembrane domain [3]. Ycf4 co-fractionates with a protein complex larger than PSI upon sucrose density gradient centrifugation of solubilised thylakoids [3]. The Ycf3 protein is loosely associated with the thylakoid membrane and can be released from the membrane with sodium carbonate. This suggests that Ycf3 is not part of a stable complex and that it probably interacts transiently with its partners [7]. Ycf3 contains a number of tetratrico peptide repeats (TPR, IPR001440); TPR is a structural motif present in a wide range of proteins, which mediates protein-protein interactions.