InterPro domain: IPR003359

Photosystem I (PSI) is a large protein complex embedded within the photosynthetic thylakoid membrane formed by a core complex, peripheral light-harvesting complexes (LHCIs) and cofactors. It consists of 15 core and 4 LHCI subunits, ~150 chlorophylls (a and b) molecules, 2 phylloquinones, and 3 Fe4S4-clusters [ 1 ]. The three dimensional structure of the PSI complex has been resolved at 2.5 A [ 2 ], 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 ) [ 3 ], Ycf3 [ 4 , 5 ], and Ycf4 [ 6 ]. 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. The BtpA protein appears to act at the level of PSI stabilisation [ 6 ]. It is an extrinsic membrane protein located on the cytoplasmic side of the thylakoid membrane [ 7 , 8 ]. 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 N-terminal domain [ 8 , 8 ]. Together with Ycf3, it forms a core PSI assembly apparatus as an auxiliary factor. The Ycf3 is a TPR-containing protein loosely associated with the thylakoid membrane and, with its interacting partner Y3IP1, facilitates the assembly of reaction centre subunits. Ycf4 facilitates the integration of peripheral PSI subunits and LHCIs into the PSI reaction centre subcomplex [ 8 ].

1. The photosystem I assembly apparatus consisting of Ycf3-Y3IP1 and Ycf4 modules. Nat Commun 9, 2439
2. Three-dimensional structure of cyanobacterial photosystem I at 2.5 A resolution. Nature 411, 909-17
3. Molecular identification of a novel protein that regulates biogenesis of photosystem I, a membrane protein complex. J. Biol. Chem. 272, 6382-7
4. The chloroplast ycf3 and ycf4 open reading frames of Chlamydomonas reinhardtii are required for the accumulation of the photosystem I complex. EMBO J. 16, 6095-104
5. Targeted inactivation of a tobacco intron-containing open reading frame reveals a novel chloroplast-encoded photosystem I-related gene. J. Cell Biol. 139, 95-102
6. The BtpA protein stabilizes the reaction center proteins of photosystem I in the cyanobacterium Synechocystis sp. PCC 6803 at low temperature. Plant Physiol. 123, 215-22
7. Subcellular localization of the BtpA protein in the cyanobacterium Synechocystis sp. PCC 6803. Eur. J. Biochem. 261, 311-6