Is FMO-protein related to PscA in the reaction center of green sulfur bacteria?

Abstract

FMO protein is a water-soluble protein found only in green sulfur bacteria. Each subunit (366 amino acids) contains 7 bacteriochlorophyll a molecules wrapped in a string bag of protein consisting of 15 strands of beta-sheet, short lengths of alpha-helix, and a few regions of irregular conformation. PscA is a membrane-bound protein (705 amino acids) with 11 transmembrane helices, 8 bacteriochlorophyll a molecules and 2 chlorophyll a molecules esterified with delta 2,6 phytandienol. FMO protein sequence was compared to PscA, PshA (RC protein of heliobacteria), CP43 and CP47 (core proteins of PS II). Identity scores for the four proteins (570, 350, 195, and 140 respectively) show that FMO protein is most closely related to PscA. When the FMO sequence is mapped onto the PscA sequence, 4 out of 7 bacteriochlorophyll a binding sites in FMO match corresponding residues in PscA. Three long segments of PscA show homology to the FMO sequence: 1) Helix IV (54%)+loop 4, 2) Loop 5+helix VI+loop 6, and 3) Helix VIII (84%)+loop 8 (87%)+helix IX (65%)+loop 9+helix X (27%)+loop 10. There are 47 identities in the homologous sequences (similarity = 0.13). Regions of PscA that show no homology to FMO protein are N-terminal+helix I+loop1+helix II+loop 2+helix III+loop 3+helix IV(46%); most of helix V, parts of helix VII and loop 7, most of helix X, and most of helix XI+C-terminal. I propose that FMO protein shares a common ancestor with PscA, and that the common ancestor was a reaction center protein.