3.E.2.1.3
The Reaction Center (RC)-Light Harvesting (LH) complex of Roseiflexus castenholzii, PufA-PufB-PufC-PufM, of 42 aas and 1 TMS, of 55 aas and 1 TMS, of 320 aas and 1 N-terminal TMS, and of 641 aas with 11 or 12 TMSs, respectively.  Carotenoid (Car) pigments perform central roles in photosynthesis-related light harvesting (LH), photoprotection, and assembly of functional pigment-protein complexes. Xin et al. 2023 analyzed native RC-LH (nRC-LH) and Car-depleted RC-LH (dRC-LH) complexes in Roseiflexus castenholzii, a chlorosome-less filamentous anoxygenic phototroph that forms the deepest branch of photosynthetic bacteria. Newly identified exterior Cars functioned with the bacteriochlorophyll B800 to block the proposed quinone channel between LHα and LHβ subunits in the nRC-LH, forming a sealed LH ring that was disrupted by transmembrane helices from cytochrome c and subunit X to allow quinone shuttling. dRC-LH lacked subunit X, leading to an exposed LH ring with a larger opening, which together accelerated the quinone exchange rate. The authors also assigned amino acid sequences to subunit X and two hypothetical proteins (Y and Z) that functioned in forming the quinone channel and stabilizing the RC-LH interactions. The structural basis by which Cars assembly regulates the architecture and quinone exchange of bacterial RC-LH complexes (Xin et al. 2023).