Sulfoquinovosyl diacylglycerol is required for dimerization of the Rhodobacter sphaeroides RC-LH1 core complex

The reaction centre-light harvesting 1 (RC-LH1) core complex is indispensable for anoxygenic photosynthesis. In the purple bacterium Rhodobacter (Rba.) sphaeroides RC-LH1 is produced both as a monomer in which 14 LH1 subunits form a crescent-shaped antenna around one RC, and as a dimer, where 28 LH1 subunits form an S-shaped antenna surrounding two RCs. The PufX polypeptide augments the five RC and LH subunits, and in addition to providing an interface for dimerization, PufX also prevents LH1 ring closure, introducing a channel for quinone exchange that is essential for photoheterotrophic growth. Structures of Rba. sphaeroides RC-LH1 complexes revealed several new components; protein-Y, which helps to form a quinone channel; protein-Z, of unknown function but which is unique to dimers; and a tightly bound sulfoquinovosyl diacylglycerol (SQDG) lipid that interacts with two PufX arginines. This lipid lies at the dimer interface alongside weak density for a second molecule, previously proposed to be an ornithine lipid. In this work we have generated strains of Rba. sphaeroides lacking protein-Y, protein-Z, SQDG or ornithine lipids to assess the roles of these previously unknown components in the assembly and activity of RC-LH1. We show that whilst the removal of either protein-Y, protein-Z or ornithine lipids has only subtle effects, SQDG is essential for the formation of RC-LH1 dimers but its absence has no functional effect on the monomeric complex.