The solution structure of Rhodobacter sphaeroides LH1β reveals two helical domains separated by a more flexible region: structural consequences for the LH1 complex

Here, the solution structure of the Rhodobacter sphaeroides core light-harvesting complex β polypeptide solubilised in chloroform:methanol is presented. The structure, determined by homonuclear NMR spectroscopy and distance geometry, comprises two alpha helical regions (residue −34 to −15 and −11 to +6, using the numbering system in which the conserved histidine residue is numbered zero) joined by a more flexible four amino acid residue linker. The C-terminal helix forms the membrane spanning region in the intact LH1 complex, whilst the N-terminal helix must lie in the lipid head groups or in the cytoplasm, and form the basis of interaction with the α polypeptide. The structure of a mutant β polypeptide W +9F was also determined. This mutant, which is deficient in a hydrogen bond donor to the bacteriochlorophyll, showed an identical structure to the wild-type, implying that observed differences in interaction with other LH1 polypeptides must arise from cofactor binding. Using these structures we propose a modification to existing models of the intact LH1 complex by replacing the continuous helix of the β polypeptide with two helices, one of which lies at an acute angle to the membrane plane. We suggest that a key difference between LH1 and LH2 is that the β subunit is more bent in LH1. This modification puts the N terminus of LH1β close to the reaction centre H subunit, and provides a rationale for the different ring sizes of LH1 and LH2 complexes.