Fig. 4. Protein-protein and protein-pigment interactions within the two rings of light-harvesting antenna complexes.

(A) Key interactions involved in stabilizing a single α/β subunit of the 16-member LH1 ring. Histidine residues that are involved in coordination with central Mg atom of the BChls are indicated by red arrows. (B) Single α/β subunit of the 24-member LHh ring. (C) Interaction between BChl a molecules and protein in LH1 subunit. The density map is shown in blue. (D) Interaction between BChl a molecules and protein in the LHh subunit. (E) Three consecutive subunits of the LH1 ring. Interfaces between subunits are primarily mediated by the bound pigments, with few specific protein-protein interactions. (F) Three consecutive subunits of the outer LHh ring, showing H-bonds between the C-terminal domains in three LHh α/β units. GTX, gemmatoxanthin. Colors as in Fig. 1 with the exception of B816 and B868 BChls, which are colored by chain, and B800 is colored pink.