Asymmetric Structure of the Native Rhodobacter sphaeroides Dimeric LH1-RC Complex

The light-harvesting-reaction center (LH1-RC) core complex of purple photosynthetic bacterium Rhodobacter (Rba.) sphaeroides is characterized by the presence of both a dimeric form and a monomeric form. Following structure determination of the monomeric LH1-RC including its previously unrecognized component designated protein-U (Nat. Commun. 12, 6300, 2021), here we present cryo-EM structures of the dimeric LH1-RC from native Rba. sphaeroides IL106 at 2.75 [A] resolution and from an LH1-RC monomer lacking protein-U ({Delta}U) at 2.64 [A] resolution. The native dimeric core complex reveals many asymmetric features in the arrangement of its two monomeric components including the structural integrity of protein-U, the overall LH1 organization, and the rigidities of the proteins and pigments that form the complex. PufX polypeptides play a critical role in connecting two monomers, with one PufX interacting at its N-terminus with another PufX and an LH1 {beta}-polypeptide in another monomer, in good agreement with biochemical analyses. One of the proteins-U was only partially identified in the dimeric structure, signaling significantly different degrees of disorder in the two monomers. The {Delta}U LH1-RC monomer revealed a half-moon-shaped structure containing 11 - and 10 {beta}-polypeptides (compared with 14 of each in the wild type), indicating a critical role for protein-U in controlling the number of {beta}-subunits required for correct assembly and stabilization of the LH1-RC dimer. The structural features are discussed in relation to the unusual topology of intracytoplasmic photosynthetic membranes and an assembly model proposed for the native Rba. sphaeroides dimeric LH1-RC complex in membranes of wild-type cells.