Asymmetric structure of the native Rhodobacter sphaeroides dimeric LH1–RC complex

Abstract

Rhodobacter sphaeroides is a model organism in bacterial photosynthesis, and its light-harvesting-reaction center (LH1–RC) complex contains both dimeric and monomeric forms. Here we present cryo-EM structures of the native LH1–RC dimer and an LH1–RC monomer lacking protein-U (ΔU). The native dimer reveals several asymmetric features including the arrangement of its two monomeric components, the structural integrity of protein-U, the overall organization of LH1, and rigidities of the proteins and pigments. PufX plays a critical role in connecting the two monomers in a dimer, with one PufX interacting at its N-terminus with another PufX and an LH1 β-polypeptide in the other monomer. One protein-U was only partially resolved in the dimeric structure, signaling different degrees of disorder in the two monomers. The ΔU LH1–RC monomer was half-moon-shaped and contained 11 α- and 10 β-polypeptides, indicating a critical role for protein-U in controlling the number of αβ-subunits required for dimer assembly and stabilization. These features are discussed in relation to membrane topology and an assembly model proposed for the native dimeric complex.

Advances in the Spectroscopic and Structural Characterization of Core Light-Harvesting Complexes from Purple Phototrophic Bacteria

“…Purple phototrophic bacteria utilize NIR light captured by LH complexes and transfer the energy to the RC BChl dimer (the “special pair”) against an energetically “uphill” gap (“uphill” because LH1 BChls absorb lower-energy wavelengths than do RC BChls); this event initiates photosynthetic charge separation in the special pair followed by subsequent redox events (Figure A). The RC special pair contains only two BChls (in contrast to the 28–34 BChls present in the LH1 complex) and exhibits its Q y band at 860–870 nm although these were obtained from isolated RC-only complexes free of their LH1 component. − Recent structural analyses − have detected close associations between RC and LH1 complexes, indicating that the special pair’s Q y band peaks reported for RC-only complexes may not be the same as those of the RC in the native LH1–RC. Unfortunately, however, the intensive LH1 Q y band prevents accurate determination of the special pair peak in an LH1–RC complex.…”

“…This red-shifts to 780 nm upon binding to an LH1 α- or β-polypeptide, to 820 nm by forming an αβ-subunit, and to 860–970 nm (depending on the species) by forming LH1 complexes containing 14–16 αβ-subunits arranged in a circle (Figure C). In the intact LH1–RC complex, intra-BChl a dimers and inter-BChl a dimers interact to generate exciton coupling over the LH1 BChl a ring, which is involved in the LH1 Q y transition energy. , Structural information on the LH1–RC complexes of several species of purple bacteria − indicates that the distances between the intra- and inter-BChl a dimers may also contribute to the red-shifting of the LH1 Q y bands. For example, in the Tch.…”

“…In addition, BChl a bound to α n -polypeptide and BChl a bound to β n +1 -polypeptide interact to form inter-BChl a dimers (Figure A, black-dashed line). Based on structural data, − the effects of the distances between BChls on the Q y absorption band were compared. Figure B exhibits plots of the distances for the intra- and inter-BChl dimers against the LH1 Q y peak of purple bacterial species whose LH1–RC structures are known (Table S1).…”

“…sphaeroides LH1–RC contains twice the content of carotenoid bound to each αβ-subunit than does that of Tch. tepidum LH1–RC) and are thus unable to function as quinone transport pathways. , To explore this topic, we detected light-induced Fourier transform infrared (FTIR) signals responsible for changes in Tch. tepidum Q B and its binding site upon photosynthetic quinone reduction and have identified Q B and Q B H 2 marker bands based on their 15 N- and 13 C-isotopic band shifts .…”

“…Recent X-ray crystallographic , and cryo-EM − structural analyses have revealed the architecture of several purple bacterial LH1–RCs. An RC is enclosed by an open or closed LH1 ring comprised of 14–17 αβ (γ)-subunits, each of which binds two (or rarely, three) BChls coordinated with conserved His residues in the αβ -polypeptides and one or two carotenoid molecules (Figure B).…”