Architecture of a Nanotube Assembly of Light-Harvesting LH2 Complexes from Rhodobacter sphaeroides

Pu Qiana, Kenneth Goldieb, Nikhil Biyanib, Miroslav Papizc, Abhishek Singha Royd, Per A. Bullougha, Klaus Schultend, Henning Stahlbergb, C. Neil Huntera
aThe University of Sheffield, UK; bBasel University, Switzerland; cLiverpool University, UK, dUniversity of Illinois, USA

Light-harvesting complex 2 (LH2) is the major component in the intracytoplasmic membrane (ICM) of the purple bacterium Rhodobacter sphaeroides. Arrays of LH2 complexes acts as antenna to absorb and transfer light energy to the reaction center, and this array also plays a role in curving the ICM to form vesicular structures. In order to understand how the light energy is absorbed and transferred in a densely-packed LH2 cluster and how the LH2 molecules induce membrane curvature we dialysed detergent-solubilised, purified LH2 complexes from Rba. sphaeroides. During dialysis arrays of LH2 assembled to form tubular LH2 crystals typically a few microns in length, several of which were selected for structural determination using cryo-electron microscopy.

The projection map shows the crystal lattice a = 126 Å, b =146 Å, γ = 90 (Fig. 1, left), comprising linear, helical assemblies of LH2 complexes, with each zigzag line having the same orientation, and the LH2s in neighboring lines adopting the reverse orientation. The 3D electron density map shows that a 1.8 nm height difference between LH2 molecules in adjacent zigzag lines. Measurements of negatively stained, flattened LH2 nanotubes showed they have a diameter of 155 nm and a helical pitch of 281 nm, enabling the calculation of the 3D structure of the LH2 complex and the 3D organization of a complete LH2 nanotube at ~7 Å (Fig. 1, right) that defines the positions of each of the 3060 LH2 complexes. Analysis of energy migration in such structures, representing 82,620 bacteriochlorophylls, by lifetime imaging microscopy will provide new insights into exciton migration in antenna arrays.

Fig. 1. (left) 3.5 Å projection map from cryo-EM of a LH2 nanotube. (right) 3D Atomic level model of an LH2 nanotube.