Cryo-EM structure of the Blastochloris viridis LH1-RC complex at 2.9 angstrom

The light-harvesting 1-reaction centre (LH1-RC) complex is a key functional component of bacterial photosynthesis. Here we present a 2.9 angstrom resolution cryo-electron microscopy structure of the bacteriochlorophyll b-based LH1-RC complex from Blastochloris viridis that reveals the structural basis for absorption of infrared light and the molecular mechanism of quinone migration across the LH1 complex. The triple-ring LH1 complex comprises a circular array of 17 beta-polypeptides sandwiched between 17 alpha-and 16 gamma-polypeptides. Tight packing of the gamma-apoproteins between beta-polypeptides collectively interlocks and stabilizes the LH1 structure; this, together with the short Mg-Mg distances of bacteriochlorophyll b pairs, contributes to the large redshift of bacteriochlorophyll b absorption. The 'missing' 17th gamma-polypeptide creates a pore in the LH1 ring, and an adjacent binding pocket provides a folding template for a quinone, Q(P), which adopts a compact, export-ready conformation before passage through the pore and eventual diffusion to the cytochrome bc(1) complex.