Quinone Pathways in Entire Photosynthetic Chromatophores of Rhodospirillum photometricum

In photosynthetic organisms, membrane pigment–protein complexes [light-harvesting complex 1 (LH1) and light-harvesting complex 2 (LH2)] harvest solar energy and convert sunlight into an electrical and redox potential gradient (reaction center) with high efficiency. Recent atomic force microscopy studies have described their organization in native membranes. However, the cytochrome (cyt) bc1 complex remains unseen, and the important question of how reduction energy can efficiently pass from core complexes (reaction center and LH1) to distant cyt bc1 via membrane-soluble quinones needs to be addressed. Here, we report atomic force microscopy images of entire chromatophores of Rhodospirillum photometricum. We found that core complexes influence their molecular environment within a critical radius of ∼ 250 Å. Due to the size mismatch with LH2, lipid membrane spaces favorable for quinone diffusion are found within this critical radius around cores. We show that core complexes form a network throughout entire chromatophores, providing potential quinone diffusion pathways that will considerably speed the redox energy transfer to distant cyt bc1. These long-range quinone pathway networks result from cooperative short-range interactions of cores with their immediate environment.