Delocalised electron transport and chemiosmosis in Escherichia coli

The membrane organisation of electron transport and chemiosmosis remains a topic of intense debate, with current models in many bioenergetic membranes favouring the assembly of multiple chemiosmotic components into supercomplexes that could control the pathways of electron flow and utilisation of the proton-motive force. We set out to investigate the distribution and dynamics of OXPHOS components in the plasma membrane of Escherichia coli using a combination of fluorescent protein tagging and fluorescence microscopy with dynamic tracking and single-particle analysis. Complexes investigated included NDH-1, SDH, Cytochrome bd-I and the proton-translocating ATPase, which could all be tagged with a variety of fluorescent proteins, with minimal loss of function [1,2]. Fluorescence microscopy in vivo showed that all complexes tested are concentrated in mobile domains in the membrane, with dimensions of about 100–200 nm and containing 10 s to 100 s of the tagged complex. Simultaneous visualisation of pairs of tagged complexes showed that different complexes are concentrated in separate domains, with no significant co-localisation and therefore no supercomplexes [2]. Since the pairs of complexes tested include the two complexes involved in one of the major respiratory electron transport pathways, and a major source and sink for the proton-motive force, it follows that both electron transport and the proton motive force are largely delocalised over the entire membrane area in E. coli. Consistent with this model, we observed rapid long-range diffusion of a fluorescent quinone analogue. We suggest that long-range quinone diffusion serves to carry electrons between islands of distinct electron transport complexes in the membrane.