Cross-protomer interaction with the photoactive site in oligomeric proteorhodopsin complexes

The first crystal structure of a proteorhodopsin shows a hexametric ring of protein in which the conserved photoactive-site histidine forms a hydrogen bond to the Schiff base proton acceptor from the same molecule and also to a tryptophan residue of a neighboring protomer. The structure and mutant studies reveal novel aspects of the ion translocation mechanism and cooperative behavior between protomers involving the inter-molecular His-Trp pair.