Edited by Richard Cogdell Keywords:Microbial rhodopsin Histidine-Aspartate cluster Quaternary structure Protonation Size-exclusion chromatography Circular dichroism spectroscopy a b s t r a c t Gloeobacter rhodopsin (GR) is a eubacterial proton pump having a highly conserved histidine near the retinal Schiff base counter-ion, aspartate. Various interactions between His and Asp of the eubacterial proton pump have been reported. Here, we showed the pH-dependent trimer/monomer transition of GR in the presence of dodecyl-b-D-maltoside by size-exclusion chromatography. The pH dependence was closely related to the protonation state of the counter-ion, Asp121. For the H87M mutant, pH dependence disappeared and a monomer became dominant. We concluded that the formation or breaking of the salt bridge between His87 and Asp121 inside the protein changes the quaternary structure. Structured summary of protein interactions:Rhodopsin and Rhodopsin bind by molecular sieving (View interaction) Rhodopsin and Rhodopsin bind by molecular sieving (View interaction: 1, 2)