title: Functional consequences of the oligomeric assembly of proteorhodopsin.
abstract: The plasma membrane is the crucial interface between the cell and its exterior, packed with embedded proteins experiencing simultaneous protein-protein and protein-membrane interactions. A prominent example of cell membrane complexity is the assembly of transmembrane proteins into oligomeric structures, with potential functional consequences that are not well understood. From the study of proteorhodopsin (PR), a prototypical seven-transmembrane light-driven bacterial proton pump, we find evidence that the inter-protein interaction modulated by self-association yields functional changes observable from the protein interior. We also demonstrate that the oligomer is likely a physiologically relevant form of PR, as crosslinking of recombinantly expressed PR reveals an oligomeric population within the Escherichia coli membrane (putatively hexameric). Upon chromatographic isolation of oligomeric and monomeric PR in surfactant micelles, the oligomer exhibits distinctly different optical absorption properties from monomeric PR, as reflected in a prominent decrease in the pKa of the primary proton acceptor residue (D97) and slowing of the light-driven conformational change. These functional effects are predominantly determined by specific PR-PR contacts over nonspecific surfactant interactions. Interestingly, varying the surfactant type alters the population of oligomeric states and the proximity of proteins within an oligomer, as determined by sparse electron paramagnetic resonance distance measurements. Nevertheless, the dynamic surfactant environment retains the key function-tuning property exerted by oligomeric contacts. A potentially general design principle for transmembrane protein function emerges from this work, one that hinges on specific oligomeric contacts that can be modulated by protein expression or membrane composition.
authors.affiliation: Department of Chemical Engineering, University of California, Santa Barbara, CA 93106-5080, USA.
authors.forename: Sunyia
authors.initials: S
authors.lastname: Hussain
authors.affiliation: College of Creative Studies, Biology Department, University of California, Santa Barbara, CA 93106-6110, USA.
authors.forename: Maia
authors.initials: M
authors.lastname: Kinnebrew
authors.affiliation: Department of Chemical Engineering, University of California, Santa Barbara, CA 93106-5080, USA.
authors.forename: Nicole S
authors.initials: NS
authors.lastname: Schonenbach
authors.affiliation: Department of Chemistry and Biochemistry, University of California, Santa Barbara, CA 93106-9510, USA.
authors.forename: Emily
authors.initials: E
authors.lastname: Aye
authors.affiliation: Department of Chemical Engineering, University of California, Santa Barbara, CA 93106-5080, USA; Department of Chemistry and Biochemistry, University of California, Santa Barbara, CA 93106-9510, USA. Electronic address: songi@chem.ucsb.edu.
authors.forename: Songi
authors.initials: S
authors.lastname: Han
mesh_terms: D002462:Cell Membrane / Q000378:metabolism; D003432:Cross-Linking Reagents / Q000494:pharmacology; D004578:Electron Spin Resonance Spectroscopy; D004926:Escherichia coli / Q000235:genetics / Q000378:metabolism; D006863:Hydrogen-Ion Concentration; D008958:Models, Molecular; D050505:Mutant Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D009154:Mutation / Q000235:genetics; D055503:Protein Multimerization; D012243:Rhodopsin / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism*; D025604:Rhodopsins, Microbial; D013113:Spin Labels; D013501:Surface-Active Agents / Q000378:metabolism; D013696:Temperature
chemical_list: D003432:Cross-Linking Reagents; D050505:Mutant Proteins; D025604:Rhodopsins, Microbial; D013113:Spin Labels; D013501:Surface-Active Agents; C413548:proteorhodopsin; D012243:Rhodopsin
keywords: electron paramagnetic resonance; oligomers; protein–detergent complex; seven-transmembrane proteins