(781a) Understanding the Roles of Surfactants and Oligomeric Assembly on the Functionality of the Light-Activated Membrane Protein Proteorhodopsin

Membrane proteins fulfill diverse functional roles that include sensing, catalysis and transport that are attractive for technological applications, such as chemical or biological sensing, separations, and energy conversion. The functionalities of membrane proteins depend on interactions with molecular species in the proteinâ??s local environments, which typically include surfactants (e.g., lipids) and other proteins. An objective for optimally exploiting membrane proteins for technological applications is to understand how protein-surfactant and protein-protein interactions independently influence protein functionalities. However, this goal remains a major experimental challenge because adjustment of protein-surfactant interactions by surfactant quantity or composition also influences protein oligomerization, and thus protein-protein interactions. We hypothesize that membrane protein functionality is mediated primarily by protein-protein interactions because the diversity of amino acid residues at protein-protein interfaces support stronger and more specific interactions than surfactants, which interact with membrane proteins largely by relatively weak hydrophobic interactions. As a model membrane protein for this study we chose proteorhodopsin, which functions as a light-activated H⁺-ion pump and can be stabilized in surfactant solutions in monomeric or oligomeric forms, which can be isolated by using size-exclusion chromatography. Solution-state ¹H nuclear magnetic resonance measurements are used to identify and quantify the surfactant species that stabilize proteorhodopsin in solutions, while complementary blue native polyacrylamide gel electrophoresis establishes the distributions of monomeric and oligomeric proteorhodopsin. UV-visible absorption measurements provide information about the light-activated functionalities of proteorhodopsin monomers and oligomers in solutions with different surfactant compositions. The correlation of surfactant composition with functionalities of proteorhodopsin monomers and oligomers yields insight into the functional roles of surfactants and oligomeric assembly. These results provide information about conditions that support the optimal function of membrane proteins, which is broadly important for designing systems that exploit membrane protein functionalities.