Role of the Lipid Membrane on the Oligomeric Assembly and Function of Proteorhodopsin

Abstract

Transmembrane proteins are biomolecules in the lipid bilayers of cells that accomplish critical tasks such as sensing, transport, and enzymatic catalysis. Previous studies indicated that transmembrane proteins could tune their function by forming oligomeric complexes. However, it remains unclear whether oligomerization modulates function significantly more than other factors in native-like lipid bilayer environments. In this study, we aim to elucidate how protein oligomerization and other environmental factors tune the function of transmembrane proteins in lipid membrane systems. Proteorhodopsin (PR), a model transmembrane protein, was used to examine this functional modulation because of its assessable light-responsive proton transferring functions. Our results showed that oligomerization remained an important factor in tuning PR's functions in a liposome environment. Analyses of PR photocycle kinetics measured by a time-resolved optical absorbance technique indicated that faster photocycle kinetics can be observed with the monomeric PR E50Q mutant than with the pentameric/hexameric wild type PR in a liposome environment. These results concur with what has been observed for surfactant-solubilized PR. Interestingly, the pKa of the PR D97 residue that determines the pH at which proton transport switches direction was insensitive to oligomeric distribution in the lipid bilayer environment, but was sensitive to changes in the hydration gradient across lipid bilayers. Therefore, our work suggested that besides oligomerization, the lipid hydration gradient could play a role in modulating PR's function. The new understanding developed here is expected to yield guidelines for functional studies of other therapeutically relevant transmembrane proteins.