Proteorhodopsin Function Is Primarily Mediated by Oligomerization in Different Micellar Surfactant Solutions

The diverse functionalities of membrane proteins (MPs) have garnered much interest in leveraging these biomolecules for technological applications. One challenge of studying MPs in artificial micellar surfactant environments is that many factors modulate their structures and functionalities, including the surfactants that interact with the MP or their assembly into oligomers. As oligomerization offers a means by which MPs could selectively interact among the copious environmental factors in biological environments, we hypothesized that MP function is predominantly modified by oligomerization rather than interactions with local surfactants that, by comparison, largely interact with MPs nonspecifically. To test this, we study the light-activated proton pump proteorhodopsin (PR) in micellar surfactant solutions because it is functionally active in monomeric and oligomeric forms, the light-activated functionalities of which can be assessed in detail. The surfactant composition and oligomerization are correlated with PR function, as measured by the protonation behaviors of aspartic acid residue 97, which mediates light-activated proton transport, and the associated photocycle kinetics. The results demonstrate that oligomerization dominantly mediates PR function in different surfactant environments, whereas some surfactants can subtly modulate proton-pumping kinetics. This work underscores the importance of understanding and controlling oligomerization of MPs to study and exploit their function.