Lipid membrane mimetics and oligomerization tune functional properties of proteorhodopsin

The functional properties of proteorhodopsin (PR) have been found to be strongly modulated by oligomeric distributions and lipid membrane mimetics. This study aims to distinguish and explain their effects by investigating how oligomer formation impacts PR's function of proton transport in lipid-based membrane mimetic environments. We find that PR forms stable hexamers and pentamers in both E. coli membranes and synthetic liposomes. Compared with the monomers, the photocycle kinetics of PR oligomers is ∼2 and ∼4.5 times slower for transitions between the K and M and the M and N photointermediates, respectively, indicating that oligomerization significantly slows PR's rate of proton transport in liposomes. In contrast, the apparent pKa of the key proton acceptor residue D97 (pKa_D97) of liposome-embedded PR persists at 6.2-6.6, regardless of cross-protomer modulation of D97, suggesting that the liposome environment helps maintain PR's functional activity at neutral pH. By comparison, when extracted directly from E. coli membranes into styrene-maleic acid lipid particles, the pKa_D97 of monomer-enriched E50Q PR drastically increases to 8.9, implying that there is a very low active PR population at neutral pH to engage in PR's photocycle. These findings demonstrate that oligomerization impacts PR's photocycle kinetics, while lipid-based membrane mimetics strongly affect PR's active population via different mechanisms.