Proteorhodopsin (PR), the first identified true "bacterio"-rhodopsin is a light-driven proton pump from marine bacteria with photocycle similar to bacteriorhodopsin (BR) at neutral pH, including the occurrence of an M-like intermediate. At acidic pH no M is detected spectroscopically or electrically and proton pumping inverts at acidic pH. Using two-electrode voltage-clamping after expression in Xenopus oocytes we determined the voltage dependence of stationary photocurrents (I-V curves) at different intra- and extracellular pH. Furthermore, we studied the effects of mutations at the putative proton acceptor and donor groups, D97 and E108, respectively. Mutants D97N and -T mediate only inward transient and stationary currents. Mutant E108G behaves similarly to BR mutant D96G, with stationary proton pumping restored upon addition of azide.

Laser flashes applied during or after continuous illumination with green light reveal different behaviours in PR and BR under identical conditions. In contrast to BR, blue laser flashes on PR evoked much larger outwardly-directed transient currents upon blue compared to green flashes and transient inward proton translocation was observed with blue and green flashes at acidic extracellular pH. The results confirm earlier conclusions drawn by our laboratory that the direction of proton transport by PR can be switched by changes in pH or the membrane potential.