The voltage-dependent proton pumping in bacteriorhodopsin is characterizedby optoelectric behavior

light-driven proton pump bacteriorhodopsin (bR) was functionally expressed in Xenopus laevis oocytes and in HEK-293 cells. The latter expression syste m allowed high time resolution of light-induced current signals. A detailed voltage clamp and patch clamp study was performed to investigate the Delta pH versus Delta psi dependence of the pump current. The following results were obtained. The current voltage behavior of bR is linear in the measurab le range between -160 mV and +60 mV. The pH dependence is less than expecte d from thermodynamic principles, i.e., one Delta pH unit produces a shift o f the apparent reversal potential of 34 mV (and not 58 mV). The M-2-BR deca y shows a significant voltage dependence with time constants changing from 20 ms at +60 mV to 80 ms at -160 mV. The linear I-V curve can be reconstruc ted by this behavior. However, the slope of the decay rate shows a weaker v oltage dependence than the stationary photocurrent, indicating that an addi tional process must be involved in the voltage dependence of the pump. A sl owly decaying M intermediate (decay time > 100 ms) could already be detecte d at zero voltage by electrical and spectroscopic means. In effect, bR show s optoelectric, behavior. The long-lived M can be transferred into the acti ve photocycle by depolarizing voltage pulses. This is experimentally demons trated by a distinct charge displacement. From the results we conclude that the transport cycle of bR branches via a long-lived M-1* in a voltage-depe ndent manner into a nontransporting cycle, where the proton release and upt ake occur on the extracellular side.