Prokaryotic proton pumping in eukaryotes

Stephen Waschuk et al. report that the fungus Leptosphaeria maculans possesses a rhodopsin protein able to translocate protons across membranes in response to light, a function thought to be unique to prokaryotes. Traditionally, respiration and chlorophyll-based photosynthesis are the two methods used by eukaryotes to create a proton gradient, whereas archaebacteria undergo retinal-based photosynthesis dependent on the membrane protein bacteriorhodopsin. Although rhodopsin proteins have been found in fungi and algae, they appear to function only in a photosensing manner. According to the authors, however, L. maculans appears to possess a rhodopsin with high sequence homology to bacteriorhodopsin’s membranespanning regions. Time-resolved laser spectroscopy of the fungal opsin expressed in Pichia revealed rapid kinetics relative to the slow photocycle of previously characterized fungal opsins. To test the ability of Leptosphaeria rhodopsin to function as a protein pump, the researchers embedded the protein in liposomes. In the presence of light, the authors noted a change of pH between the inside and outside of the liposome. Some lower eukaryotes may thus share the prokaryotic ability to use lightsensitive retinal proteins as electrical gradient generators.