A primitive energy pathway in fungus

"The most surprising thing for me is that the fungi have kept this primitive energy pathway even though they possess normal respiration," said coauthor Leonid Brown, of the University of Guelph in Canada.Eukaryotes are classically thought to create proton gradients for energy via respiration and chlorophyll. Over the last 30 years, scientists have found that archaea and eubacteria possess another form of photosynthesis, where bacteriorhodopsin-like membrane proteins drive light-driven translocation of protons. Homologous rhodopsins examined in eukaryotes until now apparently only had photosensory functions.In the current study, Brown and colleagues analyzed the fungus Leptosphaeria maculans opsin gene and found a high degree of homology with bacteriorhodopsin. After expressing the opsin in membranes of the yeast Pichia pastoris, time-resolved laser spectroscopy in the visible range revealed its photocycle turnover was very fast, at a few tens of milliseconds."As a rule, fast photocycle turnover is what you see for efficient proton transport, while slow photocycles are linked to photosensors," Brown told The Scientist.To characterize the opsin, Brown and colleagues reconstituted it into proteoliposomes. Time-resolved spectroscopic analysis revealed that when investigators immersed these proteoliposomes in deuterated water, the opsin photocycle slowed down compared to normal water, implying proton transfer steps which the higher mass of the deuterons would delay. Rapid-scan Fourier transform infrared difference spectroscopy on proteoliposomes confirmed the opsin photocycle was bacteriorhodopsin-like, with all major retinal and carboxylic acid vibrational bonds matching closely.To test whether light-driven proton pumping occurs, Brown and colleagues illuminated proteoliposomes and found acidification inside them and alkalinization of their surrounding medium, which fully reversed in the dark. Addition of the proton uncoupler CCCP abolished nearly all these light-induce