Photochemical characterization of a novel fungal rhodopsin from Phaeosphaeria nodorum

Abstract

Eukaryotic microbial rhodopsins are widespread bacteriorhodopsin-like proteins found in many lower eukaryotic groups including fungi. Many fungi contain multiple rhodopsins, some significantly diverged from the original bacteriorhodopsin template. Although few fungal rhodopsins have been studied biophysically, both fast-cycling light-driven proton pumps and slow-cycling photosensors have been found. The purpose of this study was to characterize photochemically a new subgroup of fungal rhodopsins, the so-called auxiliary group. The study used the two known rhodopsin genes from the fungal wheat pathogen, Phaeosphaeria nodorum. One of the genes is a member of the auxiliary group while the other is highly similar to previously characterized proton-pumping Leptosphaeria rhodopsin. Auxiliary rhodopsin genes from a range of species form a distinct group with a unique primary structure and are located in carotenoid biosynthesis gene cluster. Amino acid conservation pattern suggests that auxiliary rhodopsins retain the transmembrane core of bacteriorhodopsins, including all residues important for proton transport, but have unique polar intramembrane residues. Spectroscopic characterization of the two yeast-expressed Phaeosphaeria rhodopsins showed many similarities: absorption spectra, conformation of the retinal chromophore, fast photocycling, and carboxylic acid protonation changes. It is likely that both Phaeosphaeria rhodopsins are proton-pumping, at least in vitro. We suggest that auxiliary rhodopsins have separated from their ancestors fairly recently and have acquired the ability to interact with as yet unidentified transducers, performing a photosensory function without changing their spectral properties and basic photochemistry.

Polyextremotolerant black fungi: oligotrophism, adaptive potential, and a link to lichen symbioses

“…Bacteriorhodopsin has been first discovered four decades ago in the cell membrane of the archaeon Halobacterium sp., where it functions as a light driven proton pump (Grote and O'Malley, 2011). Although numerous homologues of bacteriorhodopsins have been described in fungal genomes since then, little information exists on their (probably versatile) functions (Fan et al, 2011). Some of them may act as photosensors, however, in the case of the black fungus Leptosphaeria maculans a bacteriorhodopsin-like proton pump was shown to use the energy of photons to pump protons out of the cell and therefore transform radiation energy into the chemical form—the transmembrane proton gradient (Waschuk et al, 2005).…”

The CarO rhodopsin of the fungus Fusarium fujikuroi is a light-driven proton pump that retards spore germination

“…In chytridiomycetes rhodopsin is involved in the phototaxis of swimming zoospores 17 and recently a unique gene fusion was described in this fungal group, which combines the action of rhodopsin and guanylyl cyclase in a single protein 18 . Some fungal rhodopsins have been purified, and photochemically analysed, but their physiological roles are unknown 19 20 21 . Moreover, the analyses of their functions through targeted deletion of the encoding genes provided no clear clues so far 3 22 .…”

“…In contrast, CarO is among the auxiliary ORP-like rhodopsins, which together with LR-like rhodopsins exhibit fast photocycles and possess all conserved amino acid residues required for proton pumping. LR exhibits proton-pumping function, but the auxiliary ORP-like rhodopsins were not investigated in this respect 4 19 .…”

Marine Bacterial and Archaeal Ion-Pumping Rhodopsins: Genetic Diversity, Physiology, and Ecology

“…Thus, epileptic seizures have been prevented in mice via channelrhodopsin-enabled, light-directed mediation of nerve impulses. Many more channelrhodopsins have been found in algae by screening transcriptomes (25), while both phototaxis receptors and proton pumps have been found in fungi (26,27).…”