Abstract
The Rhizoclosmatium globosum genome encodes three rhodopsin-guanylyl cyclases (RGCs), which are predicted to facilitate visual orientation of the fungal zoospores. Here, we show that RGC1 and RGC2 function as light-activated cyclases only upon heterodimerization with RGC3 (NeoR). RGC1/2 utilize conventional green or blue-light-sensitive rhodopsins (λmax = 550 and 480 nm, respectively), with short-lived signaling states, responsible for light-activation of the enzyme. The bistable NeoR is photoswitchable between a near-infrared-sensitive (NIR, λmax = 690 nm) highly fluorescent state (QF = 0.2) and a UV-sensitive non-fluorescent state, thereby modulating the activity by NIR pre-illumination. No other rhodopsin has been reported so far to be functional as a heterooligomer, or as having such a long wavelength absorption or high fluorescence yield. Site-specific mutagenesis and hybrid quantum mechanics/molecular mechanics simulations support the idea that the unusual photochemical properties result from the rigidity of the retinal chromophore and a unique counterion triad composed of two glutamic and one aspartic acids. These findings substantially expand our understanding of the natural potential and limitations of spectral tuning in rhodopsin photoreceptors.
Highlights
The Rhizoclosmatium globosum genome encodes three rhodopsin-guanylyl cyclases (RGCs), which are predicted to facilitate visual orientation of the fungal zoospores
Phylogenetic analysis and multiple sequence alignment of the three R. globosum RGC rhodopsin modules with other microbial rhodopsins from distinct families revealed that RGC1/2 and RGC3(NeoR) from R. globosum separate as two distinct phylogenetic branches (Fig. 1a and Supplementary Fig. 1)
Heterodimerization has not been reported for any rhodopsin, it does occur with the rhodopsin-related odorant receptors (ORs) in Drosophila olfactory sensory neurons[8]
Summary
The Rhizoclosmatium globosum genome encodes three rhodopsin-guanylyl cyclases (RGCs), which are predicted to facilitate visual orientation of the fungal zoospores. Site-specific mutagenesis and hybrid quantum mechanics/molecular mechanics simulations support the idea that the unusual photochemical properties result from the rigidity of the retinal chromophore and a unique counterion triad composed of two glutamic and one aspartic acids. These findings substantially expand our understanding of the natural potential and limitations of spectral tuning in rhodopsin photoreceptors. We describe two RGCs (RGC1 and RGC2) of the fungus Rhizoclosmatium globosum[6], from the phylum Chytridiomycota (Fig. 1a and Supplementary Fig. 1), each containing a classic blue-green absorbing rhodopsin, which heterodimerize with another uncharacterized rhodopsin, RGC3 or neorhodopsin (NeoR), that we show has sensitivity in the near-infrared (NIR) spectrum
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