Channelrhodopsins ofVolvox carteriAre Photochromic Proteins That Are Specifically Expressed in Somatic Cells under Control of Light, Temperature, and the Sex Inducer

Abstract

Channelrhodopsins are light-gated ion channels involved in the photoresponses of microalgae. Here, we describe the characterization of two channelrhodopsins, Volvox channelrhodopsin-1 (VChR1) and VChR2, from the multicellular green alga Volvox carteri. Both are encoded by nuclear single copy genes and are highly expressed in the small biflagellated somatic cells but not in the asexual reproductive cells (gonidia). Expression of both VChRs increases after cell cleavage and peaks after completion of embryogenesis, when the biosynthesis of the extracellular matrix begins. Likewise, expression of both transcripts increases after addition of the sex-inducer protein, but VChR2 is induced much more than VChR1. The expression of VChR1 is specifically promoted by extended dark periods, and heat stress reduces predominantly VChR1 expression. Expression of both VChRs increased under low light conditions, whereas cold stress and wounding reduced expression. Both VChRs were spectroscopically studied in their purified recombinant forms. VChR2 is similar to the ChR2 counterpart from Chlamydomonas reinhardtii with respect to its absorption maximum (460 nm) and photocycle dynamics. In contrast, VChR1 absorbs maximally at 540 nm at low pH (D540), shifting to 500 nm at high pH (D500). Flash photolysis experiments showed that after light excitation, the D540 dark state bleaches and at least two photoproducts, P600 and P500, are sequentially populated during the photocycle. We hypothesize that VChR2 is a general photoreceptor that is responsible for the avoidance of blue light and might play a key role in sexual development, whereas VChR1 is the main phototaxis photoreceptor under vegetative conditions, as it is more specifically adapted to environmental conditions and the developmental stages of Volvox.

Algal photoreceptors: in vivo functions and potential applications

“…The pattern of proteins synthesized in both potential cell types is almost the same during the dark period and it changes rapidly when the light comes back on (Kirk and Kirk 1985). The action spectrum of this light-dependent protein synthesis is not the same as that for photosynthesis but it is shaped like the action spectrum of rhodopsin with a maximum around 510 nm (Kirk and Kirk 1985); it also correlates with the absorption maximum of VChR1 (500 nm at high pH, 540 nm at low pH) (Kianianmomeni et al 2009;Zhang et al 2008), a homolog of ChR1. Therefore, VChR1 might be responsible for light-dependent cellular differentiation or at least might be involved in this process.…”

“…Both ChR1 and ChR2 are located at the eyespot apparatus of Chlamydomonas (Berthold et al 2008;Boyd et al 2011a;Mittelmeier et al 2011;Suzuki et al 2003;Wagner et al 2008). The extended C-terminal domain of channelrhodopsins is possibly involved in the subcellular localization and in anchoring of these proteins at the eyespot apparatus (Kato et al 2012;Kianianmomeni et al 2009;Mittelmeier et al 2011). The eyespot apparatus is a photoreceptive organelle found in many green flagellates; under the microscope the eyespot appears as an orange-red spot or stigma.…”

Structure-guided SCHEMA recombination generates diverse chimeric channelrhodopsins

“…Since the initial discovery and characterization of channelrhodopsins ChR1 (31) and ChR2 (32) from the alga Chlamydomonas reinhardtii, a number of ChRs have been isolated and characterized, for example, VChR1 (33), VChR2 (34,35), MvChR1 (36), CaChR1 (37), DChR (4), and PsChR (38). De novo transcriptome sequencing of 127 species of algae led to the discovery of 14 ChRs that express and function in mammalian neurons (39).…”

Identification of a Natural Green Light Absorbing Chloride Conducting Channelrhodopsin from Proteomonas sulcata

“…The initial spectroscopic characterization of PsACR1 reveals substantial differences to photocycles of cation conducting ChRs (11,(21)(22)(23)(24). As in virtual all microbial rhodopsin also in PsACR1 the early red-shifted K-like photoproduct reflects an energetically elevated electronic ground-state after photoisomerization (25).…”