Structural basis for ion selectivity in potassium-selective channelrhodopsins.

KCR channelrhodopsins (K+-selective light-gated ion channels) have received attention as potential inhibitory optogenetic tools but more broadly pose a fundamental mystery regarding how their K+ selectivity is achieved. Here, we present 2.5-2.7 Å cryo-electron microscopy structures of HcKCR1 and HcKCR2 and of a structure-guided mutant with enhanced K+ selectivity. Structural, electrophysiological, computational, spectroscopic, and biochemical analyses reveal a distinctive mechanism for K+ selectivity; rather than forming the symmetrical filter of canonical K+ channels achieving both selectivity and dehydration, instead, three extracellular-vestibule residues within each monomer form a flexible asymmetric selectivity gate, while a distinct dehydration pathway extends intracellularly. Structural comparisons reveal a retinal-binding pocket that induces retinal rotation (accounting for HcKCR1/HcKCR2 spectral differences), and design of corresponding KCR variants with increased K+ selectivity (KALI-1/KALI-2) provides key advantages for optogenetic inhibition in vitro and in vivo. Thus, discovery of a mechanism for ion-channel K+ selectivity also provides a framework for next-generation optogenetics.

Structural basis for channel conduction in the pump-like channelrhodopsin ChRmine.

ChRmine, a recently discovered pump-like cation-conducting channelrhodopsin, exhibits puzzling properties (large photocurrents, red-shifted spectrum, and extreme light sensitivity) that have created new opportunities in optogenetics. ChRmine and its homologs function as ion channels but, by primary sequence, more closely resemble ion pump rhodopsins; mechanisms for passive channel conduction in this family have remained mysterious. Here, we present the 2.0 Å resolution cryo-EM structure of ChRmine, revealing architectural features atypical for channelrhodopsins: trimeric assembly, a short transmembrane-helix 3, a twisting extracellular-loop 1, large vestibules within the monomer, and an opening at the trimer interface. We applied this structure to design three proteins (rsChRmine and hsChRmine, conferring further red-shifted and high-speed properties, respectively, and frChRmine, combining faster and more red-shifted performance) suitable for fundamental neuroscience opportunities. These results illuminate the conduction and gating of pump-like channelrhodopsins and point the way toward further structure-guided creation of channelrhodopsins for applications across biology.

The mitochondrial intermembrane space protein mitofissin drives mitochondrial fission required for mitophagy.

Mitophagy plays an important role in mitochondrial homeostasis by selective degradation of mitochondria. During mitophagy, mitochondria should be fragmented to allow engulfment within autophagosomes, whose capacity is exceeded by the typical mitochondria mass. However, the known mitochondrial fission factors, dynamin-related proteins Dnm1 in yeasts and DNM1L/Drp1 in mammals, are dispensable for mitophagy. Here, we identify Atg44 as a mitochondrial fission factor that is essential for mitophagy in yeasts, and we therefore term Atg44 and its orthologous proteins mitofissin. In mitofissin-deficient cells, a part of the mitochondria is recognized by the mitophagy machinery as cargo but cannot be enwrapped by the autophagosome precursor, the phagophore, due to a lack of mitochondrial fission. Furthermore, we show that mitofissin directly binds to lipid membranes and brings about lipid membrane fragility to facilitate membrane fission. Taken together, we propose that mitofissin acts directly on lipid membranes to drive mitochondrial fission required for mitophagy.

Phototrophy by antenna-containing rhodopsin pumps in aquatic environments.

Energy transfer from light-harvesting ketocarotenoids to the light-driven proton pump xanthorhodopsins has been previously demonstrated in two unique cases: an extreme halophilic bacterium1 and a terrestrial cyanobacterium2. Attempts to find carotenoids that bind and transfer energy to abundant rhodopsin proton pumps3 from marine photoheterotrophs have thus far failed4-6. Here we detected light energy transfer from the widespread hydroxylated carotenoids zeaxanthin and lutein to the retinal moiety of xanthorhodopsins and proteorhodopsins using functional metagenomics combined with chromophore extraction from the environment. The light-harvesting carotenoids transfer up to 42% of the harvested energy in the violet- or blue-light range to the green-light absorbing retinal chromophore. Our data suggest that these antennas may have a substantial effect on rhodopsin phototrophy in the world's lakes, seas and oceans. However, the functional implications of our findings are yet to be discovered.

Microbial Rhodopsins: The Last Two Decades.

Microbial rhodopsins are diverse photoreceptive proteins containing a retinal chromophore and are found in all domains of cellular life and are even encoded in genomes of viruses. These rhodopsins make up two families: type 1 rhodopsins and the recently discovered heliorhodopsins. These families have seven transmembrane helices with similar structures but opposing membrane orientation. Microbial rhodopsins participate in a portfolio of light-driven energy and sensory transduction processes. In this review we present data collected over the last two decades about these rhodopsins and describe their diversity, functions, and biological and ecological roles.

Crystal structure of heliorhodopsin.

Heliorhodopsins (HeRs) are a family of rhodopsins that was recently discovered using functional metagenomics1. They are widely present in bacteria, archaea, algae and algal viruses2,3. Although HeRs have seven predicted transmembrane helices and an all-trans retinal chromophore as in the type-1 (microbial) rhodopsin, they display less than 15% sequence identity with type-1 and type-2 (animal) rhodopsins. HeRs also exhibit the reverse orientation in the membrane compared with the other rhodopsins. Owing to the lack of structural information, little is known about the overall fold and the photoactivation mechanism of HeRs. Here we present the 2.4-Å-resolution structure of HeR from an uncultured Thermoplasmatales archaeon SG8-52-1 (GenBank sequence ID LSSD01000000). Structural and biophysical analyses reveal the similarities and differences between HeRs and type-1 microbial rhodopsins. The overall fold of HeR is similar to that of bacteriorhodopsin. A linear hydrophobic pocket in HeR accommodates a retinal configuration and isomerization as in the type-1 rhodopsin, although most of the residues constituting the pocket are divergent. Hydrophobic residues fill the space in the extracellular half of HeR, preventing the permeation of protons and ions. The structure reveals an unexpected lateral fenestration above the ß-ionone ring of the retinal chromophore, which has a critical role in capturing retinal from environment sources. Our study increases the understanding of the functions of HeRs, and the structural similarity and diversity among the microbial rhodopsins.

Crystal structure of the natural anion-conducting channelrhodopsin GtACR1.

The naturally occurring channelrhodopsin variant anion channelrhodopsin-1 (ACR1), discovered in the cryptophyte algae Guillardia theta, exhibits large light-gated anion conductance and high anion selectivity when expressed in heterologous settings, properties that support its use as an optogenetic tool to inhibit neuronal firing with light. However, molecular insight into ACR1 is lacking owing to the absence of structural information underlying light-gated anion conductance. Here we present the crystal structure of G. theta ACR1 at 2.9 Å resolution. The structure reveals unusual architectural features that span the extracellular domain, retinal-binding pocket, Schiff-base region, and anion-conduction pathway. Together with electrophysiological and spectroscopic analyses, these findings reveal the fundamental molecular basis of naturally occurring light-gated anion conductance, and provide a framework for designing the next generation of optogenetic tools.

A distinct abundant group of microbial rhodopsins discovered using functional metagenomics.

Many organisms capture or sense sunlight using rhodopsin pigments1,2, which are integral membrane proteins that bind retinal chromophores. Rhodopsins comprise two distinct protein families 1 , type-1 (microbial rhodopsins) and type-2 (animal rhodopsins). The two families share similar topologies and contain seven transmembrane helices that form a pocket in which retinal is linked covalently as a protonated Schiff base to a lysine at the seventh transmembrane helix2,3. Type-1 and type-2 rhodopsins show little or no sequence similarity to each other, as a consequence of extensive divergence from a common ancestor or convergent evolution of similar structures 1 . Here we report a previously unknown and diverse family of rhodopsins-which we term the heliorhodopsins-that we identified using functional metagenomics and that are distantly related to type-1 rhodopsins. Heliorhodopsins are embedded in the membrane with their N termini facing the cell cytoplasm, an orientation that is opposite to that of type-1 or type-2 rhodopsins. Heliorhodopsins show photocycles that are longer than one second, which is suggestive of light-sensory activity. Heliorhodopsin photocycles accompany retinal isomerization and proton transfer, as in type-1 and type-2 rhodopsins, but protons are never released from the protein, even transiently. Heliorhodopsins are abundant and distributed globally; we detected them in Archaea, Bacteria, Eukarya and their viruses. Our findings reveal a previously unknown family of light-sensing rhodopsins that are widespread in the microbial world.

Structural mechanisms of selectivity and gating in anion channelrhodopsins.

Both designed and natural anion-conducting channelrhodopsins (dACRs and nACRs, respectively) have been widely applied in optogenetics (enabling selective inhibition of target-cell activity during animal behaviour studies), but each class exhibits performance limitations, underscoring trade-offs in channel structure-function relationships. Therefore, molecular and structural insights into dACRs and nACRs will be critical not only for understanding the fundamental mechanisms of these light-gated anion channels, but also to create next-generation optogenetic tools. Here we report crystal structures of the dACR iC++, along with spectroscopic, electrophysiological and computational analyses that provide unexpected insights into pH dependence, substrate recognition, channel gating and ion selectivity of both dACRs and nACRs. These results enabled us to create an anion-conducting channelrhodopsin integrating the key features of large photocurrent and fast kinetics alongside exclusive anion selectivity.

Crystal structure of schizorhodopsin reveals mechanism of inward proton pumping.

Schizorhodopsins (SzRs), a new rhodopsin family identified in Asgard archaea, are phylogenetically located at an intermediate position between type-1 microbial rhodopsins and heliorhodopsins. SzRs work as light-driven inward H+ pumps as xenorhodopsins in bacteria. Although E81 plays an essential role in inward H+ release, the H+ is not metastably trapped in such a putative H+ acceptor, unlike the other H+ pumps. It remains elusive why SzR exhibits different kinetic behaviors in H+ release. Here, we report the crystal structure of SzR AM_5_00977 at 2.1 Å resolution. The SzR structure superimposes well on that of bacteriorhodopsin rather than heliorhodopsin, suggesting that SzRs are classified with type-1 rhodopsins. The structure-based mutagenesis study demonstrated that the residues N100 and V103 around the ß-ionone ring are essential for color tuning in SzRs. The cytoplasmic parts of transmembrane helices 2, 6, and 7 are shorter than those in the other microbial rhodopsins, and thus E81 is located near the cytosol and easily exposed to the solvent by light-induced structural change. We propose a model of untrapped inward H+ release; H+ is released through the water-mediated transport network from the retinal Schiff base to the cytosol by the side of E81. Moreover, most residues on the H+ transport pathway are not conserved between SzRs and xenorhodopsins, suggesting that they have entirely different inward H+ release mechanisms.

Functional characterization of four opsins and two G alpha subtypes co-expressed in the molluscan rhabdomeric photoreceptor.

BACKGROUND: Rhabdomeric photoreceptors of eyes in the terrestrial slug Limax are the typical invertebrate-type but unique in that three visual opsins (Gq-coupled rhodopsin, xenopsin, Opn5A) and one retinochrome, all belonging to different groups, are co-expressed. However, molecular properties including spectral sensitivity and G protein selectivity of any of them are not determined, which prevents us from understanding an advantage of multiplicity of opsin properties in a single rhabdomeric photoreceptor. To gain insight into the functional role of the co-expression of multiple opsin species in a photoreceptor, we investigated the molecular properties of the visual opsins in the present study. RESULTS: First, we found that the fourth member of visual opsins, Opn5B, is also co-expressed in the rhabdomere of the photoreceptor together with previously identified three opsins. The photoreceptors were also demonstrated to express Gq and Go alpha subunits. We then determined the spectral sensitivity of the four visual opsins using biochemical and spectroscopic methods. Gq-coupled rhodopsin and xenopsin exhibit maximum sensitivity at ~ 456 and 475 nm, respectively, and Opn5A and Opn5B exhibit maximum sensitivity at ~ 500 and 470 nm, respectively, with significant UV sensitivity. Notably, in vitro experiments revealed that Go alpha was activated by all four visual opsins, in contrast to the specific activation of Gq alpha by Gq-coupled rhodopsin, suggesting that the eye photoreceptor of Limax uses complex G protein signaling pathways. CONCLUSIONS: The eye photoreceptor in Limax expresses as many as four different visual opsin species belonging to three distinct classes. The combination of opsins with different spectral sensitivities and G protein selectivities may underlie physiological properties of the ocular photoreception, such as a shift in spectral sensitivity between dark- and light-adapted states. This may be allowed by adjustment of the relative contribution of the four opsins without neural networks, enabling a simple strategy for fine-tuning of vision.

Reversible Photoreaction of a Retinal Photoisomerase, Retinal G-Protein-Coupled Receptor RGR.

Retinal G-protein-coupled receptor (RGR) plays a crucial role in the visual system of vertebrates as a retinal photoisomerase, which isomerizes all-trans-retinal to 11-cis-retinal to maintain the photosensitivity of visual rhodopsins. Despite the previous characterization of bovine RGR, little is known about the spectral properties of RGR from other species. In addition, photoreactivity of the 11-cis-retinal-binding form remains unclear. In this study, we revealed that human and chicken RGRs form blue-absorbing pigments similar to bovine RGR. Furthermore, the spectroscopic and biochemical analyses revealed that bovine and chicken RGRs are bistable rhodopsins displaying a reversible photoreaction. These findings provide insight into the behavior of RGR as a retinal photoisomerase and aid in understanding its role in the visual system.

Difference FTIR Spectroscopy of Jumping Spider Rhodopsin-1 at 77 K.

Animal visual rhodopsins can be classified into monostable and bistable rhodopsins, which are typically found in vertebrates and invertebrates, respectively. The former example is bovine rhodopsin (BovRh), whose structures and functions have been extensively studied. On the other hand, those of bistable rhodopsins are less known, despite their importance in optogenetics. Here, low-temperature Fourier-transform infrared (FTIR) spectroscopy was applied to jumping spider rhodopsin-1 (SpiRh1) at 77 K, and the obtained light-induced spectral changes were compared with those of squid rhodopsin (SquRh) and BovRh. Although chromophore distortion of the resting state monitored by HOOP vibrations is not distinctive between invertebrate and vertebrate rhodopsins, distortion of the all-trans chromophore after photoisomerization is unique for BovRh, and the distortion was localized at the center of the chromophore in SpiRh1 and SquRh. Highly conserved aspartate (D83 in BovRh) does not change the hydrogen-bonding environment in invertebrate rhodopsins. Thus, present FTIR analysis provides specific structural changes, leading to activation of invertebrate and vertebrate rhodopsins. On the other hand, the analysis of O-D stretching vibrations in D2O revealed unique features of protein-bound water molecules. Numbers of water bands in SpiRh1 and SquRh were less and more than those in BovRh. The X-ray crystal structure of SpiRh1 observed a bridged water molecule between the protonated Schiff base and its counterion (E194), but strongly hydrogen-bonded water molecules were never detected in SpiRh1, as well as SquRh and BovRh. Thus, absence of strongly hydrogen-bonded water molecules is substantial for animal rhodopsins, which is distinctive from microbial rhodopsins.

Structural characterization of proton-pumping rhodopsin lacking a cytoplasmic proton donor residue by X-ray crystallography.

DTG/DTS rhodopsin, which was named based on a three-residue motif (DTG or DTS) that is important for its function, is a light-driven proton-pumping microbial rhodopsin using a retinal chromophore. In contrast to other light-driven ion-pumping rhodopsins, DTG/DTS rhodopsin does not have a cytoplasmic proton donor residue, such as Asp, Glu, or Lys. Because of the lack of cytoplasmic proton donor residue, proton directly binds to the retinal chromophore from the cytoplasmic solvent. However, mutational experiments that showed the complicated effects of mutations were not able to clarify the roles played by each residue, and the detail of proton uptake pathway is unclear because of the lack of structural information. To understand the proton transport mechanism of DTG/DTS rhodopsin, here we report the three-dimensional structure of one of the DTG/DTS rhodopsins, PspR from Pseudomonas putida, by X-ray crystallography. We show that the structure of the cytoplasmic side of the protein is significantly different from that of bacteriorhodopsin, the best-characterized proton-pumping rhodopsin, and large cytoplasmic cavities were observed. We propose that these hydrophilic cytoplasmic cavities enable direct proton uptake from the cytoplasmic solvent without the need for a specialized cytoplasmic donor residue. The introduction of carboxylic residues homologous to the cytoplasmic donors in other proton-pumping rhodopsins resulted in higher pumping activity with less pH dependence, suggesting that DTG/DTS rhodopsins are advantageous for producing energy and avoiding intracellular alkalization in soil and plant-associated bacteria.

Comparing pedigree and genomic inbreeding coefficients, and inbreeding depression of reproductive traits in Japanese Black cattle.

BACKGROUND: Pedigree-based inbreeding coefficients have been generally included in statistical models for genetic evaluation of Japanese Black cattle. The use of genomic data is expected to provide precise assessment of inbreeding level and depression. Recently, many measures have been used for genome-based inbreeding coefficients; however, with no consensus on which is the most appropriate. Therefore, we compared the pedigree- ([Formula: see text]) and multiple genome-based inbreeding coefficients, which were calculated from the genomic relationship matrix with observed allele frequencies ([Formula: see text]), correlation between uniting gametes ([Formula: see text]), the observed vs expected number of homozygous genotypes ([Formula: see text]), runs of homozygosity (ROH) segments ([Formula: see text]) and heterozygosity by descent segments ([Formula: see text]). We quantified inbreeding depression from estimating regression coefficients of inbreeding coefficients on three reproductive traits: age at first calving (AFC), calving difficulty (CD) and gestation length (GL) in Japanese Black cattle. RESULTS: The highest correlations with [Formula: see text] were for [Formula: see text] (0.86) and [Formula: see text] (0.85) whereas [Formula: see text] and [Formula: see text] provided weak correlations with [Formula: see text], with range 0.33-0.55. Except for [Formula: see text] and [Formula: see text], there were strong correlations among genome-based inbreeding coefficients ([Formula: see text] 0.94). The estimates of regression coefficients of inbreeding depression for [Formula: see text] was 2.1 for AFC, 0.63 for CD and -1.21 for GL, respectively, but [Formula: see text] had no significant effects on all traits. Genome-based inbreeding coefficients provided larger effects on all reproductive traits than [Formula: see text]. In particular, for CD, all estimated regression coefficients for genome-based inbreeding coefficients were significant, and for GL, that for [Formula: see text] had a significant.. Although there were no significant effects when using overall genome-level inbreeding coefficients for AFC and GL, [Formula: see text] provided significant effects at chromosomal level in four chromosomes for AFC, three chromosomes for CD, and two chromosomes for GL. In addition, similar results were obtained for [Formula: see text]. CONCLUSIONS: Genome-based inbreeding coefficients can capture more phenotypic variation than [Formula: see text]. In particular, [Formula: see text] and [Formula: see text] can be considered good estimators for quantifying inbreeding level and identifying inbreeding depression at the chromosome level. These findings might improve the quantification of inbreeding and breeding programs using genome-based inbreeding coefficients.

Converting a Natural-Light-Driven Outward Proton Pump Rhodopsin into an Artificial Inward Proton Pump.

Microbial rhodopsins are a large family of photoreceptive membrane proteins with diverse light-regulated functions. While the most ubiquitous microbial rhodopsins are light-driven outward proton (H+) pumps, new subfamilies of microbial rhodopsins transporting H+ inwardly, i.e., light-driven inward H+ pumps, have been discovered recently. Although structural and spectroscopic studies provide insights into their ion transport mechanisms, the minimum key element(s) that determine the direction of H+ transport have not yet been clarified. Here, we conducted the first functional conversion study by substituting key amino acids in a natural outward H+-pumping rhodopsin (PspR) with those in inward H+-pumping rhodopsins. Consequently, an artificial inward H+ pump was constructed by mutating only three residues of PspR. This result indicates that these residues govern the key processes that discriminate between outward and inward H+ pumps. Spectroscopic studies revealed the presence of an inward H+-accepting residue in the H+ transport pathway and direct H+ uptake from the extracellular solvent. This finding of the simple element for determining H+ transport would provide a new basis for understanding the concept of ion transport not only by microbial rhodopsins but also by other ion-pumping proteins.

Twisting and Protonation of Retinal Chromophore Regulate Channel Gating of Channelrhodopsin C1C2.

Channelrhodopsins (ChRs) are light-gated ion channels and central optogenetic tools that can control neuronal activity with high temporal resolution at the single-cell level. Although their application in optogenetics has rapidly progressed, it is unsolved how their channels open and close. ChRs transport ions through a series of interlocking elementary processes that occur over a broad time scale of subpicoseconds to seconds. During these processes, the retinal chromophore functions as a channel regulatory domain and transfers the optical input as local structural changes to the channel operating domain, the helices, leading to channel gating. Thus, the core question on channel gating dynamics is how the retinal chromophore structure changes throughout the photocycle and what rate-limits the kinetics. Here, we investigated the structural changes in the retinal chromophore of canonical ChR, C1C2, in all photointermediates using time-resolved resonance Raman spectroscopy. Moreover, to reveal the rate-limiting factors of the photocycle and channel gating, we measured the kinetic isotope effect of all photoreaction processes using laser flash photolysis and laser patch clamp, respectively. Spectroscopic and electrophysiological results provided the following understanding of the channel gating: the retinal chromophore highly twists upon the retinal Schiff base (RSB) deprotonation, causing the surrounding helices to move and open the channel. The ion-conducting pathway includes the RSB, where inflowing water mediates the proton to the deprotonated RSB. The twisting of the retinal chromophore relaxes upon the RSB reprotonation, which closes the channel. The RSB reprotonation rate-limits the channel closing.

Rhodopsins at a glance.

Rhodopsins are photoreceptive membrane proteins consisting of a common heptahelical transmembrane architecture that contains a retinal chromophore. Rhodopsin was first discovered in the animal retina in 1876, but a different type of rhodopsin, bacteriorhodopsin, was reported to be present in the cell membrane of an extreme halophilic archaeon, Halobacterium salinarum, 95 years later. Although these findings were made by physiological observation of pigmented tissue and cell bodies, recent progress in genomic and metagenomic analyses has revealed that there are more than 10,000 microbial rhodopsins and 9000 animal rhodopsins with large diversity and tremendous new functionality. In this Cell Science at a Glance article and accompanying poster, we provide an overview of the diversity of functions, structures, color discrimination mechanisms and optogenetic applications of these two rhodopsin families, and will also highlight the third distinctive rhodopsin family, heliorhodopsin.

Time-resolved detection of light-induced conformational changes of heliorhodopsin.

Heliorhodopsins (HeRs) are a new category of rhodopsins. They exist as a dimer and exhibit a characteristic inverted topology. HeRs bind all-trans-retinal as a chromophore in the dark, and its isomerization to the 13-cis form by light illumination leads to a photocyclic reaction involving several photo-intermediates: K, L, M, and O. In this study, the kinetics of conformational changes of HeR from Thermoplasmatales archaeon SG8-52-1 (TaHeR) were studied by the transient grating (TG) and circular dichroism (CD) methods. The TG method reveals that the diffusion coefficient (D) does not change until the O formation suggesting no significant conformation change at the surface of the protein during the early steps of the reaction. Subsequently, D decreases upon the O formation. Although two time constants (202 µs and 2.6 ms) are observed for the conversion from the M to O by the absorption detection, D decreases only at the first step (202 µs). Light-induced unfolding of helical structure is detected by the CD method. To examine the contribution of a characteristic helix in the intracellular loop 1 (ICL1 helix), Tyr93 on the ICL1 helix was replaced by Gly (Y93G), and the reaction of this mutant was also investigated. It was found that this replacement partially suppresses the D-change, although the CD-change is almost the same as that of the wild type. These results are interpreted in terms of different sensitivities of TG and CD methods, that is, D is sensitive to the structure of the solvent-exposed surface and selectively observes the conformational change in the ICL1 region. It is suggested that the structure of hydrophilic residues in the ICL1 helix is changed during this process.

Diverse heliorhodopsins detected via functional metagenomics in freshwater Actinobacteria, Chloroflexi and Archaea.

The recently discovered rhodopsin family of heliorhodopsins (HeRs) is abundant in diverse microbial environments. So far, the functional and biological roles of HeRs remain unknown. To tackle this issue, we combined experimental and computational screens to gain some novel insights. Here, 10 readily expressed HeR genes were found using functional metagenomics on samples from two freshwater environments. These HeRs originated from diverse prokaryotic groups: Actinobacteria, Chloroflexi and Archaea. Heterologously expressed HeRs absorbed light in the green and yellow wavelengths (543-562 nm) and their photocycles exhibited diverse kinetic characteristics. To approach the physiological function of the HeRs, we used our environmental clones along with thousands of microbial genomes to analyze genes neighbouring HeRs. The strongest association was found with the DegV family involved in activation of fatty acids, which allowed us to hypothesize that HeRs might be involved in light-induced membrane lipid modifications.