Unique structure and function of viral rhodopsins.

Recently, two groups of rhodopsin genes were identified in large double-stranded DNA viruses. The structure and function of viral rhodopsins are unknown. We present functional characterization and high-resolution structure of an Organic Lake Phycodnavirus rhodopsin II (OLPVRII) of group 2. It forms a pentamer, with a symmetrical, bottle-like central channel with the narrow vestibule in the cytoplasmic part covered by a ring of 5 arginines, whereas 5 phenylalanines form a hydrophobic barrier in its exit. The proton donor E42 is placed in the helix B. The structure is unique among the known rhodopsins. Structural and functional data and molecular dynamics suggest that OLPVRII might be a light-gated pentameric ion channel analogous to pentameric ligand-gated ion channels, however, future patch clamp experiments should prove this directly. The data shed light on a fundamentally distinct branch of rhodopsins and may contribute to the understanding of virus-host interactions in ecologically important marine protists.

Biosynthetic production of fully carbon-13 labeled retinal in E. coli for structural and functional studies of rhodopsins.

The isomerization of a covalently bound retinal is an integral part of both microbial and animal rhodopsin function. As such, detailed structure and conformational changes in the retinal binding pocket are of significant interest and are studied in various NMR, FTIR, and Raman spectroscopy experiments, which commonly require isotopic labeling of retinal. Unfortunately, the de novo organic synthesis of an isotopically-labeled retinal is complex and often cost-prohibitive, especially for large scale expression required for solid-state NMR. We present the novel protocol for biosynthetic production of an isotopically labeled retinal ligand concurrently with an apoprotein in E. coli as a cost-effective alternative to the de novo organic synthesis. Previously, the biosynthesis of a retinal precursor, ß-carotene, has been introduced into many different organisms. We extended this system to the prototrophic E. coli expression strain BL21 in conjunction with the inducible expression of a ß-dioxygenase and proteo-opsin. To demonstrate the applicability of this system, we were able to assign several new carbon resonances for proteorhodopsin-bound retinal by using fully 13C-labeled glucose as the sole carbon source. Furthermore, we demonstrated that this biosynthetically produced retinal can be extracted from E. coli cells by applying a hydrophobic solvent layer to the growth medium and reconstituted into an externally produced opsin of any desired labeling pattern.

NEUROSCIENCE. Natural light-gated anion channels: A family of microbial rhodopsins for advanced optogenetics.

Light-gated rhodopsin cation channels from chlorophyte algae have transformed neuroscience research through their use as membrane-depolarizing optogenetic tools for targeted photoactivation of neuron firing. Photosuppression of neuronal action potentials has been limited by the lack of equally efficient tools for membrane hyperpolarization. We describe anion channel rhodopsins (ACRs), a family of light-gated anion channels from cryptophyte algae that provide highly sensitive and efficient membrane hyperpolarization and neuronal silencing through light-gated chloride conduction. ACRs strictly conducted anions, completely excluding protons and larger cations, and hyperpolarized the membrane of cultured animal cells with much faster kinetics at less than one-thousandth of the light intensity required by the most efficient currently available optogenetic proteins. Natural ACRs provide optogenetic inhibition tools with unprecedented light sensitivity and temporal precision.

Proteorhodopsin from Dokdonia sp. PRO95 is a light-driven Na+-pump.

The gene encoding proteorhodopsin AEX55013 from Dokdonia sp. PRO95 was cloned and expressed in Escherichia coli cells. Illumination of the proteorhodopsin-producing E. coli cells in Na+-containing media resulted in alkalinization of the media. This response was accelerated by uncoupler CCCP and inhibited by penetrating anion SCN(-). Illumination of the cells in a sodium-free medium (made by substituting Na+ with K+) resulted in SCN(-)-stimulated and CCCP-sensitive acidification of the medium. Illumination of the proteorhodopsin-containing E. coli cells caused CCCP-resistant transmembrane sodium export from these cells. We conclude that the proteorhodopsin from the marine flavobacterium Dokdonia sp. PRO95 is a primary light-driven Na+-pump. A high level of the heterologous production in E. coli cells as well as stability and purity of the isolated protein makes this proteorhodopsin an attractive model for studying the mechanism of active sodium transmembrane translocation.

Microbial rhodopsins in the spotlight.

The discovery of the light-gated cation channel Channelrhodopsin-2 (ChR2) and the use of the rediscovered light-driven Cl-pump halorhodopsin (HR) as optogenetic tools--genetically encoded switches that enable neurons to be turned on or off with bursts of light--refines the functional study of neurons in larger networks. Cell-specific expression allows a fast optical scanning approach to determine neuronal crosstalk following plasticity at the single synapse level or long-range projections in locomotion and somatosensory networks. Both rhodopsins proved to work functionally and could evoke behavioral responses in lower model organisms, reinstall rudimentary visual perception in blind mice and were set in a biomedical context with the investigation of neurodegenerative diseases.

Light-induced activation of distinct modulatory neurons triggers appetitive or aversive learning in Drosophila larvae.

During classical conditioning, a positive or negative value is assigned to a previously neutral stimulus, thereby changing its significance for behavior. If an odor is associated with a negative stimulus, it can become repulsive. Conversely, an odor associated with a reward can become attractive. By using Drosophila larvae as a model system with minimal brain complexity, we address the question of which neurons attribute these values to odor stimuli. In insects, dopaminergic neurons are required for aversive learning, whereas octopaminergic neurons are necessary and sufficient for appetitive learning. However, it remains unclear whether two independent neuronal populations are sufficient to mediate such antagonistic values. We report the use of transgenically expressed channelrhodopsin-2, a light-activated cation channel, as a tool for optophysiological stimulation of genetically defined neuronal populations in Drosophila larvae. We demonstrate that distinct neuronal populations can be activated simply by illuminating the animals with blue light. Light-induced activation of dopaminergic neurons paired with an odor stimulus induces aversive memory formation, whereas activation of octopaminergic/tyraminergic neurons induces appetitive memory formation. These findings demonstrate that antagonistic modulatory subsystems are sufficient to substitute for aversive and appetitive reinforcement during classical conditioning.

The multitalented microbial sensory rhodopsins.

Sensory rhodopsins are photoactive, membrane-embedded seven-transmembrane helix receptors that use retinal as a chromophore. They are widespread in the microbial world in each of the three domains of life: Archaea, Bacteria and Eukarya. A striking characteristic of these photoreceptors is their different modes of signaling in different organisms, including interaction with other membrane proteins, interaction with cytoplasmic transducers and light-controlled Ca(2+) channel activity. More than two decades since the discovery of the first sensory rhodopsins in the archaeon Halobacterium salinarum, genome projects have revealed a widespread presence of homologous photosensors. New work on cyanobacteria, algae, fungi and marine proteobacteria is revealing how evolution has modified the common design of these proteins to produce a remarkably rich diversity in their signaling biochemistry.

Microbial rhodopsins: functional versatility and genetic mobility.

The type 1 (microbial) rhodopsins are a diverse group of photochemically reactive proteins that span the three domains of life. Their broad phylogenetic distribution has motivated conjecture that rhodopsin-like functionality was present in the last common ancestor of all life. Here, we discuss the evolution of the type 1 microbial rhodopsins and document five cases of lateral gene transfer (LGT) between domains. We suggest that, thanks to the functional versatility of these retinylidene proteins and the relative ease with which they can complement the existing energy-generating or photosensory repertoires of many organisms, LGT is in fact the principal force that determines their broad but patchy distribution.

Prokaryotic photosynthesis and phototrophy illuminated.

Genome sequencing projects are revealing new information about the distribution and evolution of photosynthesis and phototrophy. Although coverage of the five phyla containing photosynthetic prokaryotes (Chlorobi, Chloroflexi, Cyanobacteria, Proteobacteria and Firmicutes) is limited and uneven, genome sequences are (or soon will be) available for >100 strains from these phyla. Present knowledge of photosynthesis is almost exclusively based on data derived from cultivated species but metagenomic studies can reveal new organisms with novel combinations of photosynthetic and phototrophic components that have not yet been described. Metagenomics has already shown how the relatively simple phototrophy based upon rhodopsins has spread laterally throughout Archaea, Bacteria and eukaryotes. In this review, we present examples that reflect recent advances in phototroph biology as a result of insights from genome and metagenome sequencing.

Modeling light-induced currents in the eye of Chlamydomonas reinhardtii.

Rhodopsin-mediated electrical events in green algae have been recorded in the past from the eyes of numerous micro-algae like Haematococcus pluvialis, Chlamydomonas reinhardtii and Volvox carteri. However, the electrical data gathered by suction-pipette techniques could be interpreted in qualitative terms only. Here we present two models that allow a quantitative analysis of such results: First, an electrical analog circuit for the cell in suction pipette configuration is established. Applying this model to experimental data from unilluminated cells of C. reinhardtii yields a membrane conductance of about 3 Sm(-2). Furthermore, an analog circuit allows the determination of the photocurrent fraction that is recorded under experimental conditions. Second, a reaction scheme of a rhodopsin-type photocycle with an early Ca(2+) conductance and a later H(+) conductance is presented. The combination of both models provides good fits to light-induced currents recorded from C. reinhardtii. Finally, it allowed the calculation of the impact of each model parameter on the time courses of observable photocurrent and of inferred transmembrane voltage. The reduction of the flash-to-peak times at increasing light intensities are explained by superposition of two kinetically distinct rhodopsins and by assuming that the Ca(2+)-conducting state decays faster at more positive membrane voltages.