Commentary and Perspective
Function of animal rhodopsins and related proteins: Report for the session 9
2023 年 20 巻 Supplemental 号 論文ID: e201018
詳細
2023 年 20 巻 Supplemental 号 論文ID: e201018
In the session of “Function of animal rhodopsins and related proteins”, 4 speakers gave a talk about their recent studies.
Dr. Masahiro Ueda (Osaka University, Japan) talked on “G-protein-coupled chemoattractant receptors activate, recruit and capture G-proteins for wide range chemotaxis.” Cells exhibit directional migration under a chemical concentration gradient, a property known as chemotaxis. In eukaryotic cells, G protein-coupled receptors (GPCRs) mediate gradient sensing for chemotaxis, which shares a common molecular mechanism from slime mold amoebae to human leukocytes. One of the characteristic features of chemotactic cells is that they have the ability for gradient sensing over a wide concentration range of 105-fold. They revealed that the receptor-mediated activation, recruitment, and capturing of G proteins mediate chemotactic signaling at the lower, middle and higher concentration ranges, respectively. These multiple mechanisms of G protein dynamics can successfully cover distinct ranges of ligand concentrations, resulting in seamless and broad chemotaxis. Furthermore, single-molecule imaging analysis showed that the activated Gα subunit forms an unconventional complex with the chemoattractant-bound receptor. This complex formation increased the membrane-binding time of individual Gα molecules and thus resulted in the local accumulation of activated Gα subunit to the membrane facing to higher concentration of chemoattractant. Their findings provide an additional chemotactic dynamic range mechanism, in which multiple G protein dynamics contribute to the production of gradient information.
Dr. Shuji Tachibanaki (Osaka University, Japan) talked on “Analysis of molecular bases of rod and cone differences using isolated carp photoreceptor cells” In the vertebrate retina, there are two types of visual photoreceptors, rods and cones. They differ in functional aspects such as light-sensitivity and temporal resolution: rods are highly light sensitive and their responses to a brief light flash is slow, while cones are less light-sensitive, and their responses are much briefer. For these differences, rods are responsible for night vision and cones for day vision. Both photoreceptors use a photo-transduction cascade, consisting of a series of enzymatic reactions, to convert photon capture to an electrical signal. The reason for their distinctive responses despite utilizing a common mechanism is thought to be caused by differences between rods and cones in the efficiency of enzymatic reactions and expression levels of the enzymes in the photo-transduction cascade. To evaluate this, they developed a method to isolate and purify rods and cones from carp retina, and compared biochemically the efficiency of each reaction in the photo-transduction cascade between them. The results showed that the activities of some enzymes together with their expression levels are mostly different between rods and cones. In general, reactions responsible for generation of a response are somewhat less effective in cones than in rods, but those for termination and recovery of a response are much more effective in cones. These findings are consistent with lower light-sensitivity and briefer responses in cones than in rods. In addition, using isolated rods and cones, they developed a method to purify membranes of outer segments (OS). This allowed to search for unknown factors involved in rod- and cone-specific responses [1]. With proteomic and lipidomic analysis of rod and cone OS membranes, possible two factors that could influence their responses were identified: neurocalcin-deltaB and cell-specific lipid distribution patterns. In the presentation, he discussed these novel factors such as the different lipid composition between cones and rods. Cones contain more cholesterols than rods, while rod contain more unsaturated fatty acids. Because both factors affect the fluidity of the membranes, it is possible that the lipid composition is one of the factors to produce the different responses between rods and cones.
Dr. Judith Klein-Seetharaman (Arizona State University, United States) talked on “Opsin family members in the stony coral Pocillopora damicornis” With the ease of gene sequencing and the technology available to study and manipulate non-model organisms, the extension of the methodological toolbox required to translate our understanding of model organisms to non-model organisms has become an urgent problem. For example, mining of large coral and their symbiont sequence data is a challenge, but also provides an opportunity for understanding functionality and evolution of these and other nonmodel organisms. Much more information than for any other eukaryotic species is available for humans, especially related to signal transduction and diseases. However, the coral cnidarian host and human have diverged over 700 million years ago and homologies between proteins in the two species are therefore often in the gray zone, or at least often undetectable with traditional BLAST searches. Using a two-stage approach to identifying putative coral homologues of human proteins [2], Her group mapped membrane receptors in humans to membrane receptors in corals, with specific focus on the stony coral, Pocillopora damicornis. More than 1000 human membrane receptors mapped to 335 coral receptors, including 151 GPCRs. To validate specific sub-families, they chose opsin proteins. Through detailed structure-function analysis of their ligand-binding pockets and downstream signaling cascades, they selected those candidate remote homologues likely to carry out visual functions in the corals. This pipeline may prove generally useful for other non-model organisms.
Dr. Yusuke Sakai (Osaka Metropolitan University, Japan) was selected from poster presentation and talked on the topic of “Investigation of spectroscopic properties and spectral tuning of anthozoan opsins in a reef-building coral.” Animal opsins belong to a GPCR family and serve as photoreceptive proteins in animals by binding the chromophore retinal. Anthozoans such as corals and sea anemones possess a set of unique opsins, “anthozoan opsins”, which are phylogenetically distant from known animal opsins. It has been suggested that anthozoan opsins might function as photoreceptive proteins to regulate behavior and reproduction light-dependently. However, a lack of knowledge on the molecular characteristics of anthozoan opsins, such as chromophore binding abilities and absorption spectra, limits our ability to predict and test their functions. In this study, they conducted spectroscopic analyses of anthozoan opsins expressed in mammalian cultured cells which transfected the isolated genes from a reef-building coral, Acropora tenuis. They found that the respective anthozoan opsins have an ability to absorb light in a specific wavelength region between 360 nm to 500 nm. They subsequently focused on a visible light-sensitive anthozoan opsin and investigated a counterion of the opsin, which is involved in the spectral tuning of visible light absorption via stabilization of the positive charge of the protonated Schiff base linkage, by analyzing a series of its site-directed mutans. On the basis of the results, they proposed a novel counterion system in the anthozoan opsin, and discussed a possible mechanism for visible light absorption of anthozoan opsins.
