Abstract
Opn3/TMT opsins belong to one of the opsin groups with vertebrate visual and non-visual opsins, and are widely distributed in eyes, brains and other internal organs in various vertebrates and invertebrates. Vertebrate Opn3/TMT opsins are further classified into four groups on the basis of their amino acid identities. However, there is limited information about molecular properties of these groups, due to the difficulty in preparing the recombinant proteins. Here, we successfully expressed recombinant proteins of TMT1 and TMT2 opsins of medaka fish (Oryzias latipes) in cultured cells and characterized their molecular properties. Spectroscopic and biochemical studies demonstrated that TMT1 and TMT2 opsins functioned as blue light-sensitive Gi/Go-coupled receptors, but exhibited spectral properties and photo-convertibility of the active state different from each other. TMT1 opsin forms a visible light-absorbing active state containing all-trans-retinal, which can be photo-converted to 7-cis- and 9-cis-retinal states in addition to the original 11-cis-retinal state. In contrast, the active state of TMT2 opsin is a UV light-absorbing state having all-trans-retinal and does not photo-convert to any other state, including the original 11-cis-retinal state. Thus, TMT opsins are diversified so as to form a different type of active state, which may be responsible for their different functions.
Highlights
Most animals have light-sensing G protein-coupled receptors called opsins to utilize light from the outer environment as various information sources
We characterized the molecular properties of medaka TMT1A and TMT2 opsins and found that they are both blue light-sensitive Gi/Go-coupled receptors, but form active states having different spectral and photochemical properties
TMT1A opsin photo-converts to an active state that has λmax in the visible region and is photo-convertible to the original state (Fig 2E)
Summary
Most animals have light-sensing G protein-coupled receptors called opsins to utilize light from the outer environment as various information sources. The agonist and inverse agonist of opsin are all-trans- and 11-cis-retinal, respectively. Opsins in the resting state contain inverse agonist 11-cis-retinal, which works as a light-absorbing chromophore. Light causes activation of opsin by changing 11-cis-retinal to agonist all-trans-retinal through photoisomerization. Several thousands of opsins have been identified from various animals, and have been classified into several groups based on their amino acid sequence identities [1, 2]. Opn (encephalopsin) was first identified in the deep brain and internal organs in human and mouse [3, 4]. After the discovery of mammalian Opn, its homolog was found from neural tissues and a variety of non-neural tissues of teleosts, and was named teleost multiple tissue opsin (TMT opsin) [5].
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