Spectral tuning mediated by helix III in butterfly long wavelength-sensitive visual opsins revealed by heterologous action spectroscopy

Tomohiro Sugihara,Akihisa Terakita,Kentaro Arikawa,Tomoka Saito,Mitsumasa Koyanagi,Takashi Nagata

doi:10.1186/s40851-019-0150-2

Tomohiro Sugihara, Akihisa Terakita + Show 4 more

Open Access

https://doi.org/10.1186/s40851-019-0150-2

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Abstract

Absorption spectra of opsin-based pigments are tuned from the UV to the red regions by interactions of the chromophore with surrounding amino acid residues. Both vertebrates and invertebrates possess long-wavelength-sensitive (LWS) opsins, which underlie color vision involving “red” sensing. The LWS opsins have independently evolved in each lineage, which suggests the existence of diverse mechanisms in spectral tuning. In vertebrate LWS opsins, the mechanisms underlying spectral tuning have been well characterized by spectroscopic analyses with recombinant pigments of wild type (WT) and mutant opsins. However in invertebrate LWS opsins including insect ones, the mechanisms are largely unknown due to the difficulty in obtaining recombinant pigments. Here we have overcome the problem by analyzing heterologous action spectra based on light-dependent changes in the second messenger in opsin-expressing cultured cells. We found that WTs of two LWS opsins of the butterfly, Papilio xuthus, PxRh3 and PxRh1 have the wavelengths of the absorption maxima at around 570 nm and 540 nm, respectively. Analysis of a series of chimeric mutants showed that helix III is crucial to generating a difference of about 15 nm in the wavelength of absorption maxima of these LWS opsins. Further site-directed mutations in helix III revealed that amino acid residues at position 116 and 120 (bovine rhodopsin numbering system) are involved in the spectral tuning of PxRh1 and PxRh3, suggesting a different spectral tuning mechanism from that of primate LWS opsins.

Highlights

Many animals use light information for both visual and non-visual functions
We recently found that heterologous action spectroscopy is useful for estimating the absorption spectrum of pigments for which it is difficult to obtain purified recombinant pigments
We concluded that it was not possible to proceed with a comprehensive comparation of the absorption spectra of purified wild type (WT) and mutant PxRh1 and PxRh3 proteins

Summary

Introduction

Many animals use light information for both visual and non-visual functions. In most cases, animals capture light through an opsin, which binds to a chromophore retinal to form a light-sensitive pigment. Thousands of opsins have been identified from a wide variety of animals far These are classified into at least eight groups, which diversified early in the evolution of animals [2, 3]. Each group contains opsins that form the pigments with different absorption spectra, suggesting that multiple diversification events occurred independently in each group lineage, and even in each animal lineage. Interaction of retinal, a chromophore, with its surrounding amino acid residues tunes the absorption spectrum of the chromophore, namely the absorption spectrum of the opsin-based pigment. Because substitutions of amino acid residues surrounding the chromophore occurred during opsin evolution, absorption spectra of opsin (2019) 5:35 pigments belonging to different opsin groups may be tuned by different mechanisms

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