The binding of secondary chromophore for thermally stable rhodopsin makes more stable with temperature

Microbial rhodopsin is a transmembrane protein that functions with a chromophore and is regulated by light. In nature, regulation through the retinal chromophore plays an important role in physiological phenomena. However, living organisms in soil, sea, and freshwater synthesize carotenoids preferentially over retinal in the biosynthetic pathway. Evolution has extended the energy conversion photosystem with additional pigments that act as antennae. Previously, Gloeobacter rhodopsin and xanthorhodopsin have been reported to form secondary chromophores with carotenoids. In this study, we report that a thermophilic rhodopsin (TR) and Tara76 rhodopsin, the latter of which is classified as a blue light-absorbing proteorhodopsin, can form secondary chromophores with canthaxanthin (CAN). Tara76 rhodopsin and TR were found to exhibit high thermal stabilities and photophysical properties following their interaction with CAN. Isothermal titration calorimetry analysis, spectral shift measurements, and exciton analysis were used to examine the interactions of these rhodopsins with CAN. It was found that these interactions increased the stability toward temperature and pH through highly efficient chromophore formation, in addition to rapidly recruiting the retinal at a rate approximately twice as high as that obtained in the absence of CAN.