Difference FTIR Spectroscopy of Jumping Spider Rhodopsin-1 at 77 K

Animal visual rhodopsins can be classified into monostable and bistable rhodopsins, which are typically found in vertebrates and invertebrates, respectively. The former example is bovine rhodopsin (BovRh), whose structures and functions have been extensively studied. On the other hand, those of bistable rhodopsins are less known, despite their importance in optogenetics. Here, low-temperature Fourier transform infrared (FTIR) spectroscopy was applied to jumping spider rhodopsin-1 (SpiRh1) at 77 K, and the obtained light-induced spectral changes were compared with those of squid rhodopsin (SquRh) and BovRh. Although chromophore distortion of the resting state monitored by HOOP vibrations is not distinctive between invertebrate and vertebrate rhodopsins, distortion of the all -trans chromophore after photoisomerization is unique for BovRh, and the distortion was localized at the center of the chromophore in SpiRh1 and SquRh. Highly conserved aspartate (D83 in BovRh) does not change the hydrogen-bonding environment in invertebrate rhodopsins. Thus, present FTIR analysis provides specific structural changes, leading to activation of invertebrate and vertebrate rhodopsins. On the other hand, the analysis of O-D stretching vibrations in D2O revealed unique features of protein bound water molecules. Numbers of water bands in SpiRh1 and SquRh were less and more than those in BovRh. The X-ray crystal structure of SpiRh1 observed a bridged water molecule between the protonated Schiff base and its counterion (E194), but strongly hydrogen-bonded water molecules were never detected in SpiRh1, as well as SquRh and BovRh. Thus, absence of strongly hydrogen bonded water molecules is substantial for animal rhodopsins, which is distinctive from microbial rhodopsins.

Automated summary

Animal visual rhodopsins can be classified into monostable and bistable rhodopsins, typically found in vertebrates and invertebrates respectively. Bovine rhodopsin (BovRh) is an example of monostable rhodopsin with extensively studied structures and functions. On the other hand, the structures and functions of bistable rhodopsins, despite their importance in optogenetics, are less known. In this study, low-temperature Fourier transform infrared (FTIR) spectroscopy was used to compare the spectral changes of jumping spider rhodopsin-1 (SpiRh1), squid rhodopsin (SquRh), and BovRh. The analysis showed that while the chromophore distortion in the resting state is not distinctive between invertebrate and vertebrate rhodopsins, the distortion after photoisomerization is unique for BovRh. Moreover, the analysis of O-D stretching vibrations revealed unique features of protein-bound water molecules. The absence of strongly hydrogen-bonded water molecules is a distinct characteristic of animal rhodopsins compared to microbial rhodopsins.