Rhodopsins with Similar Absorbance Characteristics but Different Rates of Thermal Activation: Amino Acid Sequences

Thermal activation of the visual pigment is thought to be an important factor that in many cases limits the absolute sensitivity of vision in darkness. It has been suggested that pigments with high \lambda_max (ie with good absorbance at long wavelengths, allowing ‘red-sensitive’ vision) are associated with a cost in terms of high thermal activation rates, degrading signal/noise and hence visual sensitivity [Barlow, 1957 Nature (London) 179 255 – 256]. While rhodopsins in different species do show a general correlation between red-sensitivity and high thermal activation rates as measured electrophysiologically in whole rods (Firsov and Govardovskii, 1990 Sensornye Sistemy 4 25 – 34), a comparison of the toads Bufo marinus and B. bufo with the bull-frog Rana catesbeiana has suggested that the two properties are not tightly coupled. Toad and bull-frog rhodopsins have almost the same \lambda_max, yet the thermal activation rate in bull-frog rods is lower by almost one order of magnitude [Donner et al, 1990 Journal of Physiology (London) 428 673 – 692]. We have sequenced the cDNA coding for the rhodopsins of the two toad species and that of the common frog R. temporaria and aligned them to previously sequenced rhodopsins of R. catesbeiana, R. pipiens and Xenopus laevis in an attempt to identify substitutions that could underlie the greater thermal stability of bull-frog rhodopsin compared with toad rhodopsins.

The open reading frame predicted proteins of 354 amino acids. There was 96% identity between species of the same genus and 90% identity between genera. Across the six species studied, there is a total of 22 non-conserved substitutions and changes that include gain or loss of hydroxyl groups. Our study produced a list of substitutions that apparently do not significantly affect absorbance characteristics. We could not, however, unequivocally identify substitutions important for thermal stability.