Abstract
Low dark noise is a prerequisite for rod cells, which mediate our dim-light vision. The low dark noise is achieved by the extremely stable character of the rod visual pigment, rhodopsin, which evolved from less stable cone visual pigments. We have developed a biochemical method to quickly evaluate the thermal activation rate of visual pigments. Using an isomerization locked chromophore, we confirmed that thermal isomerization of the chromophore is the sole cause of thermal activation. Interestingly, we revealed an unexpected correlation between the thermal stability of the dark state and that of the active intermediate MetaII. Furthermore, we assessed key residues in rhodopsin and cone visual pigments by mutation analysis and identified two critical residues (E122 and I189) in the retinal binding pocket which account for the extremely low thermal activation rate of rhodopsin.
Highlights
Vertebrate eyes utilize two types of photoreceptor cells for dim- and bright-light conditions
We first investigated whether or not thermal activation of rhodopsin and cone pigments really originates from the thermal cis-trans isomerization of their chromophores
The possibility that thermal activation is achieved without cis-trans isomerization arises in the framework of the two-state model, where the receptor fluctuates between active and inactive states, even in the presence of an inverse agonist such as 11-cis-retinal
Summary
Vertebrate eyes utilize two types of photoreceptor cells for dim- and bright-light conditions. Differences in the thermal activation rate (kth) between rhodopsin and cone pigments originate from differences in their amino acid sequences. Thermal activation of visual pigments originates exclusively from thermal cis-trans isomerization of the retinal chromophore.
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