| Abstract
| - The Thr94 → Ile mutation in the second transmembrane segment of rhodopsin has been reportedto be associated with a congenital night blindness phenotype in a large Irish pedigree. Previously, twoother known rhodopsin mutants that cause congenital night blindness, A292E and G90D, have been shownin vitro to constitutively activate the G protein transducin in the absence of a chromophore. The proposedmechanism of constitutive activation of these two mutants is an electrostatic disruption of the active sitesalt bridge between Glu113 and Lys296 that contributes to stabilization of the protein in the inactivestate. Here, the T94I rhodopsin mutant is characterized and compared to the two other known rhodopsinnight blindness mutants. The T94I mutant opsin is shown also to constitutively activate transducin. TheT94I mutant pigment (with a bound 11-cis-retinal chromophore), like the other known rhodopsin nightblindness mutants, is not active in the dark and has wild-type activity upon exposure to light. Similar tothe Gly90 → Asp substitution, position 94 is close enough to the Schiff base nitrogen that an Asp at thisposition can functionally substitute for the Glu113 counterion. However, in contrast to the other nightblindness mutants, the T94I MII intermediate decays with a half-life that is approximately 8-fold slowerthan in the wild-type MII intermediate. Thus, the one phenotype shared by all congenital night blindnessmutants that is different from the wild-type protein is constitutive activation of the apoprotein.
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