Naturally occurring point mutations in the opsin gene cause the retinal diseases retinitis pigmentosa and congenital night blindness. Although these diseases involve similar mutations in very close locations in rhodopsin, their progression is very different, with retinitis pigmentosa being severe and causing retinal degeneration. We report on the expression and characterization of the recently found T94I mutation associated with congenital night blindness, in the second transmembrane helix or rhodopsin, and mutations at the same site. T94I mutant rhodopsin folded properly and was able to bind 11-cis-retinal to form chromophore, but it showed a blue-shifted visible band at 478 nm and reduced molar extinction coefficient. Furthermore, T94I showed dramatically reduced thermal stability, extremely long lived metarhodopsin II intermediate, and highly increased reactivity toward hydroxylamine in the dark, when compared with wild type rhodopsin. The results are consistent with the location of Thr-94 in close proximity to Glu-113 counterion in the vicinity of the Schiff base linkage and suggest a role for this residue in maintaining the correct dark inactive conformation of the receptor. The reported results, together with previously published data on the other two known congenital night blindness mutants, suggest that the molecular mechanism underlying this disease may not be structural misfolding, as proposed for retinitis pigmentosa mutants, but abnormal functioning of the receptor by decreased thermal stability and/or constitutive activity.Naturally occurring mutations in rhodopsin (most of them single amino acid replacements) are associated with retinal disease. Most of these are the cause of retinitis pigmentosa (RP), 1 a group of inherited retinal degenerative diseases (1-3) that leads to blindness by causing photoreceptor cell death (4). Over 100 mutations have been found to date in the opsin gene associated with RP, most of them being inherited as an autosomal dominant trait (1). These are located in all the three domains of rhodopsin, namely the intradiscal, the transmembrane, and the cytoplasmic domains of the protein (5). Mutations in the transmembrane and intradiscal domains of rhodopsin that cause RP have been shown to cause misfolding of the mutant proteins (6 -8). Only a very small number of mutations have been associated with the retinal disease characterized by a congenital night blindness (CNB) phenotype. CNB appears to be a stable condition that does not seem to cause photoreceptor degeneration resulting mainly in night vision impairment. Two of these mutations were previously studied, namely G90D (9) and A292E (10) in transmembrane helices II and VII of rhodopsin, respectively. The mechanism of action of the G90D and A292E mutations was proposed to be persistent activation of the phototransduction pathway by constitutive activity of the mutant proteins (9 -11). Another possible explanation for the observed G90D mutant phenotype has been proposed, i.e. enhanced rate of thermal isomerization due to lo...