Mutations in the visual receptor rhodopsin are a common cause of inherited retinal diseases. Many of these mutations occur within the transmembrane core of the receptor and can impact in its folding and function. In this study, we investigate two mutations associated with the retinal degenerative disease retinitis pigmentosa, T108 3.23 P and G121 3.36 R, both located in the third transmembrane helix of rhodopsin and in proximity to key structural residues essential for rhodopsin stability and proper folding. We find that the G121 3.36 R mutant fails to regenerate with 11- cis -retinal, suggesting that the mutation severely impairs chromophore binding and proper folding. In contrast, the T108 3.23 P mutant regenerates efficiently, shows normal photobleaching and acidification profiles, and similar chromophore regeneration to that of the wild-type protein. However, the T108 3.23 P mutation leads to reduced thermal and chemical stability of its dark state, and a delayed formation of the active conformation. These conformational alterations correlate with a slower and less efficient activation of transducin, indicating that T108 3.23 P partially decouples the light-induced response from downstream G-protein signaling. Together, these findings demonstrate that amino acid substitutions in the third transmembrane helix of rhodopsin can have diverse molecular consequences: from the severe impairment of chromophore binding in G121 3.36 R to selective destabilization and signaling defects in the T108 3.23 P case. Our results emphasize the importance of the third transmembrane helix in supporting the conformational changes necessary for rhodopsin activation. and contribute to a deeper understanding of how specific mutations in rhodopsin may elicit distinct pathogenic mechanisms underlying retinal degeneration.
Fernandez-Gonzalez et al. (Wed,) studied this question.