Mutations in the visual photoreceptor rhodopsin are the cause of the retinal degenerative disease retinitis pigmentosa. Some naturally occurring mutations can lead to protein conformational instability. Two such mutations, NSSK and G90V, in the first and second transmembrane helices of the receptor, have been associated with sector and classical retinitis pigmentosa, respectively, and showed enhanced thermal sensitivity. We have carefully analyzed the effect of phospholipid bicelles on the stability and ligand binding properties of these two mutants and compared it with those of the detergent-solubilized samples. We have used a phospholipid bilayer consisting of 1,2-dimyristoyl-sn-glycero-3-phosphatidylcholine (DMPC) and 1,2-dihexanoyl-sn-glycero-3-phosphocholine (DHPC). We find that DMPC/DHPC 0 50 100 bicelles dramatically increase the thermal stability of the rhodopsin mutants G90V and N55K. The chromophore stability and regeneration of the mutants were also increased in bicelles when compared to their behavior in a dodecyl maltoside detergent solution. The retinal release process was slowed in bicelles, and chromophore entry, after illumination, was improved for the G90V mutant but not for N55K. Furthermore, fluorescence spectroscopy measurements showed that bicelles allowed more exogenous retinal binding to the photoactivated G90V mutant than in a detergent solution. In contrast, N55K could not reposition any chromophore either in the detergent or in bicelles. The results demonstrate that DMPC/DHPC bicelles can counteract the destabilizing effect of the disease-causing mutations and can modulate the structural changes in the ensuing receptor photoactivation in a distinct specific manner for different retinitis pigmentosa mutant phenotypes.
