Structural insights into retinitis pigmentosa from unfolding simulations of rhodopsin mutants

Disease-causing missense mutations in membrane proteins, such as rhodopsin mutations associated with the autosomal dominant form of retinitis pigmentosa (ADRP), are often linked to defects in folding and/or trafficking. The mechanical unfolding of wild-type rhodopsin was compared with that of 20 selected ADRP-linked mutants more or less defective in folding and retinal binding. Rhodopsin fold is characterized by networks of amino acids in the retinal and G-protein binding sites likely to play a role in the stability and function of the protein. The distribution of highly connected nodes in the network reflects the existence of a diffuse intramolecular communication inside and between the 2 poles of the helix bundle, which makes pathogenic mutations share similar phenotypes irrespective of topological and physicochemical differences between them. Because of this communication, the ADRP-linked rhodopsin mutations share a more or less marked ability to impair selected hubs in the protein structure network. The extent of this structural effect relates to the severity of the biochemical defect caused by mutation. The investigative strategy employed in this study is likely to apply to all structurally known membrane proteins particularly susceptible to misassembly-causing mutations.