Angelo Felline, Michele Seeber, Francesco Rao, and Francesca Fanelli
Abstract: Retinitis pigmentosa (RP) refers to a group of debilitating, hereditary disorders that cause severe visual impairment in as many as 1.5 million patients worldwide. Rhodopsin mutations account for >25% of the autosomal dominant form of the disease (ADRP). Forty artificial and ADRP-associated mutations located in the second extracellular loop (EL2) that folds into a twisted Î²-hairpin were screened through replica exchange molecular dynamics (REMD) simulations using the FACTS implicit solvent model. According to in vitro experiments, ADRP-linked mutants fail to express at the plasma membrane and/or to reconstitute with 11- cis-retinal, indicative of variable defects in protein folding and/or stability. The computational protocol was first probed on the protein G C-terminal Î²-hairpin, proving the effectiveness of the implicit solvent model in reproducing the free energy landscape of Î²-hairpin formation. Eight out of the 40 EL2 mutants resulted in misfolding effects on the native Î²-hairpin structure, consistent with in vitro evidence that they all share severe impairments in folding/expression. Five mutants displayed moderate misfolding attitudes, whereas the remaining 27 mutants, overall characterized by milder effects on rhodopsin expression, did not perturb significantly the conformational behavior of the native Î²-hairpin but are expected to exert variably disturbing effects on the native interactions of the loop with the chromophore and/or the surrounding receptor domains. Collectively, the results of this study add structural insight to the poorly resolved biochemical behavior of selected class II ADRP mutations, a fundamental step toward an understanding of the atomistic causes of the disease.