Cysteine residues of photoreceptor peripherin/rds: role in subunit assembly and autosomal dominant retinitis pigmentosa

Abstract

Peripherin/rds is a tetraspanning membrane glycoprotein that is essential for the morphogenesis and stabilization of outer segments of vertebrate rod and cone photoreceptor cells. Mutations in the gene for peripherin/rds are responsible for retinal degeneration in the rds mouse and a variety of progressive human retinal degenerative diseases including autosomal dominant retinitis pigmentosa and macular dystrophy. Peripherin/rds associates with rom-1, a homologous subunit, to form a heterotetrameric complex. This study examines the importance of cysteine residues for the structure of peripherin/rds and its assembly with rom-1. Each of the 13 cysteine residues in bovine peripherin/rds was individually replaced with a serine residue by site-directed mutagenesis, and the resulting mutants were expressed individually or together with rom-1 in COS-1 cells. SDS-polyacrylamide gel electrophoresis, immunoprecipitation, and velocity sedimentation were carried out to evaluate the ability of these mutants to form disulfide-linked homodimers, associate with rom-1, and assemble into tetramers characteristic of wild-type peripherin/rds. Substitution of each of the six nonconserved cysteines had no apparent effect on dimer formation, folding, or subunit assembly. In contrast, replacement of any of the seven conserved cysteine residues predicted to lie within a 150 amino acid intradiscal loop significantly altered these properties. Six of these mutants, including a C214S mutant linked to autosomal dominant retinitis pigmentosa, were unable to fold normally, interact with rom-1, or self-assemble into tetramers but instead formed a mixture of large aggregates and a smaller component, most likely a dimer. The C150S mutant, on the other hand, was incapable of forming intermolecular disulfide bonds but did associate with rom-1 into a heterotetramer. These results suggest that (1) the conserved C150 residue is required for intermolecular disulfide bonding but not subunit assembly; (2) the six other conserved cysteine residues are crucial for proper folding and subunit assembly, possibly through formation of intramolecular disulfide bonds; and (3) the misfolding and defective subunit assembly of the C214S mutant is responsible for a form of monogenic autosomal dominant retinitis pigmentosa.