Peripherin/rds is an integral membrane glycoprotein found in the rim regions of vertebrate photoreceptor cell discs. Natural mutations of the encoding gene result in degenerative retinal disorders, such as retinitis pigmentosa. The retinal degeneration slow (rds) phenotype, observed in mice, is considered to be an appropriate model for peripherin/rds-mediated retinitis pigmentosa. Associated abnormalities in the outer segment of photoreceptor cells have implicated peripherin/rds in some aspect of disc morphology, yet it remains unclear whether such morphological effects are the cause or the result of this condition. Here we present the first direct evidence to support a role for peripherin/rds in maintaining the flattened vesicle morphology characteristic of photoreceptor outer segments. In vitro expression yields a 36-kDa immunoreactive species, which is inserted into membranes and undergoes N-glycosylation, inter-and intramolecular disulfide bonding, and dimerization. Electron microscopy reveals that peripherin/rds flattens microsomal vesicles. This effect appears to be dependent on disulfide bond formation but not N-glycosylation. The inability of two pathogenic peripherin/rds mutants (P216L and C165Y) to flatten membrane vesicles implicates such mutations as the primary cause of the retinal degeneration observed in retinitis pigmentosa.The outer segment of the vertebrate rod photoreceptor cell exhibits a highly specialized structure, comprised of a stack of 1000 or more flattened vesicles or discs (1). This organization is vital to the visual process, as it maximizes the area available for photoreception and allows efficient renewal of the photoreceptor outer segments. Any elements involved in the formation and maintenance of this structure are, therefore, of vital importance to the visual process. One protein that has been implicated to have such an involvement is peripherin/rds. Peripherin/rds is a 39-kDa integral membrane glycoprotein localized exclusively to the rim regions of photoreceptor cell discs (2, 3). A topological model has been proposed (4) in which the protein possesses four transmembrane helices and cytosolically oriented N and C termini. The protein has been shown to form disulfide-linked homodimers (2). These peripherin/rds dimers and homodimers of the related disc rim protein ROM-1 (5) are believed to non-covalently associate to form a functional heterotetrameric complex (6, 7).The clinical significance of the human peripherin/rds gene is illustrated by its involvement in a wide range of degenerative retinal disorders, including retinitis pigmentosa (8). To date, over 50 pathogenic mutations within the human gene have been identified. In addition there is a mutation in the murine gene that causes the related disorder, retinal degeneration slow (rds) (9). Phenotypically rds mice exhibit distinctive photoreceptor degeneration (10, 11), and hence, murine rds is considered an important animal model for the inherited human retinal degenerative condition. Mice homozygous for the rds condition ha...

Peripherin/ rds is an integral membrane glycoprotein found in the rim regions of vertebrate photoreceptor cell discs. The protein is believed to be involved in both formation and maintenance of the characteristic flattened morphology of the outer segment discs and its essential nature is demonstrated by the wide range of retinal degenerative disorders in which the protein has an involvement. Little structural data has been determined for peripherin/ rds, but a topological model of the protein has been proposed. In this paper, we present the first direct evidence for the topology of the protein through the use of scanning glycosylation mutagenesis. Both the topological data and the observation that only the Asn(229) site is efficiently glycosylated in this in vitro transcription/translation system support the common hypotheses. Additionally, expression of the Pro(216)-->Leu mutant demonstrates an abnormal glycosylation pattern, which may explain the mechanism by which this mutation precipitates a retinal degenerative phenotype.

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