An intramembrane glutamic acid governs peripherin/rds function for photoreceptor disk morphogenesis.

PURPOSE: Peripherin/rds (P/rds), the product of the retinal degeneration slow (rds) gene, is a tetraspanin protein that plays a pivotal role for photoreceptor outer segment (OS) structure and is involved in a broad spectrum of inherited retinal degenerations. P/rds interacts with the homologous protein rom-1, previously proposed to regulate P/rds function. The authors examined the significance of an intramembrane glutamic acid conserved in all P/rds proteins (and many other tetraspanins) but absent in all rom-1 orthologs.
METHODS: The authors performed isosteric glutamine substitution of the conserved glutamate at position 276, in the fourth transmembrane domain of bovine P/rds, and expressed E276Q P/rds in COS-1 cells and in transgenic mouse photoreceptors of rds +/+, -/+, and -/- backgrounds. Western blot, immunoprecipitation, and sedimentation analyses were used to assess protein structure and interactions. Microscopy and electroretinography were used to characterize transgenic protein localization and retinal photoreceptor structure and function.
RESULTS: E276Q P/rds was expressed, assembled, and properly localized in photoreceptor OSs of transgenic mice. In contrast to wild-type (WT) P/rds, however, this mutant did not rescue the OS structural defects observed in rds -/- and -/+ mice. Moreover, E276Q expression did not prevent the retinal degeneration that occurred as a consequence of OS disruption.
CONCLUSIONS: E276 plays a critical role in P/rds support of photoreceptor OS structure. This finding provides a molecular rationale for asymmetry in P/rds and rom-1 function and for rom-1 regulation of P/rds activity. These findings also suggest that ionizable intramembrane residues may serve regulatory roles for tetraspanin proteins more generally.