Peripherin-2 and ROM1 Incorporate Directly Into the Rims of Enclosing Photoreceptor Discs Without Accumulating in the Nascent Disc Lamellae.

Purpose: Oligomeric complexes of peripherin-2 and ROM1 support the rim structure of membrane discs stacked inside the light-sensitive outer segment of vertebrate photoreceptor cells. We investigated the route by which peripherin-2 and ROM1 reach their destination within the disc rims. We addressed two possible mechanisms: first, whether these proteins accumulate within the lamellae of newly forming discs prior to their enclosure, after which they incorporate into the rims, or second, whether they incorporate directly into the rims of discs undergoing the process of enclosure. Methods: Subcellular localization of endogenous and myc-tagged peripherin-2, ROM1, and the cyclic nucleotide-gated (CNG) channel in rod photoreceptors of wild type mice was analyzed by their co-immunostaining with prominin-1, a protein marker labeling the expanding edges of nascent discs. Myc-tagged peripherin-2 was introduced into rods by adeno-associated virus transduction. The preservation of outer segment ultrastructure in immunostained tissue was confirmed by electron microscopy. Results: We observed a minimal overlap between immunostaining of peripherin-2/ROM1 and prominin-1. Whereas prominin-1 was primarily detected at the base of the outer segment, where nascent discs are formed, peripherin-2 and ROM1 were observed more distally at a location where discs undergo enclosure. The CNG channel subunits were similarly not detected in the lamellae of newly forming discs but were robustly stained in the plasma membrane enclosing mature outer segments. Conclusions: Our data confirm the previously established strong affinity of peripherin-2 for highly curved disc rims and demonstrate that ROM1 exhibits a similar preference; neither protein significantly accumulates in disc lamellae prior to enclosure.