The GARP Domain of the Rod CNG Channel's β1-subunit Contains Distinct Sites for Outer Segment Targeting and Connecting to the Photoreceptor Disc Rim

Abstract

Vision begins when light is captured by the outer segment organelle of photoreceptor cells in the retina. Outer segments are modified cilia filled with hundreds of flattened disc-shaped membranes. Disc membranes are separated from the surrounding plasma membrane and each membrane type has unique protein components. The mechanisms underlying this protein sorting remain entirely unknown. In this study, we investigated the outer segment delivery of the rod cyclic nucleotide-gated (CNG) channel, which is located in the outer segment plasma membrane where it mediates the electrical response to light. We now show that the targeted delivery of the CNG channel to the outer segment requires pre-assembly of its constituent 1 and {beta}1 subunits and that CNG{beta}1 contains specific targeting information encoded within the glutamic acid-rich region of its N-terminal GARP domain. We also found that the GARP domain connects the CNG channel to photoreceptor disc rims likely through an interaction with peripherin-2 and demonstrated that this function is performed by a proline-enriched region adjacent to the GARP domain. Our data reveal fine functional specializations within the structural domains of the CNG channel and suggest that channel delivery to the outer segment is independent of peripherin-2 interactions. Significance StatementThe precise delivery and organization of signaling proteins in the ciliary outer segment organelle of photoreceptor cells is critical for light detection. We report that the CNG channel, mediating the electrical response to light in rods, contains a region within the N-terminus of its CNG{beta}1 subunit that encodes the outer segment targeting information for the entire channel. This targeting region is adjacent to a region that connects CNG{beta}1 to the rims of photoreceptor discs, likely determining the subcellular compartmentalization of the CNG channel into the outer segment plasma membrane.