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 α1 and β1 subunits and that CNGβ1 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.

Compartmentalization of Photoreceptor Sensory Cilia

“…Both PRCD and GC1 require rhodopsin for their stability and trafficking to the OS (Pearring et al, 2015 ; Spencer et al, 2016 ). A recent study confirmed trafficking of GC1 through the conventional pathway (Pearring et al, 2020 ).…”

“…Until recently, peripherin 2 was the only OS-destined protein identified that is transported through the unconventional pathway. However, evidence suggests that R9AP traffics to the OS independently of rhodopsin, suggesting that it may transport via the unconventional secretory pathway (Pearring et al, 2014 ) and a recent study has shown that ABCA4 also traffics via the unconventional pathway (Pearring et al, 2020 ).…”

“…However, a recent study claims to cast doubt on the role of peripherin 2 for CNG channel trafficking to the OS (Pearring et al, 2020 ). This study showed that CNG channels are delivered via the conventional secretory pathway, whereas peripherin 2 is delivered exclusively through the unconventional pathway (Tian et al, 2014 ).…”