Targeted delivery of rhodopsin's assembled core is required for outer segment extension in mouse rod photoreceptors

Vertebrate vision in dim-light environments is initiated by rod photoreceptor cells that express the photopigment rhodopsin, a G protein-coupled receptor. To ensure efficient light capture, rhodopsin is densely packed into hundreds of tightly stacked membrane discs within the rod-shaped outer segment (OS) compartment. Along with its role in eliciting the visual response, rhodopsin serves as a building block necessary for proper OS formation and a trafficking guide for a few OS resident membrane proteins. An interesting aspect of rod homeostasis is that mutations that affect the localization of rhodopsin to the OS result in photoreceptor degeneration. In this study, we focus on determining the properties of rhodopsin's cytosolic C terminus required for either proper OS trafficking or the capacity to extend the rudimentary OS in rhodopsin KO rods. We find that the well-described C-terminal QVAPA OS targeting motif also plays a role in endoplasmic reticulum exit and is necessary for elongation of the OS compartment. We identify that rhodopsin's core, helix-8, CC anchor, and QVAPA targeting motif are the minimal requirements to extend the rudimentary OS in rhodopsin KO rods. Our findings provide useful insights into rhodopsin's molecular features needed for OS delivery and subsequent elongation of this membrane-rich compartment.