Membrane curvature generation by a C-terminal amphipathic helix in peripherin-2/rds, a tetraspanin required for photoreceptor sensory cilium morphogenesis

Abstract

Vertebrate vision requires photon absorption by photoreceptor outer segments (OSs), structurally elaborate membranous organelles derived from non-motile sensory cilia. OS structure/function depends on a precise stacking of hundreds of membranous disks. Each disk is fully (as in rods) or partially (as in cones) bounded by a rim, at which the membrane is distorted into an energetically unfavorable high curvature bend; however, the mechanism(s) underlying disk rim structure are not established. Here we demonstrate that the intrinsically disordered cytoplasmic C-terminus of the photoreceptor tetraspanin peripherin-2/rds (P/rds) can directly generate membrane curvature. A P/rds C-terminal domain (CTER) and a peptide mimetic of an amphipathic helix (AH) contained within it (CHR) each generated curvature in liposomes with a composition similar to that of OS disks and in liposomes generated from native OS lipids. CTER association with liposomes required conical phospholipids, and was promoted by membrane curvature and anionic surface charge, results suggesting that the P/rds C-terminal AH can partition into the cytosolic membrane leaflet to generate curvature via a hydrophobic insertion (wedging) mechanism. This activity was evidenced in full-length P/rds by its induction of small diameter tubulovesicular membrane foci in cultured cells. In sum, the findings suggest that curvature generation by the P/rds C-terminus may contribute to the distinctive structure of OS disk rims, and provide insight into how inherited defects in P/rds can disrupt organelle structure to cause retinal disease. They also raise the possibility that tethered AHs can function for shaping cellular membranes more generally.