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

Abstract

SummaryVertebrate vision requires photon absorption by photoreceptor outer segments (OSs), structurally elaborate membranous organelles derived from non-motile sensory cilia. The structure and function of OSs 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 is (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 and a peptide mimetic of an amphipathic helix contained within it each generated curvature in liposomes with a composition similar to that of OS disks and in liposomes generated from native OS lipids. Association of the C-terminal domain with liposomes required conical phospholipids, and was promoted by membrane curvature and anionic surface charge, results suggesting that the P/rds C-terminal amphipathic helix can partition into the cytosolic membrane leaflet to generate curvature by 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 Cterminus contributes 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 amphipathic helices can function for shaping cellular membranes more generally.

Submembrane Assembly and Renewal of Rod Photoreceptor cGMP-Gated Channel: Insight into the Actin-Dependent Process of Outer Segment Morphogenesis

“…Peripherin/rds is a structural protein required for the integrity of the disk membrane structure. Trafficking of peripherin/rds to the evagination is consistent with the role of peripherin/rds in inducing membrane curvature (Wrigley et al, 2000;Kevany et al, 2013;Khattree et al, 2013). Overgrown evaginations, as observed by electron microscopy in this and previous studies (Williams et al, 1988), would require a larger edge with high membrane curvature, to which additional peripherin/rds can contribute.…”

Tetraspanins at a glance

“…With more than 50 residues, the C-terminal cytoplasmic domains of peripherin/RDS and ROM-1 are, among tetraspanins, exceptionally long. This domain in peripherin/RDS has been suggested to have a fusogenic activity (Boesze-Battaglia et al, 2003) and was recently shown to contain an amphiphatic helix that is able to generate membrane curvature (Khattree et al, 2013), suggesting that there is a direct role of peripherin/RDS in generating the high-curvature bend of the disk rim.…”

Gene Therapy for PRPH2-Associated Ocular Disease: Challenges and Prospects

“…Although the complete role of peripherin-2 in this process is not yet fully elucidated, the carboxy-terminal domain of peripherin-2 has been shown to play a role in membrane fusion (Boesze-Battagliaa andStefano 2002, 2003), initiation of rim membrane curvature (Khattree et al 2013), and OS targeting (Tam et al 2004;Tian et al 2014). The D2 loop and covalently linked peripherin-2 complexes are required for the maintenance of the flattened rim morphology (Wrigley et al 2000;Chakraborty et al 2009Chakraborty et al , 2010.…”