Phospholipid scrambling by rhodopsin

Abstract

Rhodopsin has been intensively characterized in its role as a visual pigment and G protein-coupled receptor responsible for dim-light vision. We recently discovered that it also functions as an ATP-independent phospholipid scramblase: when reconstituted into large unilamellar vesicles, rhodopsin accelerates the normally sluggish transbilayer translocation of common phospholipids by more than 1000-fold, to rates in excess of 10,000 phospholipids transported per rhodopsin per second. Here we summarize the work leading to this discovery and speculate on the mechanism by which rhodopsin scrambles phospholipids. We also present a hypothesis that rhodopsin’s scramblase activity is necessary for the function of the ABC transporter ABCA4 that is responsible for mitigating the toxic accumulation of 11-cis-retinal and bis-retinoids in the retina.

Structural basis for phospholipid scrambling in the TMEM16 family

“…Since this cleft is of appropriate size to accommodate their headgroups, lipids may diffuse along this path on their way across the membrane. Alternatively, also other mechanisms to lower the barrier for lipid flip-flop are conceivable (see for example [68]). The structure has revealed a regulatory calcium binding site that is located within the membrane, in proximity to the subunit cavity, but the mechanism by which Ca 2+ activates the protein is still ambiguous.…”

“…Recently, rhodopsin was also associated with scrambling processes. However, compared to TMEM16 proteins, its structure and activity profile suggests a fundamentally different scrambling mechanism [68][69][70].…”

On the molecular mechanism of flippase- and scramblase-mediated phospholipid transport

“…Therefore, there must be a mechanism that makes rhodopsin silent in membranes where asymmetry must be preserved. It is possible that the lipid bilayer composition restricts rhodopsin scramblase activity at the plasma membrane, for example through the high sterol content found in these membranes [180].…”

A Fluorescence-based Assay of Phospholipid Scramblase Activity

“…Scramblases are phospholipid transporters that increase the intrinsically slow rate of transbilayer phospholipid movement to physiologically appropriate levels in a bidirectional, ATP-independent manner 4–6 . Examples of their actions can be found in the endoplasmic reticulum and bacterial cytoplasmic membrane where constitutive scrambling is needed for membrane homeostasis and growth, as well as for a variety of glycosylation pathways 5 .…”

“…Regulated phospholipid scrambling is needed to expose phosphatidylserine (PS) on the surface of apoptotic cells where it acts as an “eat-me”-signal for macrophages 7 and provides a procoagulant surface on activated blood platelets to catalyze the production of protein factors needed for blood clotting. In photoreceptor disc membranes, rhodopsin’s scrambling activity has been suggested to counteract the phospholipid imbalance between the two membrane leaflets of the bilayer that is generated by the ATP-dependent, unidirectional lipid flippase ABCA4 4,8,910–12 .…”