A cholesterol switch controls phospholipid scrambling by G protein-coupled receptors

Abstract

Class A G protein-coupled receptors (GPCRs), a superfamily of cell membrane signaling receptors, moonlight as constitutively active phospholipid scramblases. The plasma membrane of metazoan cells is replete with GPCRs, yet has a strong resting trans-bilayer phospholipid asymmetry, with the signaling lipid phosphatidylserine confined to the cytoplasmic leaflet. To account for the persistence of this lipid asymmetry in the presence of GPCR scramblases, we hypothesized that GPCR-mediated lipid scrambling is regulated by cholesterol, a major constituent of the plasma membrane. We now present a technique whereby synthetic vesicles reconstituted with GPCRs can be supplemented with cholesterol to a level similar to that of the plasma membrane and show that the scramblase activity of two prototypical GPCRs, opsin and the β1-adrenergic receptor, is impaired upon cholesterol loading. Our data suggest that cholesterol acts as a switch, inhibiting scrambling above a receptor-specific threshold concentration to disable GPCR scramblases at the plasma membrane.

GPCR surface creates a favorable pathway for membrane permeation of drug molecules

“…Although GPCRs are mainly known for their essential roles in signal transduction across the plasma membrane, recent in vitro studies [6, 7, 8] as well as molecular dynamics simulations [9] have revealed a moonlighting activity of these receptors: lipid scrambling. Flip-flops are rarely performed spontaneously due to the high energy barrier for the polar lipid headgroup to traverse the region of the non-polar tails.…”