title: Opsin is a phospholipid flippase.
abstract: Polar lipids must flip-flop rapidly across biological membranes to sustain cellular life [1, 2], but flipping is energetically costly [3] and its intrinsic rate is low. To overcome this problem, cells have membrane proteins that function as lipid transporters (flippases) to accelerate flipping to a physiologically relevant rate. Flippases that operate at the plasma membrane of eukaryotes, coupling ATP hydrolysis to unidirectional lipid flipping, have been defined at a molecular level [2]. On the other hand, ATP-independent bidirectional flippases that translocate lipids in biogenic compartments, e.g., the endoplasmic reticulum, and specialized membranes, e.g., photoreceptor discs [4, 5], have not been identified even though their activity has been recognized for more than 30 years [1]. Here, we demonstrate that opsin is the ATP-independent phospholipid flippase of photoreceptor discs. We show that reconstitution of opsin into large unilamellar vesicles promotes rapid (τ<10 s) flipping of phospholipid probes across the vesicle membrane. This is the first molecular identification of an ATP-independent phospholipid flippase in any system. It reveals an unexpected activity for opsin and, in conjunction with recently available structural information on this G protein-coupled receptor [6, 7], significantly advances our understanding of the mechanism of ATP-independent lipid flip-flop.
authors.affiliation: Department of Biochemistry, Weill Cornell Medical College, New York, NY 10065, USA.
authors.forename: Indu
authors.initials: I
authors.lastname: Menon
authors.forename: Thomas
authors.initials: T
authors.lastname: Huber
authors.forename: Sumana
authors.initials: S
authors.lastname: Sanyal
authors.forename: Sourabh
authors.initials: S
authors.lastname: Banerjee
authors.forename: Patrick
authors.initials: P
authors.lastname: Barré
authors.forename: Sam
authors.initials: S
authors.lastname: Canis
authors.forename: J David
authors.initials: JD
authors.lastname: Warren
authors.forename: John
authors.initials: J
authors.lastname: Hwa
authors.forename: Thomas P
authors.initials: TP
authors.lastname: Sakmar
authors.forename: Anant K
authors.initials: AK
authors.lastname: Menon
mesh_terms: D000255:Adenosine Triphosphate / Q000378:metabolism; D000818:Animals; D002417:Cattle; D005786:Gene Expression Regulation; D057809:HEK293 Cells; D006801:Humans; D008565:Membrane Proteins / Q000235:genetics / Q000378:metabolism; D055355:Opsins / Q000737:chemistry* / Q000378:metabolism*; D010743:Phospholipids / Q000378:metabolism*; D020419:Photoreceptor Cells, Vertebrate
chemical_list: D008565:Membrane Proteins; D055355:Opsins; D010743:Phospholipids; D000255:Adenosine Triphosphate