Lipid flipping in the omega-3 fatty-acid transporter

Abstract

AbstractMfsd2a is the primary transporter for the docosahexaenoic acid (DHA), an omega-3 fatty acid, across the blood brain barrier (BBB). Defects in Mfsd2a are linked to ailments from behavioral, learning, and motor dysfunctions to severe microcephaly. Mfsd2a typically transports long-chain unsaturated fatty-acids, including DHA and α-Linolenic acid (ALA), that are attached to the zwitterionic lysophosphatidylcholine (LPC) headgroup. Even with two recently determined structures of Mfsd2a the molecular details of how this transporter performs the energetically unfavorable task of translocating and flipping lysolipids across the lipid bilayer remained unclear. Here, we report five single-particle cryo-EM structures of the Danio rerio Mfsd2a (drMfsd2a): in the inward-open conformation in the ligand-free state and bound to ALA-LPC at four unique positions along the substrate translocation pathway. These Mfsd2a snapshots detail the Na+-dependent flipping mechanism of the lipid-LPC from outer to inner membrane leaflet during ligand translocation through the Mfsd2a substrate tunnel and release for membrane integration on the cytoplasmic side. These results also map Mfsd2a mutants that disrupt lipid-LPC transport and are associated with known disease. Together these results provide a model for omega-3 fatty-acid transport and has the potential for the design of the delivery strategies for amphipathic drugs across the BBB.