Evidence for a trap-and-flip mechanism in a proton-dependent lipid transporter

Transport of lipids across membranes is fundamental for diverse biological pathways in cells. Multiple ion-coupled transporters participate in lipid translocation, but their mechanisms remain largely unknown. Major facilitator superfamily (MFS) lipid transporters play central roles in cell wall synthesis, brain development and function, lipids recycling, and cell signaling. Recent structures of MFS lipid transporters revealed overlapping architectural features pointing towards a common mechanism. Here we used cysteine disulfide trapping, molecular dynamics simulations, mutagenesis analysis, and transport assays in vitro and in vivo, to investigate the mechanism of LtaA, a proton-dependent MFS lipid transporter essential for lipoteichoic acids synthesis in the pathogen Staphylococcus aureus. We reveal that LtaA displays asymmetric lateral openings with distinct functional relevance and that cycling through outward- and inward-facing conformations is essential for transport activity. We demonstrate that while the entire amphipathic central cavity of LtaA contributes to lipid binding, its hydrophilic pocket dictates substrate specificity. We propose that LtaA catalyzes lipid translocation by a trap-and-flip mechanism that might be shared among MFS lipid transporters.

Lipid Translocation (Flip-Flop) As One of the Least Understood Dynamical Processes in Articular Cartilage Membrane

The lipids molecules translocation in phospholipid membrane is named the (flip-flop) event. For this purpose, the translocation may take place in vivo with the surface of articular cartilage. Flip-flops are one of the least understood dynamical processes in vivo in cartilage membranes. In this work, we explain the molecular mechanism lipid translocation (flip-flop) in vivo during the drying process cartilage surface. Wettability of cartilage surface vs. drying time was determined. Our findings strongly support the idea that the process of translocation lipid (flip-flop) transformed hydrophilic surface in hydrophobic in dry-air condition for healthy and osteoarthritic cartilage. Such material can be classified cartilage as “smart material” and this fact is poorly known in the literature.

Cryo-EM structures capturing the entire transport cycle of the P4-ATPase flippase

In eukaryotic membranes, P4-ATPases mediate the translocation of phospholipids from the outer to inner leaflet and maintain lipid asymmetry, which is critical for protein trafficking and signaling pathways. Here we report the cryo-EM structures of six distinct intermediates of the human ATP8A1-CDC50a hetero-complex, at 2.6–3.3 Å resolutions, revealing the entire lipid translocation cycle of this P4-ATPase. ATP-dependent phosphorylation induces a large rotational movement of the actuator domain around the phosphorylation site, accompanied by lateral shifts of the first and second transmembrane helices, thereby allowing phosphatidylserine binding. The phospholipid head group passes through the hydrophilic cleft, while the acyl chain is exposed toward the lipid environment. These findings advance our understanding of the flippase mechanism and the disease-associated mutants of P4-ATPases.One Sentence SummaryCryo-EM reveals lipid translocation by P4-type flippase.