Mechanism of Membrane Transporters Our work aims to provide a molecular description of the transport mechanism(s) of sugar transporters from the Major Facilitator Superfamily (MFS) using a novel combination of mass-spectrometry based methods and modelling. According to the widely acknowledged alternating-access model, MFS transporters have to adopt at least two conformations open to opposite sides of the membrane to allow vectorial translocation of their substrate. However, a molecular description of the mechanism underlying such transition is not well established and the actual models fail to integrate the role of the lipid environment. By combining reconstitution in different lipid environments, mutagenesis and modelling, we hope to derive a detailed conformational landscape of the transporters XylE and LacY in lipid bilayers. We use native mass spectrometry to identify with high-resolution the binding of lipid species, ion-mobility mass spectrometry to assess the global conformational changes and the variation in stability of the protein, and hydrogen-deuterium exchange to follow the structural dynamics.