Structural Evidence for Functional Lipid Interactions in the Betaine Transporter BetP

Abstract

Bilayer lipids contribute greatly to the stability of membrane transporters and are crucially involved in their proper functioning. However, the molecular details of how these lipids affect membrane transport is limited to the few atomic resolution structures of channels and transporters revealing functional lipid interactions. As a consequence, despite their biological importance, bilayer lipid-protein interactions are rarely considered in the molecular description of a transport mechanism, which are difficult to detect in X-ray structures per se. Lipids are often depleted from the detergent-protein complex during isolation or appear too unordered to identify in the crystal. Overcoming these difficulties, we report here on a new structure of the osmotic stress-regulated betaine transporter BetP in complex with anionic lipids. Activity regulation in BetP depends strongly on the presence of negatively charged lipids. We observe eight fully resolved palmitoyl-oleoyl phosphatidyl glycerol (PG) lipids bound to one BetP trimer, mimicking parts of the membrane leaflets. Our data reveal that the PG lipids interact with key residues in transport and regulation. Lipids, while likely being involved in the correct assembly of the BetP trimer also offer important communication sites between the protomers that might be required during stress sensing and transport regulation. The lipid-protein interactions observed in BetP reiterate the influence that the surrounding membrane can have on membrane protein function and urge a more holistic approach towards understanding membrane transport, especially for LeuT-like fold transporters.