Membrane protein efflux pumps confer antibiotic resistance by extruding structurally distinct compounds and lowering their intracellular concentration. Yet there are no clinically approved drugs to inhibit efflux pumps, which would potentiate the efficacy of existing antibiotics rendered ineffective by drug efflux. Here we identified synthetic antigen-binding fragments (Fabs) that inhibit the quinolone transporter NorA from methicillin-resistant
Staphylococcus aureus
(MRSA). Structures of two NorA-Fab complexes determined using cryo-electron microscopy reveal a Fab loop deeply inserted in the substrate binding pocket of NorA. An arginine residue on this loop interacts with two neighboring aspartate and glutamate residues essential for NorA-mediated antibiotic resistance in MRSA. Peptide mimics of the Fab loop inhibit NorA with sub-micromolar potency and ablate MRSA growth in combination with the antibiotic norfloxacin. These findings establish a class of peptide inhibitors that block antibiotic efflux in MRSA by targeting indispensable residues in NorA without the need for membrane permeability.
composition are coupled to fluctuations in the local membrane curvature, as in the Leibler-Andelman mechanism. Structure factors of components in both leaves display a peak at non-zero wave vector which is a defining characteristic of a microemulsion. This indicates that the disordered fluid membrane is characterized by structure of the corresponding wavelength. The scale is given primarily by membrane properties: its bending modulus and its surface tension that arises from the membrane's connections to the cytoskeleton. From measurements on the plasma membrane, this scale is on the order of 100 nm. We find that the membrane can be divided into two different kinds of domains that differ not only in their composition but also in their curvature. The first domain in the outer, exoplasmic, leaf is rich in cholesterol and sphingomyelin, while the inner, cytoplasmic, leaf is rich in phosphatidylserine and phosphatidylcholine. The second kind of domain is rich in phosphatidylcholine in the outer leaf, and in cholesterol and phosphatidylethanolamine in the inner leaf. The theory provides a tenable basis for the origin of structure in the plasma membrane, and an illuminating picture of the organization of the lipids therein.
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