A nucleotide-sugar transporter supplies an activated donor across a membrane to a compartment containing glycosyltransferases. Whole-cell donor abundance does not establish donor availability at a lumen-facing enzyme.
A scramblase facilitates relatively nonselective bidirectional movement of lipids between membrane leaflets, tending to dissipate asymmetry when active. Its action differs from directional transport used to maintain asymmetry.
A flippase catalyzes directional movement of selected lipids between membrane leaflets. Such activity can establish or maintain lipid asymmetry, subject to specificity and energy coupling.
An ATP-binding cassette transporter couples nucleotide-binding-domain activity to transmembrane transport. For an exporter, productive substrate movement must be measured separately from substrate binding and ATP hydrolysis.
AcrB is a bacterial proton-coupled multidrug transporter that functions as part of the AcrAB–TolC export assembly. Its asymmetric trimer and multiple drug-binding regions are structural models for studying broad specificity.[ref]
Crystal structures of a multidrug transporter reveal a functionally rotating mechanism by Satoshi Murakami and others. Figs. 1–5, drug-bound asymmetric AcrB trimer
A multidrug efflux pump exports multiple chemical substrates using a coupled source of free energy. Broad recognition can arise from overlapping binding sites and conformational flexibility, but binding alone does not establish productive transport.
A membrane carrier protein binds a solute and changes its exposed binding site from one side of a cell membrane to the other. Depending on its mechanism and coupling, it can mediate passive transport or active transport.
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