Dissecting the function of a protruding loop in AcrB trimerization

The resistance-nodulation-cell division family multidrug transporter Acriflavine resistance protein B (AcrB) from Eschericha coli is an obligate homotrimer. Approximately, 45% of the inter-subunit interface is contributed by a protruding loop (also referred to as the thumb) and its corresponding binding tunnel in the neighboring subunit. In an earlier study, we have demonstrated that a single Pro to Gly mutation in the loop drastically destabilized AcrB trimer and reduced its substrate efflux activity. To further dissect the role of the loop during AcrB trimerization, we performed Ala scanning of the loop and examined the effect of each mutation on protein activity. We found that not all conserved residues are important for AcrB function and likewise not all critical residues are conserved. In addition, we replaced the loop of AcrB with the loop of MexB, which is a highly conserved homolog of AcrB. The resultant chimeric protein remained partly active. Structural characterization of the chimeric protein i...