a new potential lipopolysaccharide transport partner

What is it about?

The Escherichia coli MacAB-TolC transporter has been implicated in efflux of macrolide antibiotics and secretion of enterotoxin STII. In this study, we found that purified MacA, a periplasmic membrane fusion protein, contains one tightly bound rough core lipopolysaccharide (R-LPS) molecule per MacA molecule. R-LPS was bound specifically to MacA protein with affinity exceeding that of polymyxin B. Sequence analyses showed that MacA contains two high-density clusters of positively charged amino acid residues located in the cytoplasmic N-terminal domain and the periplasmic C-terminal domain. Substitutions in the C-terminal cluster reducing the positive-charge density completely abolished binding of R-LPS. At the same time, these substitutions significantly reduced the functionality of MacA in the protection of E. coli against macrolides in vivo and in the in vitro MacB ATPase stimulation assays. Taken together, our results suggest that R-LPS or a similar glycolipid is a physiological substrate of MacAB-TolC.

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Why is it important?

MacAB-TolC compelx in E.coli was identified as the first ABC-type transporter involved in macrolide resistance and secretion of STII. Interestingly, a combination of several observations suggested that under specific growth conditions, MacAB-TolC is involved in the assembly or maintenance of the outer membrane of E.coli. My studied revealed for the first time that the periplasmic membrane fusion protein MacA binds to outer membrane component R-LPS with high affinity and the LPS binding ability of MacA directly affected the drug resistance function of MacAB-TolC complex, suggesting that MacAB-TolC exporter has a combined role in LPS transport and drug resistance. More importantly, I identified LPS binding motifs on the N- and C- terminal of MacA and demonstrated their roles in the complex formation and functions of MacAB-TolC. It is possible that the transport of LPS by MacAB-TolC is analogous to other LPS transport machineries. Therefore, MacAB-TolC can be considered as a potential drug target and further study the MacAB-TolC function under specific conditions could lead us to the discovery of new LPS translocate pathway.