Preacinetobactin not acinetobactin is essential for iron uptake by the BauA transporter of the pathogen Acinetobacter baumannii

Abstract

New strategies are urgently required to develop antibiotics. The siderophore uptake system has attracted considerable attention, but rational design of siderophore antibiotic conjugates requires knowledge of recognition by the cognate outer-membrane transporter. Acinetobacter baumannii is a serious pathogen, which utilizes (pre)acinetobactin to scavenge iron from the host. We report the structure of the (pre)acinetobactin transporter BauA bound to the siderophore, identifying the structural determinants of recognition. Detailed biophysical analysis confirms that BauA recognises preacinetobactin. We show that acinetobactin is not recognised by the protein, thus preacinetobactin is essential for iron uptake. The structure shows and NMR confirms that under physiological conditions, a molecule of acinetobactin will bind to two free coordination sites on the iron preacinetobactin complex. The ability to recognise a heterotrimeric iron-preacinetobactin-acinetobactin complex may rationalize contradictory reports in the literature. These results open new avenues for the design of novel antibiotic conjugates (trojan horse) antibiotics.

Acinetobacter baumannii can use multiple siderophores for iron acquisition, but only acinetobactin is required for virulence

“…However, whilst the affinity of the baumannoferrins for iron has not been elucidated, pre-acinetobactin binds iron with a comparable affinity to fimsbactins A (log K Fe = 27.1 ± 0.2 versus 27.4 ± 0.2) and acinetobactin binds iron with a lower affinity (log K Fe = 26.2 ± 0.2) [ 38 , 55 ], suggesting that iron affinity alone does not readily explain the importance of acinetobactin. Alternatively, ferric-acinetobactin may be captured and utilized more efficiently than the other two siderophores, supported partly by the observation that the receptor protein for acinetobactin (BauA) binds a heterotrimeric acinetobactin:preacinetobactin:Fe(III) complex or a preacinetobactin 2 :Fe(III) complex with nanomolar affinity prior to uptake of the siderophore [ 69 ]. The specificity and affinity of the predicted baumannoferrin (BfnH) and fimsbactins (FbsN) receptors have not been defined, and thus further research is required to determine if efficacy in siderophore transport contributes to the utility of the siderophores.…”

CDR-H3 loop ensemble in solution – conformational selection upon antibody binding

“…54 The box size in MD simulations can influence the resulting dynamics if sampling is insufficient. 55,56 For all crystal structures parameters of the AMBER force field 14SB were used. 57 The antibodies were carefully equilibrated using a multistep equilibration protocol.…”

Key innovations in transition from homospory to heterospory

“…Recently, Fu et al reported the earliest fossil flower from the Early Jurassic. 1 This finding brought forward the time of angiosperm origination to the Early Jurassic, almost 50 million y before the previous record. Some questions regarding to key evolutionary innovations such as how the earliest flower looks like can be solved by discovery of fossils.…”