3.A.1.21.2
The Fe³⁺-mycobactin/carboxymycobactin transporter, IrtAB (Rodriguez and Smith, 2006). IrtA contains an FAD-binding domain (Ryndak et al., 2010). M. tuberculosis produces two classes of siderophores, lipid-bound mycobactin and water-soluble carboxymycobactin. Iron-loaded carboxymycobactin is imported into the cytoplasm by IrtAB which has an additional cytoplasmic siderophore interaction domain that may serve as a periplasmic receptor. Membrane-reconstituted IrtAB is sufficient for the import of mycobactins, which are then reduced by the siderophore interaction domain to facilitate iron release (Arnold et al. 2020). Structure determination by X-ray crystallography and cryo-EM not only confirmed that IrtAB has an ABC exporter fold (as indicated by TC-BLAST searches), but also revealed structural peculiarities at the transmembrane region of IrtAB that result in a partially collapsed inward-facing substrate-binding cavity. The siderophore interaction domain is positioned in close proximity to the inner membrane leaflet, enabling the reduction of membrane-inserted mycobactin. Enzymatic ATPase activity and in vivo growth assays revealed that IrtAB has a preference for mycobactin over carboxymycobactin as its substrate (Arnold et al. 2020). The large transporter (Rv1348-1349) of 859 aas seems to have three domains, an extracytoplasmic receptor domain (IrtA), a central membrane domain of 6 TMSs (IrtB) and a C-terminal ATPase (IrtC). If so, there may be two potential receptors (see Mandal et al. 2021 for a review). Cryo-EM structures for the M. tuberculosis iron-loaded siderophore transporter IrtAB has been reported (Sun et al. 2023). It adopts the canonical type IV exporter fold. The structures of unliganded Mtb IrtAB in complex with ATP, ADP, or the ATP analogue (AMP-PNP) at resolutions ranging from 2.8 to 3.5 Å. The structure of IrtAB bound ATP-Mg²⁺ shows a "head-to-tail" dimer of nucleotide-binding domains (NBDs), a closed amphipathic cavity within the TMSs, and a metal ion liganded to three histidine residues of IrtA in the cavity. Cryo-EM structures and ATP hydrolysis assays showed that the NBD of IrtA has a higher affinity for nucleotides and increased ATPase activity compared with IrtB (Sun et al. 2023).