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Intracellular replication of the deadly pathogen Mycobacterium tuberculosis relies on the production of small organic molecules called siderophores that scavenge iron from host proteins.sup.1. M. tuberculosis produces two classes of siderophore, lipid-bound mycobactin and water-soluble carboxymycobactin.sup.2,3. Functional studies have revealed that iron-loaded carboxymycobactin is imported into the cytoplasm by the ATP binding cassette (ABC) transporter IrtAB.sup.4, which features an additional cytoplasmic siderophore interaction domain.sup.5. However, the predicted ABC exporter fold of IrtAB is seemingly contradictory to its import function. Here we show that membrane-reconstituted IrtAB is sufficient to import mycobactins, which are then reduced by the siderophore interaction domain to facilitate iron release. Structure determination by X-ray crystallography and cryo-electron microscopy not only confirms that IrtAB has an ABC exporter fold, but also reveals 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 show that IrtAB has a preference for mycobactin over carboxymycobactin as its substrate. Our study provides insights into an unusual ABC exporter that evolved as highly specialized siderophore-import machinery in mycobacteria. The mycobacterial ABC transporter IrtAB functions as a siderophore importer despite exhibiting an exporter fold in its structure, and contains a siderophore interaction domain capable of siderophore reduction and iron release inside the cell., Author(s): Fabian M. Arnold [sup.1] , Miriam S. Weber [sup.2] , Imre Gonda [sup.1] , Marc J. Gallenito [sup.3] , Sophia Adenau [sup.1] , Pascal Egloff [sup.1] [sup.5] , Iwan [...]

Mycobacterium marinum is a model organism for pathogenic Mycobacterium species, including Mycobacterium tuberculosis, the causative agent of tuberculosis. These pathogens enter phagocytes and replicate within the Mycobacterium‐containing vacuole, possibly followed by vacuole exit and growth in the host cell cytosol. Mycobacteria release siderophores called mycobactins to scavenge iron, an essential yet poorly soluble and available micronutrient. To investigate the role of M. marinum mycobactins, we purified by organic solvent extraction and identified by mass spectrometry the lipid‐bound mycobactin (MBT) and the water‐soluble variant carboxymycobactin (cMBT). Moreover, we generated by specialised phage transduction a defined M. marinum ΔmbtB deletion mutant predicted to be defective for mycobactin production. The M. marinum ΔmbtB mutant strain showed a severe growth defect in broth and phagocytes, which was partially complemented by supplying the mbtB gene on a plasmid. Furthermore, purified Fe‐MBT or Fe‐cMBT improved the growth of wild type as well as ΔmbtB mutant bacteria on minimal plates, but only Fe‐cMBT promoted the growth of wild‐type M. marinum during phagocyte infection. Finally, the intracellular growth of M. marinum ΔmbtB in Acanthamoeba castellanii amoebae was restored by coinfection with wild‐type bacteria. Our study identifies and characterises the M. marinum MBT and cMBT siderophores and reveals the requirement of mycobactins for extra‐ and intracellular growth of the pathogen. [ABSTRACT FROM AUTHOR]