The mycobacterial ATP-binding cassette (ABC) transporter IrtAB has drawn significant attention from researchers for its unique ability to import iron-charged siderophores known as mycobactins. Traditionally, ABC transporters are known for exporting substances from the cell; IrtAB, conversely, functions distinctly by facilitating the uptake of these iron-binding molecules. Recent investigations have uncovered the transporter's conformational dynamics and the role of specific structural features, particularly emphasizing its membrane-facing crevice.
Iron is indispensable for the survival of many microorganisms, including pathogenic mycobacteria such as Mycobacterium tuberculosis, which causes tuberculosis. To evade host immune defenses, these pathogens have evolved sophisticated iron-scavenging systems utilizing solubilized small molecules termed siderophores. Among these, mycobactins represent the most pivotal class of siderophores for mycobacteria. Understanding how IrtAB transports iron-loaded mycobactin could illuminate new approaches to combat mycobacterial infections.
The research team employed advanced cryo-electron microscopy (cryo-EM) and double electron-electron resonance (DEER) measurements to evaluate how IrtAB operates at the molecular level. The resulting structural analyses revealed significant insights: IrtAB exhibits alternating inward-facing (IF) and outward-occluded (OFoccl) conformations but does not adopt the typical outward-facing structure. This unique behavior sets the stage for its specific import mechanism. Notably, cryo-EM structures indicated the transporter has a specialized binding site located at the heterodimer interface, which serves as the entry for mycobactin.
Extensive evidence indicates mycobactin likely traverses this membrane-facing crevice, penetrating through to internal pockets where it facilitates iron release. The study presented compelling data supporting this ‘credit-card’ style import mechanism, where the polar head group of mycobactin is engaged during passage, leaving the lipophilic tail embedded within the lipid bilayer. The presence of the HHH-motif, which coordinates zinc ions, is integral to initiating and regulating this transport process. Mutations affecting this motif resulted in decreased transport activity, reinforcing its necessity for maintaining IrtAB's function as both iron carrier and energy regulator.
Critically, the researchers highlighted the importance of specific residues associated with the transporter’s binding strategies. For example, the mutation of Q249IrtB—a residue involved significantly with mycobactin binding—demonstrated enhanced ATPase activity when not bound to the substrate, indicating altered regulatory dynamics. Elevated basal activities of some variants suggest there exists potential for them to sense mycobactin binding independently of its transport function.
This comprehensive study not only sheds light on the functionality of IrtAB but also points to broader implications for antimicrobial therapies. The distinct method by which IrtAB engages with its substrate could inspire new strategies for drug development targeting mycobacterial infections. The interplay between iron and zinc acquisition systems may open avenues for therapeutic modulation throughout infection cycles.
Future studies are necessary to fully elucidate how IrtAB balances iron import with zinc regulation, particularly under various physiological conditions. Investigations could explore targeting IrtAB for therapeutic interventions aimed at disrupting mycobacterial iron acquisition, offering potential pathways for novel treatments against persistent infections.