Mechanistic Insights into the Structural Asymmetry of the LanFEG Transporter NisFEG in Lantibiotic Immunity

Nisin is one of the best studied antimicrobial peptides. Still, how nisin-producing strains can protect themselves against nisin's bactericidal effects is only partially understood. Located within the nisin biosynthesis operon, the heterotetrameric ABC transporter NisFEG transports nisin to the extracellular environment, granting autoimmunity to the producer strain. NisFEG belongs to the LanFEG family of ABC transporters, members of which are found in some lantibiotic-producing bacterial strains. However, their structure has not been elucidated. In this work, we constructed a full atom model of NisFEG in the ATP-bound conformation. The architecture of the complex reveals a narrow contact interface between the two transmembrane chains, with prominent lateral clefts, similar to those observed in other exporters of hydrophobic compounds. Through molecular dynamics (MD) simulations, we observed that one of the most conserved elements of the LanFEG family, the E-loop of the nucleotide-binding domain, interacts preferentially with a small intracellular helix of the NisG transmembrane chain. By combining co-solvent MD simulations and predictions of the binding mode of the terminal segment of nisin, we could identify a putative interaction surface, located predominantly on NisE. Our results suggest that nisin extrusion operates in an asymmetric manner, where contacts between the E-loop and NisG are the driving force for the conformational changes triggered by ATP hydrolysis, whereas the NisE subunit is the main mediator of interactions with the lantibiotic. This functional asymmetry could explain why the LanFEG family has evolved two distinct transmembrane chains, where each one was selected to perform a single step in an optimal way, maximizing the immunity of lantibiotic-producing bacteria.