Unveiling the substrate specificity of the ABC transporter Tba and its role in glycopeptide biosynthesis.

Glycopeptide antibiotics (GPA) such as vancomycin are essential last-resort antibiotics produced by actinomycetes. Their biosynthesis is encoded within biosynthetic gene clusters, also harboring genes for regulation, and transport. Diverse types of GPAs have been characterized that differ in peptide backbone composition and modification patterns. However, little is known about the ATP-binding cassette (ABC) transporters facilitating GPA export. Employing a multifaceted approach, we investigated the substrate specificity of GPA ABC-transporters toward the type-I GPA balhimycin. Phylogenetic analysis suggested and -complementation experiments confirmed that balhimycin is exported only by the related type I GPA transporters Tba and Tva (transporter of vancomycin). Molecular dynamics simulations and mutagenesis experiments showed that Tba exhibits specificity toward the peptide backbone rather than the modifications. Unexpectedly, deletion or functional inactivation of Tba halted balhimycin biosynthesis. Combined with proximity biotinylation experiments, this suggested that the interaction of the active transporter with the biosynthetic machinery is required for biosynthesis.