A mycobacterial ABC transporter mediates the uptake of hydrophilic compounds

Mycobacterium tuberculosis (Mtb) is an obligate human pathogen and the causative agent of tuberculosis^1-3. Although Mtb can synthesize vitamin B₁₂ (cobalamin) de novo, uptake of cobalamin has been linked to pathogenesis of tuberculosis². Mtb does not encode any characterized cobalamin transporter^4-6; however, the gene rv1819c was found to be essential for uptake of cobalamin¹. This result is difficult to reconcile with the original annotation of Rv1819c as a protein implicated in the transport of antimicrobial peptides such as bleomycin⁷. In addition, uptake of cobalamin seems inconsistent with the amino acid sequence, which suggests that Rv1819c has a bacterial ATP-binding cassette (ABC)-exporter fold¹. Here, we present structures of Rv1819c, which reveal that the protein indeed contains the ABC-exporter fold, as well as a large water-filled cavity of about 7,700 Å³, which enables the protein to transport the unrelated hydrophilic compounds bleomycin and cobalamin. On the basis of these structures, we propose that Rv1819c is a multi-solute transporter for hydrophilic molecules, analogous to the multidrug exporters of the ABC transporter family, which pump out structurally diverse hydrophobic compounds from cells^8-11.