Structural basis of lipid-linked galactan export by the mycobacterial ABC transporter Wzm-Wzt

Abstract Mycobacteria, including Mycobacterium tuberculosis , possess a unique cell envelope containing arabinogalactan, a heteropolysaccharide critical for cell wall integrity and target of several tuberculosis drugs. The cytosolic precursor of arabinogalactan, lipid-linked galactan (LLG), is translocated across the plasma membrane by the essential ABC transporter Wzm-Wzt through a molecular mechanism that is poorly understood. Here, we present a series of cryo-EM structures of Wzm-Wzt from Mycobacterium abscessus , representing different conformations of the transport cycle. Conserved residues lining the proposed LLG translocation pathway were investigated by three orthologous functional assays, revealing that the cytosolic gate helix (GH) plays a key functional role in polysaccharide transport. Our data suggests that the hydrophobic polyprenyl-moiety is translocated first, followed by the galactan-polysaccharide, which requires Wzm-Wzt to open a continuous channel through which the sugar chain is ratcheted at the expense of ATP hydrolysis. Our results provide a rational basis for the development of drugs that inhibit mycobacterial cell wall biosynthesis.