Structural insights into terminal arabinosylation biosynthesis of the mycobacterial cell wall arabinan

Yaqi Liu, Chelsea M. Brown, Satchal Erramilli, Yi-Chia Su, Po-Sen Tseng, Yu-Jen Wang, Nam Ha Duong, Piotr Tokarz, Brian Kloss, Cheng-Ruei Han, Hung-Yu Chen, José Rodrigues, Margarida Archer, Todd L. Lowary, Anthony A. Kossiakoff, Phillip J. Stansfeld, Rie Nygaard, Filippo Mancia

bioRxiv 2024 September 17

PMID: 39345558 PMCID: PMC11429727 DOI: 10.1101/2024.09.17.613533

The emergence of drug-resistant strains exacerbates the global challenge of tuberculosis caused by Mycobacterium tuberculosis (Mtb). Central to the pathogenicity of Mtb is its complex cell envelope, which serves as a barrier against both immune system and pharmacological attacks. Two key components of this envelope, arabinogalactan (AG) and lipoarabinomannan (LAM) are complex polysaccharides that contain integral arabinan domains important for cell wall structural and functional integrity. The arabinofuranosyltransferase AftB terminates the synthesis of these arabinan domains by catalyzing the addition of the addition of β-(1→2)-linked terminal arabinofuranose residues. Here, we present the cryo-EM structures of Mycobacterium chubuense AftB in its apo and donor substrate analog-bound form, determined to 2.9 Å and 3.4 Å resolution, respectively. Our structures reveal that AftB has a GT-C fold transmembrane (TM) domain comprised of eleven TM helices and a periplasmic cap domain. AftB has an irregular tube-shaped cavity that bridges the two proposed substrate binding sites. By integrating structural analysis, biochemical assays, and molecular dynamics simulations, we elucidate the molecular basis of the reaction mechanism of AftB and propose a model for catalysis