Mycobacterium tuberculosis lipoprotein LprG (Rv1411c) binds triacylated glycolipid agonists of Toll-like receptor 2

Abstract

Toll-like receptor 2 is activated by triacylated lipoproteins, but structural and functional studies now show that Mycobacterium tuberculosis lipoprotein LprG is a TLR2 agonist even when it is non-acylated. LprG contains a hydrophobic pocket to bind triacylated glycolipids and delivers them to TLR2, perhaps via CD14. In addition, LprG may serve as a glycolipid chaperone in cell wall assembly by Mycobacterium tuberculosis. Knockout of lprG results in decreased virulence of Mycobacterium tuberculosis (MTB) in mice. MTB lipoprotein LprG has TLR2 agonist activity, which is thought to be dependent on its N-terminal triacylation. Unexpectedly, here we find that nonacylated LprG retains TLR2 activity. Moreover, we show LprG association with triacylated glycolipid TLR2 agonists lipoarabinomannan, lipomannan and phosphatidylinositol mannosides (which share core structures). Binding of triacylated species was specific to LprG (not LprA) and increased LprG TLR2 agonist activity; conversely, association of glycolipids with LprG enhanced their recognition by TLR2. The crystal structure of LprG in complex with phosphatidylinositol mannoside revealed a hydrophobic pocket that accommodates the three alkyl chains of the ligand. In conclusion, we demonstrate a glycolipid binding function of LprG that enhances recognition of triacylated MTB glycolipids by TLR2 and may affect glycolipid assembly or transport for bacterial cell wall biogenesis.