Structural basis of Lewisb antigen binding by the Helicobacter pylori adhesin BabA

摘要

Helicobacter pylori is a leading cause of peptic ulceration and gastric cancer worldwide. To achieve colonization of the stomach, this Gram-negative bacterium adheres to Lewis^b (Le^b) antigens in the gastric mucosa using its outer membrane protein BabA. Structural information for BabA has been elusive, and thus, its molecular mechanism for recognizing Le^b antigens remains unknown. We present the crystal structure of the extracellular domain of BabA, from H. pylori strain J99, in the absence and presence of Le^b at 2.0- and 2.1-Å resolutions, respectively. BabA is a predominantly α-helical molecule with a markedly kinked tertiary structure containing a single, shallow Le^b binding site at its tip within a β-strand motif. No conformational change occurs in BabA upon binding of Le^b, which is characterized by low affinity under acidic [KD (dissociation constant) of ~227 μM] and neutral (KD of ~252 μM) conditions. Binding is mediated by a network of hydrogen bonds between Le^b Fuc1, GlcNAc3, Fuc4, and Gal5 residues and a total of eight BabA amino acids (C189, G191, N194, N206, D233, S234, S244, and T246) through both carbonyl backbone and side-chain interactions. The structural model was validated through the generation of two BabA variants containing N206A and combined D233A/S244A substitutions, which result in a reduction and complete loss of binding affinity to Le^b, respectively. Knowledge of the molecular basis of Le^b recognition by BabA provides a platform for the development of therapeutics targeted at inhibiting H. pylori adherence to the gastric mucosa.