Structural Insights into Polymorphic ABO Glycan Binding by Helicobacter pylori

Abstract

Summary The Helicobacter pylori adhesin BabA binds mucosal ABO/Leb blood group (bg) carbohydrates. BabA facilitates bacterial attachment to gastric surfaces, increasing strain virulence and forming a recognized risk factor for peptic ulcers and gastric cancer. High sequence variation causes BabA functional diversity, but the underlying structural-molecular determinants are unknown. We generated X-ray structures of representative BabA isoforms that reveal a polymorphic, three-pronged Leb binding site. Two diversity loops, DL1 and DL2, provide adaptive control to binding affinity, notably ABO versus O bg preference. H. pylori strains can switch bg preference with single DL1 amino acid substitutions, and can coexpress functionally divergent BabA isoforms. The anchor point for receptor binding is the embrace of an ABO fucose residue by a disulfide-clasped loop, which is inactivated by reduction. Treatment with the redox-active pharmaceutic N-acetylcysteine lowers gastric mucosal neutrophil infiltration in H. pylori-infected Leb-expressing mice, providing perspectives on possible H. pylori eradication therapies.

Dynamic Expression of the BabA Adhesin and Its BabB Paralog during Helicobacter pylori Infection in Rhesus Macaques

“…Even among strains that express BabA, they can be classified as "specialists" or "generalists," depending on whether they bind only glycans in blood group O individuals or instead bind blood group A, B, and O antigens. Recent structural analysis has demonstrated that the basis for this functional diversity lies in two "diversity loops" within the carbohydrate binding site that represent areas of strong positive selection among babA sequences (13). Similarly, SabA shows polymorphism among clinical isolates in binding affinity to sialyl Lewis x (sLex) (31) and can be modulated by phase variation (32), gene conversion (33), and even variation in the length of a poly-T tract in the promoter region that serves as a rheostat-like mechanism to alter gene expression (34).…”

“…The only known function of BabA is attachment to fucosylated ABO(H) type 1 or Leb blood group antigens expressed on gastric epithelial cells and the overlying mucin (10)(11)(12). We recently demonstrated by high-resolution structural analysis that BabA residues Cys189 and Cys197 form a redox-sensitive disulfide-clasped loop designated CL2, which is essential for binding of the ABO blood group antigen-determining ␣1.2-fucose residue of Leb (13). Cys-to-Ala replacements at BabA residues 189 and 197 in H. pylori J166 (designated the babA-CL2 mutant) were sufficient to eliminate all Leb binding activity, though the protein was expressed at levels similar to those in the WT (Fig.…”

“…Cys-to-Ala replacements at BabA residues 189 and 197 in H. pylori J166 (designated the babA-CL2 mutant) were sufficient to eliminate all Leb binding activity, though the protein was expressed at levels similar to those in the WT (Fig. 1B) and reached the cell surface as a folded protein (13). To determine if Leb binding is required for loss of BabA protein by phase variation or gene conversion, we challenged six rhesus macaques with the babA-CL2 mutant and characterized the babA locus in output strains recovered 2, 8, 14, and 20 weeks p.i.…”

The Prevalence of Helicobacter pylori Virulence Related Genes (hpa and babA2) in Iranian Patients with Gastrointestinal Disorders

“…There are 3 bab alleles (babA1, babA2, and babB) of H. pylori, but only the babA2 gene indicates Lewis-b binding function (6). The BabA adhesion of H. pylori is an external membrane protein that is attached to the fucosylated histoblood group antigens on the outer surface of gastric epithelial receptors (6,7). Duodenal users and adenocarcinoma H. pylori agglutinin (hpa) is a binding protein of H. pylori that is coded by the hpa gene and binds H. pylori to gastric mucosal cells.…”

Helicobacter pylori Strains from Duodenal Ulcer Patients Exhibit Mixed babA/B Genotypes with Low Levels of BabA Adhesin and Lewis b Binding

“…BabA belongs to a family that also includes the BabB and BabC proteins [13], that are very similar, but lack the Le b binding adhesin properties. The recent crystal structure of BabA revealed a distinct carbohydrate binding domain, which is missing in the closely related BabB protein [14,15]. The tight BabA-mediated binding and mucosal adherence result in chronic inflammation, with high levels of recruited inflammatory cells [14].…”