Genetic and functional characterization of the alpAB gene locus essential for the adhesion of Helicobacter pylori to human gastric tissue

In this study, we isolated and characterized a chromosomal locus of Helicobacter pylori previously identified by transposon shuttle mutagenesis as being involved in the adhesion of the pathogen to gastric epithelial cells, Two closely homologous genes were identified, designated as alpA and alpB, encoding outer membrane (OM) proteins of 518 amino acids each. They are members of the outer membrane protein supergene family identified in the H. pylori 26695 complete genome sequence, AlpA carries a functional lipoprotein signal sequence. AlpB carries a putative standard N-terminal signal sequence and shows a strong amino-acid sequence identity to AlpA. Transposon insertion mutagenesis, immunoblotting and primer extension studies indicate that both genes are organized in an operon, but no obvious consensus promoter sequence was found upstream of the transcriptional start site. The C-terminal portion of both proteins is predicted to form a porin-like beta-barrel in the outer membrane, consisting of 14 transmembrane amphipathic beta-strands. Adhesion experiments with defined isogenic mutants indicate that both proteins are necessary for specific adherence of H. pylori to human gastric tissue. The pattern of AlpAB-dependent adherence of H. pylori to the gastric epithelial surface shows a clear difference to the BabA2-mediated adherence to Lewis(b), suggesting that a different receptor is involved.