Structural basis for the Helicobacter pylori carcinogenic TNF〈-inducing protein

Stomach cancer is strongly associated with infection by Helicobacter pylori. In 2005, we identified a new H. pylori gene encoding a TNF-α inducing protein (Tipα) that acts as a carcinogenic factor. Tipα is secreted from H. pylori as a homodimer whose subunits are linked by disulfide bonds. We also characterized a Tipα deletion mutant (del-Tipα) that lacks the N-terminal six amino acid residues (LQACTC), including two cysteines (C5 and C7) that form disulfide bonds, but nonetheless shows a weak ability to induce TNF-α expression. Here we report that del-Tipα has a novel elongated structure containing a 40-Å-long α helix, and forms a heart-shaped homodimer via non-covalent bonds. Moreover, their circular dichroism spectra strongly suggest that the structures of the del-Tipα and Tipα homodimers are very similar. del-Tipα’s unique mode of dimer formation provides important insight into protein–protein interactions and into the mechanism underlying the carcinogenicity of H. pylori infection.