CagZ, a 23 kDa protein encoded by the cagZ gene (HP0526) of the cag pathogenicity island of Helicobacter pylori, has been cloned, over-expressed, purified and its three-dimensional structure determined. The protein consists of a single compact L-shaped domain, composed of seven alpha-helices including about 70% of the total residues. Three-dimensional homology searches did not reveal structural homologues, and CagZ can be considered representative of a new protein fold. The presence of a disordered C-terminal tail and the nature of the molecular surface suggest that CagZ may participate in the interaction of effector proteins with one or more components of the H.pylori type IV secretion system on the cytoplasmic side of the inner membrane.