Oncogenic signal transduction by the gastric pathogen Helicobacter pylori

Many Gram-negative pathogens harbor type IV secretion systems (T4SSs) that translocate bacterial virulence factors into host cells to hijack cellular processes. The pathology of the gastric pathogen Helicobacter pylori strongly depends on a T4SS. This T4SS forms a needle-like pilus, and its assembly is accomplished by multiple protein-protein interactions and various pilus-associated factors that bind to host cell integrins followed by delivery of the CagA onco-protein into gastric epithelial cells. Injected CagA becomes tyrosine-phosphorylated on EPIYA sequence motifs by Src and Abl family kinases. CagA then binds to and activates/ inactivates multiple signaling proteins in a phosphoryla-tion-dependent and phosphorylation-independent manner. A proteomic screen systematically identified eukaryotic binding partners of the EPIYA phosphorylation sites of CagA and similar sites in other bacterial effectors by high-resolution mass spectrometry. Individual phosphorylation sites recruited a surprisingly high number of interaction partners suggesting that each phosphorylation site can interfere with many downstream pathways. We now count more than 25 cellular binding partners of CagA, which represents the highest quantitiy among all yet known vir-ulence-associated effector proteins in the microbial world. This complexity generates a highly remarkable scenario, and the involved signal tranduction pathways are currently analysed. It appears that injected CagA can act as a 'masterkey' which evolved the ability to manipulate multiple host cell signalling cascades, which include the induction of membrane dynamics, actin-cytoskeletal rearrangements and the disruption of cell-to-cell junctions as well as proliferative, pro-inflammatory and anti-apoptotic nuclear responses. However, it is not yet clear how H. pylori can achieve contact with its basolateral integrin receptors in polarised epi-thelial cells. These polarized cell monolayers are commonly sealed by tight junctions, E-cadherin-based adherens junctions and other cell-to-cell contacts. We have demonstrated that H. pylori secrete an active serine protease into the ex-tracellular space, called HtrA, which can cleave E-cadherin and probably other junctional proteins. Remarkably, E-cad-herin shedding results in the local disruption of adherens junctions in polarized gastric epithelial cells allowing H. pylori entry into the intercellular space, which is in agreement with the detection of bacteria between neighboring epithelial cells and in underlying tissues of stomach biopsies obtained from gastric cancer patients. This indicates that HtrA-mediated E-cadherin cleavage may represent an entire novel mechanism how the pathogen can promote its own paracellular transmigration. We determined the cleav-age sites of HtrA in E-cadherin by Edman sequencing. Our results suggest that HtrA protease specificity is tailored to recognize an extracellular E-cadherin domain signature sequence , …