Tyrosine-Phosphorylated Bacterial Proteins

For over a century, since the discovery by Roux and Yersin that sterile culture supernatants of Corynebacterium diphtheriae contained a potent toxin able to reproduce the lesions caused by diphtheria (1), most pathogenic bacteria have been considered to be microorganisms able to intoxicate local and distant tissues by secreting toxins in the ex-tracellular medium. Recently, it has been shown that some bacteria inject toxic proteins directly into the cytoplasm of host cells using a specialized, needle-like (2) secretion apparatus (molecular syringe) known as a type III secretion system (3, 4). A report in this issue by Ashai et al. (5) and reports from Haas et al. (Haas, R., personal communication), Segal et al. (6), and our own laboratory (7) now provide definitive evidence for the existence in Helicobacter pylori of a second type of molecular syringe (type IV secretion system) that is also able to inject toxic proteins into eukaryotic cells. These papers describe the type IV–mediated delivery into eukaryotic cells and the subsequent tyrosine phosphor-ylation of CagA, an immunodominant protein of H. pylori , encoded by a 40-kb pathogenicity island (cag). The need for a functional type IV secretion system for CagA translo-cation into the membrane fraction of host cells and its ty-rosine phosphorylation is shown at several levels using a variety of isogenic mutants in the cag region (7), biochemical fractionation of the host cell compartments (7), and confo-cal microscopy (6). In addition, it has been shown that CagA present in culture supernatants or bacterial cell lysates is unable to enter eukaryotic cells, indicating that this is not a function that the soluble protein can perform on its own (5, 7). Although the correlation between expression of CagA and H. pylori virulence was described a long time ago (8, 9), the last seven years have been marked by the frustration of not finding any role for the CagA protein. Finally, the concomitant report by four independent laboratories of a role for CagA sends important messages to the scientific community. They are summarized below. Type IV and Type III Secretion Systems Are Functionally Equivalent Molecular Syringes. Gram-negative bacteria build and anchor different extracellular organelles, such as flagella and conjugative pili, using specialized supramolecular structures. These molecular engines transport the building blocks of flagella and pili across both the inner and outer membranes and polymerize the external filamentous structures by adding new monomers from the inside. In their evolution, bacteria …