Studies of two Yersinia proteins: YopJ, an inhibitor of inflammatory responses, and YpkA, a RhoA/Rac-binding kinase

Eukaryotic cells possess sensitive systems for the detection of microbes and microbial-derived products. Bacteria-eukaryotic cells contact triggers several signaling pathways, most of which are dependent on protein-protein interactions followed by phosphorylation, leading to activation of defense mechanisms. Yersinia bacteria have evolved different strategies to interfere with eukaryotic host defenses. One such strategy involves the direct injection of proteins from the bacterium into eukaryotic cytoplasm. These proteins also known as Yops are active inside eukaryotic cells and contain eukaryotic-like domains or protein motifs that are normally not found in bacterial proteins. The YopJ protein inhibits the induction of cytokine induction by Yersinia-infected eukaryotic cells and induces apoptosis on Yersinia-infected macrophages. Surprisingly, a YopJ homologous protein from Salmonella does not possess these YopJ activities. The same operon containing yopJ, contains also a gene encoding the Yersinia protein kinase A (YpkA). YpkA is a 733-residue protein that contains in its N-terminus a domain that is similar to eukaryotic ser/thr protein kinases. YpkA is capable of autophosphorylation in vitro in the presence of cofactors present in both eukaryotic cell extracts and serum. Transfected YpkA is sufficient to localise to the inner side of the plasmic membrane and to induce morphology changes on eukaryotic cells. At its carboxyl-terminal region, YpkA possesses sequence similarity to the RhoA-binding domains of a number of eukaryotic kinases and binds RhoA, both in vivo and in vitro. The YpkA C terminal region is responsible for eukaryotic cell morphology induction and the N terminal region responsible for its membrane localisation. The N-terminal region interacts with a eukaryotic protein, the Calcium and Integrin Binding protein (CIB), in vivo and in vitro. The biological significances of these interactions are still not known.