Determining the mechanism of Akt activation by the Salmonella type III effector SopB
Zusammenfassung der Projektergebnisse
Salmonella Typhimurium is a major cause of food poisoning worldwide and infections can be lethal in young children or immunocompromised individuals. These bacteria have the ability to invade host cells and grow intracellulary. To this end they secrete specialized virulence proteins directly into the cytosol of host cells. SopB, one of the many effector proteins that are translocated into host cells, modulates signal transduction pathways during the infection, including phosphoinositide (PI) metabolism, leading to activation of the prosurvival kinse Akt. Preliminary data suggested that SopB-mediated Akt activation is different from canonical Akt activation by growth factors or insulin. During my research project I could show that PI(3,4)P2/PI(3,4,5)P3 formation, initiated by SopB, is necessary for Akt activation. After PI3-kinase inhibitor treatment or knock.down of class II PI3-kinases, SopB-mediated Akt activation was still observed. In yeast, where PI(3,4,5)P3 is normally absent, PH-Akt was recruited from the cytosol to the plasma membrane after induction of SopB expression, suggesting PI(3,4)P2/PI(3,4,5)P3 formation. This indicates a novel PI3-kinase-independent mechanism, by which SopB initiates the formation of PI(3,4)P2/PI(3,4,5)P3, thereby leading to Akt activation. My data suggests that SopB-mediated Akt activation seems to follow a canonical activation pathway in a sense that it is activated by PI(3,4)P2/PI(3,4,5)P3. I did not have any evidence for a mechanism where Akt is phosphorylated at two different cellular locations and downstream of two independent PI3-kinase signaling events, as proposed in my model. In contrast, I could show that PI(3,4)P2/PI(3,4,5)P3 formation, leading to Akt activation occurs in the absence of any classical PI3-kinase. This data is supported by the fact that I could induce PI(3,4)P2/PI(3,4,5)P3 formation in yeast, an organism that lacks this lipid under normal conditions. However, formation of these lipids seems to be different than in a classical EGF-stimulated Akt activation pathway.
