Book Description

Many bacterial toxins covalently modify components of eukaryotic signalling pathways in a highly specific manner, and can be used as powerful tools to decipher the function of their molecular target(s). The Pasteurella multocida toxin (PMT) mediates its cellular effects through the activation of three of the four heterotrimeric G-protein families, Gq, G12 and Gj. -- I have investigated modification of Ga subunits mediated by PMT in vivo using 2-D gel electrophoresis and antibody detection. A change in the isoelectric point was observed for the Ga subunit of the Gq and Gi families, but not G5 , following PMT treatment of Swiss 3T3 cells. PMT induced the modification of Ga11 which is not activated by PMT. I suggest that the modification of these G-proteins is due to tyrosine phosphorylation. Ga13 is also modified by PMT treatment. Prolonged treatment with PMT led to a reduction in membrane-associated Gai, but not Gaq. I also show that PMT inhibits the GTPase activity of Gq using a trypsin protection assay that probes the conformation state of a G-protein and demonstrated that PMT inhibits the basal and receptor activated steady state GTPase activity in Swiss 3T3 membranes. -- During the course of my research Gai has recently been shown to be deamidated at Gln205 by PMT to inhibit its intrinsic GTPase activity. My findings support the concept that PMT functions as an archetypal intracellular toxin to induce the covalent modification of G-proteins and to stimulate their activation through a common mechanism.