Modification of the α-subunits of heterotrimeric guanine nucleotide–binding proteins (G proteins) by bacterial toxins is a well-known phenomenon; ADP-ribosylation of Gαs subunits by cholera toxin renders them constitutively active, whereas a similar modification of many Gαi proteins by pertussis toxin blocks their interactions with G protein–coupled receptors. Pastuerella multocida toxin (PMT) causes the activation of G proteins containing Gαq, Gαi, and Gα13 subunits. Although the mechanism involved is unknown, a C-terminal domain of PMT is required for its activity. Orth et al. showed that the guanosine triphosphatase (GTPase) activity of recombinant Gαi2 was inhibited by incubation with the C-terminal domain of wild-type PMT (PMT-Cwt) but not with that of a mutant PMT that is inactive (PMT-CC1165S). Mass spectrometric analysis showed that Gαi2 that was coexpressed in Escherichia coli with PMT-Cwt, but not PMT-CC1165S, underwent deamidation of a glutamine residue (Gln205), a conserved residue that is critical for the intrinsic GTPase activity of G protein α-subunits. Two-dimensional gel electrophoresis showed that endogenous Gαi2 in mouse embryonic fibroblast membranes was deamidated by PMT. In HEK293 cells, a mutant Gαi2 protein in which Gln205 was replaced by glutamate, thus mimicking the deamidation reaction, was constitutively active, as assessed by measurement of the abundance of adenosine 3′,5′-monophosphate (cAMP). Further studies showed that Gαq was also deamidated by PMT; however, the closely related Gα11 subunit, which is not a target of PMT, did not undergo deamidation. Although the mechanism by which PMT deamidates its specific targets is still unclear, this modification is consistent with the potent mitogenic activity of PMT, because constitutively active G protein α-subunits trigger cellular transformation.
J. H. C. Orth, I. Preuss, I. Fester, A. Schlosser, B. A. Wilson, K. Aktories, Pasteurella multocida toxin activation of heterotrimeric G proteins by deamidation. Proc. Natl. Acad. Sci. U.S.A. 106, 7179–7184 (2009). [Abstract] [Full Text]