The pasteurella multocida toxin interacts with signalling pathways to perturb cell growth and differentiation.
Some years ago we showed that the Pasteurella multocida toxin (PMT) is a potent mitogen for cells in culture. It is an intracellularly acting toxin that stimulates several signal transduction pathways. The heterotrimeric G-protein, Gq, is stimulated, which in turn causes activation of protein kinase C and an increase in inositol trisphosphates. The Rho GTPase is also activated, leading via the Rho kinase, to activation of the focal adhesion kinase and to cytoskeletal rearrangements. Analysis of the PMT sequence suggested the presence of three domains that encode receptor binding, translocation and catalytic domains. The location of all three domains has been confirmed directly. Competitive binding assays confirmed that the N-terminus of PMT encoded the receptor-binding domain, while cytoplasmic microinjection of expressed PMT fragments identified the location of the C-terminal catalytic domain. Recently, we have demonstrated the presence of key amino acids that affect membrane insertion within the putative transmembrane domain. Several lines of evidence suggest that PMT activates Galphaq, and that this is one potential molecular target for the toxin. Galphaq is known to be tyrosine phosphorylated when activated normally via a G-protein-coupled receptor (GPCR), and it has been suggested that this is an essential part of the activation process. We have shown that PMT induces Galphaq tyrosine phosphorylation, but that this is not essential for activation of the G-protein. Furthermore, a totally inactive mutant of PMT stimulates Galpha phosphorylation without leading to its activation. Phosphorylation of Galphaq triggered by the inactive mutant potentiates activation of Gq via a GPCR, demonstrating that phosphorylation of Gq cannot lead to receptor uncoupling. Natural or experimental infection of animals with toxigenic P. multocida, or injection with purified recombinant PMT causes loss of nasal turbinate bone. The effects on bone have been analysed in vitro using cultures of osteoblasts--cells that lay down bone. PMT blocks the formation of mature calcified bone nodules and the expression of differentiation markers such as CBFA-1, alkaline phosphatase and osteocalcin. These effects can be partially prevented by inhibitors of Rho or Rho kinase function, implicating this pathway in osteoblast differentiation. Indeed, inhibitors of Rho stimulate the formation of bone nodules in vitro. In summary, PMT is a novel toxin that acts via signalling pathways to promote proliferation in many cells, while specifically inhibiting differentiation in osteoblast cells.
Lax
Alistair J
AJ
Department of Microbiology, Dental Institute, King's College London, United Kingdom. alistair.lax@kcl.ac.uk
Pullinger
Gillian D
GD
Baldwin
Michael R
MR
Harmey
Dympna
D
Grigoriadis
Agamemnon E
AE
Lakey
Jeremy H
JH
D001426:Bacterial Proteins / Q000737:chemistry / Q000378:metabolism / Q000494:pharmacology / Q000502:physiology*; D001427:Bacterial Toxins / Q000737:chemistry / Q000378:metabolism / Q000494:pharmacology; D016723:Bone Remodeling / Q000187:drug effects / Q000502:physiology; D002454:Cell Differentiation / Q000187:drug effects / Q000502:physiology; D002455:Cell Division / Q000187:drug effects / Q000502:physiology; D020962:Heterotrimeric GTP-Binding Proteins / Q000502:physiology; D010006:Osteoblasts / Q000187:drug effects / Q000378:metabolism / Q000382:microbiology; D016979:Pasteurella multocida / Q000737:chemistry / Q000378:metabolism*; D010766:Phosphorylation; D011994:Recombinant Proteins / Q000737:chemistry / Q000378:metabolism / Q000494:pharmacology; D015398:Signal Transduction / Q000187:drug effects / Q000502:physiology; D020741:rho GTP-Binding Proteins / Q000378:metabolism
D001426:Bacterial Proteins; D001427:Bacterial Toxins; C062258:Pasteurella multocida toxin; D011994:Recombinant Proteins; D020962:Heterotrimeric GTP-Binding Proteins; D020741:rho GTP-Binding Proteins