Pasteurella multocida toxin is a potent mitogen for cultured Swiss 3T3 cells where it causes an accumulation of inositol phosphates and activation of protein kinase C. The gene sequence described here coded for a 146 kDa protein. The ORF was preceded by a ribosome binding site and followed by a stem loop. There was no evidence for a signal sequence. The gene had a low G + C base ratio which differs from the rest of the Pasteurella genome. There was no significant homology with other known proteins, although a motif found in certain bacterial toxins which are ADP-ribosyl transferases is present. A recombinant expressing only part of the PMT gene was not mitogenic.

Native Pasteurella multocida toxin (PMT) is shown to be an extremely potent mitogen for Swiss 3T3 fibroblasts. Half-maximal stimulation of DNA synthesis was obtained at concentrations of 1 and 2 pM for recombinant PMT (rPMT) and PMT, respectively. The degree of rPMT-induced DNA synthesis was comparable to that elicited by 10% fetal bovine serum and, moreover, was observed in the complete absence of other factors. Cell proliferation was also enhanced by rPMT. The toxin was also a potent mitogen for BALB/c and NIH 3T3 cells, 3T6 cells, and tertiary mouse embryo or human fibroblasts. The mitogenic activity of rPMT was heat-labile. A polyclonal antiserum to PMT inhibited DNA synthesis when added early, but not late, during treatment of the Swiss 3T3 cells with rPMT. A similar time-dependent action of methylamine was also observed. Furthermore, transient exposure of the cells to rPMT at 37 degrees C, but not at 4 degrees C, resulted in a stimulation of DNA synthesis. Thus, toxin action may require cell entry and processing via an acidic compartment. The toxin, at mitogenic concentrations, caused a large increase in the production of inositol phosphates. In contrast, rPMT did not increase the intracellular concentration of cyclic AMP in Swiss 3T3 cells. The basis of rPMT action may afford a unique insight into molecular signaling events involved in the control of cell proliferation.