Biological activity of a C-terminal fragment of Pasteurella multocida toxin

The protein toxin of Pasteurella multocida PMT is a potent mitogen and acti vator of phospholipase C beta. In this study different toxin fragments were investigated A C-terminal fragment encompassing amino acids 581 through 12 85 (PMT581C) was constructed, which was inactive toward intact embryonic bo vine lung (EBL) cells after addition to culture medium but caused reorganiz ation of the actin cytoskeleton and rounding up of cells when introduced in to the cells by electroporation. As the holotoxin, the toxin fragment PMT58 1C induced an increase in total inositol phosphate levels after introductio n into the cell by electroporation. A C-terminal fragment shorter than PMT5 81C as well as N-terminal fragments were inactive. Exchange of cysteine-116 5 for serine in the holotoxin resulted in a complete loss of the ability to increase inositol phosphate levels. Correspondingly, the mutated toxin fra gment PMT581C.C1165S was inactive after cell introduction by electroporatio n, suggesting an essential role of Cys-1165 in the biological activity of t he toxin.