FIG. 7.

Model for the G₂ phase cell cycle arrest induced by C. botulinum C2 toxin. Cyclin B/p34^cdc2 is activated at the G₂/M border by dephosphorylation of p34^cdc2 by the phosphatase cdc25-C. Active p34^cdc2 kinase drives the cell into mitosis. Furthermore, active p34^cdc2 kinase activates cdc25-C phosphatase via an autocatalytic loop. Treatment of HeLa cells with C2 toxin delays the G₂/M transition by preventing tyrosine dephosphorylation and thereby activation of the p34^cdc2 protein kinase. C2 toxin treatment also prevents activation of cdc25-C phosphatase, which indicates that C2 toxin may affect intracellular signal cascades upstream of the mitotic inducers p34^cdc2 and cdc25-C.