Bordetella bronchiseptica dermonecrotizing toxin, which activates a small GTP-binding protein rho, induces membrane organelle proliferation and caveolae formation.

We recently demonstrated that Bordetella bronchiseptica dermonecrotizing toxin (DNT) modifies a small GTP-binding protein rho and causes assembly of actin stress fibers and focal adhesions in MC3T3-E1 cells, indicating that DNT activates the function of rho protein (Horiguchi et al., 1995, J. Cell Sci. 108, 3243-3251). In this study, we examined by electron microscopy ultrastructural changes in DNT-treated MC3T3-E1 cells under conditions in which DNT elicited the above morphological changes. We found that DNT induced proliferation of cytoplasmic membrane organelles, such as Golgi apparatus, smooth and rough endoplasmic reticulum, and mitochondria, and formation of plasmalemmal caveolae. Therefore we examined also the effect of Clostridium botulinum C3 exoenzyme (C3), which has been known to ADP-ribosylate and inactivate rho protein, and found that C3 exoenzyme inhibited the development of membrane organelles in the MC3T3-E1 cells. These findings show that proliferation of membrane organelles and caveolae formation are controlled by the function of rho, which is the target of DNT action.