ADP-Ribosylation of Nonmuscle Actin by Component I of Botulinum C2 Toxin Inactivates the Ability to Interact with Unmodified Actin1

Botulinum C2 toxin, elaborated by Clostridium botulinum types C and D, is composed of two dissimilar unassociated proteins, designated components I and II. Component I catalyzes ADP-ribosylation of nonmuscle 0- and /-actins but not of muscle a-actin. The maximal levels of ADP-ribosylation of the actin were about 1.0 mol of ADP-ribose/mol of actin. Sedimentation velocity analysis showed that ADP-ribosylated actin remained in a monomeric state even under polymerization conditions. In addition to the inactivation of self-polymerization ability, the ADP-ribosylated actin affected neither the initial rate nor the final extent of polymerization of unmodified actin as monitored by the increase in fluorescence intensity of iV-pyrenyliodoacetamide-labeled actin. Electron microscopy revealed that no filaments or particles were formed from ADP-ribosylated actin in the conditions favorable for polymerization of unmodified actin; moreover, actin filaments produced from unmodified actin in the presence of ADP-ribosylated actin were not distinguishable from those from unmodified actin alone. These results indicate that the introduction of one ADP-ribose residue into the β/r-actin molecule by component I inactivated the actin, preventing not only the self-assembly of the modified actins but also the interaction with unmodified actin. The present results suggest that ADP-ribosylation of nonmuscle actin at arginine-177 [Vandekerckhov, J., Schering, B., Bannann, M., & Aktories, K. (1988) J. BioL Chenu 263, 696-700] plays an important role in actin-actin interaction and that the ADP-ribosylation of intracellular actin by C2 toxin shifts the dynamic equilibrium between the monomeric and polymeric forms to the depolymerized state, thereby leading to the disintegration of matrix, and consequently to the death of the cells.