ADP-ribosylation of nonmuscle actin by component I of botulinum C2 toxin inactivates the ability to interact with unmodified actin.

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 beta- and gamma-actins but not of muscle alpha-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 N-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 beta/gamma-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.(ABSTRACT TRUNCATED AT 250 WORDS)