Isolation and characterization of a Clostridium botulinum C2 toxin-resistant cell line: evidence for possible involvement of the cellular C2II receptor in growth regulation.

Clostridium botulinum C2 toxin, which consists of the binding component C2II and the enzyme component C2I, acts on eukaryotic cells by selective ADP-ribosylation of G-actin. To obtain C2 toxin-resistant cells, we mutagenized CHO-K1 cells with N-nitroso-N-methylurea and selected for C2 resistance. Cells which survived the selection procedure with 50 ng of C2I and 100 ng of C2II per ml were obtained with a frequency of 30 x 10(-6). The colony-forming ability of CHO wild-type cells was reduced to 50% with 10 ng of C2I and 20 ng of C2II per ml. In contrast, the colony-forming ability of the isolated CHO mutant cells was not influenced by up to 200 ng of C2I and 400 ng of C2II per ml. Toxin-induced ADP-ribosylation of G-actin was not impaired in lysates of mutant cells. The C2 toxin-resistant phenotype remained sensitive to the cell-rounding activities of cytotoxins from C. perfringens (iota-toxin), C. novyi, C. difficile, and C. botulinum (C3) and to cytochalasin D. Binding of component C2II was impaired in resistant CHO cells, suggesting mutation of the toxin cell surface receptor. Serum factors protected wild-type cells against the cytotoxic effect of C2 toxin. Furthermore, the C2-resistant phenotype correlated with an increased serum dependency. The data suggest that the action of C. botulinum C2 toxin is mediated by its binding and uptake via a cell surface receptor which might be involved in growth regulation.