A study was conducted into whether or not nicking of the A subunit of LT enterotoxin at position Arg192 or its neighbouring amino acids Arg192 to The 195 is required for its toxicity. The toxic activity of mutants created by substitution or deletion at this position, which lacked ADP-ribosyltransferase activity was not completely obliterated and cyclic AMP was partially induced in the target cells, showing that they still displayed enzymic activity Moreover, although the A subunit possesses three potential sites for cleavage by furin, furin was not involved in the partial toxicity and cyclic AMP induction observed. These data suggest that target cells have a nick mechanism that operates at sites other than those around Arg192 or those recognized by furin, which generates an active fragment by processing the A subunit after toxin binding to the cell membrane.


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