Glutamic acid‐112 of the A subunit of heat‐labile enterotoxin from enterotoxigenic Escherichia coli is important for ADP‐ribosyltransferase activity

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Abstract
A mutant strain of enterotoxigenic Escherichia coli (E. coli pTUH 6A) produced an abnormal heat‐labile enterotoxin (LT), the A subunit of which has a single amino acid substitution at position 112 (Glu‐112 to Lys‐112). As already reported, this mutant LT had no ileal loop and vascular permeability activities [(1990) J. Biol. Chem. 265, 22520–22525]. In this paper we report that the mutant LT showed no CHO cell elongation activity and did not activate adenylate cyclase of target cells. Moreover, no ADP‐ribosyltransferase activity was detected in the mutant LT. It is concluded that the amino acid substitution at position 112 abolished the ADP‐ribosyltransferase activity of the A subunit and this leads to the loss of toxic activities of LT.