HISTIDINE-44 OF THE A-SUBUNIT OF ESCHERICHIA-COLI ENTEROTOXIN IS INVOLVED IN ITS ENZYMATIC-ACTIVITIES AND BIOLOGICAL-ACTIVITIES

We examined the role in toxicity of histidine-44, of the A subunit of Escherichia coil enterotoxin, which is located in the active site cavi ty close to glutamic acid-112. Although amino acid substitution of his tidine-44 usually renders a mutant toxin unstable to trypsin, one muta nt, alanine-44 (His44Ala) was found to be stable. His44Ala did not sho w any agmatine:ADP-ribosyltransferase activity in the presence or abse nce of recombinant ADP-ribosylation factor. It showed no diarrheal or rabbit skin permeability activity and was a competitor in enterotoxin- ADP-ribosyltransferase assays containing recombinant ADP-ribosylation factor. These results suggest that like glutamic acid-112, histidine-4 4 plays an essential role in toxicity. A tentative model, which explai ns NAD(+) catalysis and the transfer of the ADP-ribosyl moiety to a ta rget amino acid, is proposed for histidine-44 and glutamic acid-112.