title: Role of trypsin-like cleavage at arginine 192 in the enzymatic and cytotonic activities of Escherichia coli heat-labile enterotoxin.
abstract: Previous studies of cholera toxin and Escherichia coli heat-labile enterotoxin have suggested that proteolytic cleavage plays an important role in the expression of ADP-ribosyltransferase activity and toxicity. Specifically, several studies have implicated a trypsin-like cleavage at arginine 192, which lies within an exposed region subtended by a disulfide bond in the intact A subunit, in toxicity. To investigate the role of this modification in the enzymatic and cytotonic properties of heat-labile enterotoxin, the response of purified, recombinant A subunit to tryptic activation and the effect of substituting arginine 192 with glycine on the activities of the holotoxin were examined. The recombinant A subunit of heat-labile enterotoxin exhibited significant levels of ADP-ribosyltransferase activity that were only nominally increased (approximately twofold) by prior limited trypsinolysis. The enzymatic activity also did not appear to be affected by auto-ADP-ribosylation that occurs during the high-level synthesis of the recombinant A subunit in E. coli. A mutant form of the holotoxin containing the arginine 192-to-glycine substitution exhibited levels of cytotonic activity for CHO cells that were similar to that of the untreated, wild-type holotoxin but exhibited a marked delay in the ability to increase intracellular levels of cyclic AMP in Caco-2 cells. The results indicate that trypsin-like cleavage of the A subunit of E. coli heat-labile enterotoxin at arginine 192 is not requisite to the expression of enzymatic activity by the A subunit and further reveal that this modification, although it enhances the biological and enzymatic activities of the toxin, is not absolutely required for the enterotoxin to elicit cytotonic effects.
authors.affiliation: Laboratory of Vectors and Pathogens, National Institute of Allergy and Infectious Diseases, Rocky Mountain Laboratories, Hamilton, Montana 59840.
authors.forename: C C
authors.initials: CC
authors.lastname: Grant
authors.forename: R J
authors.initials: RJ
authors.lastname: Messer
authors.forename: W
authors.initials: W
authors.lastname: Cieplak
mesh_terms: D036002:ADP Ribose Transferases / Q000378:metabolism*; D000246:Adenosine Diphosphate Ribose / Q000378:metabolism; D000818:Animals; D001427:Bacterial Toxins / Q000737:chemistry / Q000302:isolation & purification / Q000633:toxicity*; D001483:Base Sequence; D016466:CHO Cells; D006224:Cricetinae; D000242:Cyclic AMP / Q000096:biosynthesis; D004768:Enterotoxins / Q000737:chemistry / Q000302:isolation & purification / Q000633:toxicity*; D004926:Escherichia coli / Q000201:enzymology*; D029968:Escherichia coli Proteins; D008969:Molecular Sequence Data; D016297:Mutagenesis, Site-Directed; D011994:Recombinant Proteins / Q000302:isolation & purification; D013329:Structure-Activity Relationship; D014357:Trypsin / Q000494:pharmacology*
chemical_list: D001427:Bacterial Toxins; D004768:Enterotoxins; D029968:Escherichia coli Proteins; D011994:Recombinant Proteins; D000246:Adenosine Diphosphate Ribose; C027538:heat-labile enterotoxin, E coli; D000242:Cyclic AMP; D036002:ADP Ribose Transferases; D014357:Trypsin