Amino acid sequence homology between cholera toxin and Escherichia coli heat-labile toxin

Cholera toxin (CT) and the Escherichia coli heat-labile toxin (LT) are functionally, structurally and immunologically similar enterotoxins. Both toxins cause the elevation of cyclic AMP levels in gut epithelial cells 1,2 by catalysing the NAD-dependent ADP ribosylation of membrane proteins 3–6 . Each toxin is composed of two dissimilar subunits 7,8 . The A subunit has an enzymatic activity and is the adenylate cyclase-activating component of the enterotoxin 4–11 . The B subunit recognizes membrane components and binds the holotoxin to the target cell juxtaposing the A subunit with its substrates 9,12 . Binding studies and competition experiments indicate that the membrane receptors for cholera toxin B subunit (CT-B) and LT-B are similar but not identical 13 (these studies were performed before LT was purified to homogeneity). The monosialosylganglioside GMI has been shown to be the receptor for the cholera toxin 14 , and it probably composes part of the receptor for LT. Gyles and Barnum 15 first reported that LT and cholera toxin were immunologically related, and it has subsequently been shown that they share common antigenic determinants in both A and B subunits 16 . The primary structure of CT-B has been determined 17,18 . We report here a comparison between the amino acid sequences of LT-B and CT-B. The nucleotide sequence of the LT-B cistron ( elt B) was determined using a recombinant plasmid encoding LT 9 . Translation of this sequence revealed that LT-B and CT-B show significant amino acid sequence homology. In addition, several features of the elt B cistron were revealed by the sequence analysis.