CRYSTAL-STRUCTURE OF A NEW HEAT-LABILE ENTEROTOXIN, LT-IIB

Background: Cholera toxin from Vibrio cholerae and the type I heat-lab ile enterotoxins (LT-Is) from Escherichia coli are oligomeric proteins with AB(5) structures. The type II heat-labile enterotoxins (LT-IIs) from E. coli are structurally similar to, but antigenically distinct f rom, the type I enterotoxins. The A subunits of type I and type II ent erotoxins are homologous and activate adenylate cyclase by ADP-ribosyl ation of a G protein subunit, G(s) alpha. However, the B subunits of t ype I and type Il enterotoxins differ dramatically in amino acid seque nce and ganglioside-binding specificity. The structure of LT-IIb was d etermined both as a prototype for other LT-IIs and to provide addition al insights into structure/function relationships among members of the heat-labile enterotoxin family and the superfamily of ADP-ribosylatin g protein toxins. Results: The 2.25 Angstrom crystal structure of the LT-IIb holotoxin has been determined, The structure reveals striking s imilarities with LT-I in both the catalytic A subunit and the ganglios ide-binding B subunits. The latter form a pentamer which has a central pore with a diameter of 10-18 Angstrom. Despite their similarities, t he relative orientation between the A polypeptide and the B pentamer d iffers by 24 degrees in LT-I and LT-IIb. A common hydrophobic ring was observed at the A-B-5 interface which may be important in the cholera toxin family for assembly of the AB(5) heterohexamer. A cluster of ar ginine residues at the surface of the A subunit of LT-I and cholera to xin, possibly involved in assembly, is also present in LT-IIb, The gan glioside receptor binding sites are localized, as suggested by mutagen esis, and are in a position roughly similar to the sites where LT-I bi nds its receptor, Conclusions: The structure of LT-IIb provides insigh t into the sequence diversity and structural similarity of the AB(5) t oxin family, New knowledge has been gained regarding the assembly of A B(5) toxins and their active-site architecture.