Crystal structure of a new heat-labile enterotoxin, LT-IIb

Background: Cholera toxin from Vibrio cholerae and the type I heat-labile enterotoxins (LT-Is) from Escherichia coli are oligomeric proteins with AB₅ structures. The type II heat-labile enterotoxins (LT-IIs) from E. coli are structurally similar to, but antigenically distinct from, the type I enterotoxins. The A subunits of type I and type II enterotoxins are homologous and activate adenylate cyclase by ADP-ribosylation of a G protein subunit, G_Sα. However, the B subunists of type I and type II enterotoxins differ dramatically in amino acid sequence and ganglioside-binding specificity. The structure of LT-IIb was determined both as a prototype for other LT-IIs and to provide additional insights into structure/function relationships among members of the heat-labile enterotoxin family and the superfamily of ADP-ribosylating protein toxins. Results: The 2.25 Å crystal structure of the LT-IIb holotoxin has been determined. The structure reveals striking similarities with LT-I in both the catalytic A subunit and the ganglioside-binding B subunits. The latter form a pentamer which has a central pore with a diameter of 10-18 Å. Despite their similarities, the relative orientation between the A polypeptide and the B pentamer differs by 24° in LT-I and LT-IIb. A common hydrophobic ring was observed at the A-B₅ interface which may be important in the cholera toxin the surface of the A subunit of LT-I and cholera toxin, possibly involved in the sites where LT-I binds its receptor. Conclusions: The structure of LT-IIb provides insight into the sequence diversity and structural similarity of the AB₅ toxin family. New knowledge has been gained regarding the assembly of AB₅ toxins and their active-site architecture.