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, G8 alpha. However, the B subunits 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 A 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 A. Despite their similarities, the relative orientation between the A polypeptide and the B pentamer differs by 24 degrees 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 family for assembly of the AB5 heterohexamer. A cluster of arginine residues at the surface of the A subunit of LT-I and cholera toxin, possibly involved in assembly, is also present in LT-IIb. The ganglioside receptor binding sites are localized, as suggested by mutagenesis, and are in a position roughly similar to 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.
van den Akker
F
F
Department of Biological Structure and Biochemistry, University of Washington, Seattle 98195, USA.
Sarfaty
S
S
Twiddy
E M
EM
Connell
T D
TD
Holmes
R K
RK
Hol
W G
WG
D000595:Amino Acid Sequence; D001427:Bacterial Toxins / Q000737:chemistry* / Q000302:isolation & purification; D001665:Binding Sites; D017124:Conserved Sequence / Q000235:genetics; D018360:Crystallography, X-Ray; D004768:Enterotoxins / Q000737:chemistry* / Q000302:isolation & purification; D004926:Escherichia coli / Q000737:chemistry; D029968:Escherichia coli Proteins; D006162:Guanylate Cyclase / Q000737:chemistry; D006358:Hot Temperature; D008958:Models, Molecular; D008969:Molecular Sequence Data; D009243:NAD / Q000737:chemistry; D011487:Protein Conformation; D017433:Protein Structure, Secondary; D000074261:Receptors, Enterotoxin; D050705:Receptors, Guanylate Cyclase-Coupled; D018000:Receptors, Peptide / Q000737:chemistry; D016415:Sequence Alignment
D001427:Bacterial Toxins; D004768:Enterotoxins; D029968:Escherichia coli Proteins; D018000:Receptors, Peptide; D009243:NAD; C027538:heat-labile enterotoxin, E coli; D006162:Guanylate Cyclase; D000074261:Receptors, Enterotoxin; D050705:Receptors, Guanylate Cyclase-Coupled