1S5B: Cholera Holotoxin With An A-Subunit Y30s Mutation Form 3

Cholera toxin (CT) is a heterohexameric bacterial protein toxin belonging to a larger family of A/B ADP-ribosylating toxins. Each of these toxins undergoes limited proteolysis and/or disulfide bond reduction to form the enzymatically active toxic fragment. Nicking and reduction render both CT and the closely related heat-labile enterotoxin from Escherichia coli (LT) unstable in solution, thus far preventing a full structural understanding of the conformational changes resulting from toxin activation. We present the first structural glimpse of an active CT in structures from three crystal forms of a single-site A-subunit CT variant, Y30S, which requires no activational modifications for full activity. We also redetermined the structure of the wild-type, proenzyme CT from two crystal forms, both of which exhibit (i) better geometry and (ii) a different A2 "tail" conformation than the previously determined structure [Zhang et al. (1995) J. Mol. Biol. 251, 563-573]. Differences between wild-type CT and active CTY30S are observed in A-subunit loop regions that had been previously implicated in activation by analysis of the structure of an LT A-subunit R7K variant [van den Akker et al. (1995) Biochemistry 34, 10996-11004]. The 25-36 activation loop is disordered in CTY30S, while the 47-56 active site loop displays varying degrees of order in the three CTY30S structures, suggesting that disorder in the activation loop predisposes the active site loop to a greater degree of flexibility than that found in unactivated wild-type CT. On the basis of these six new views of the CT holotoxin, we propose a model for how the activational modifications experienced by wild-type CT are communicated to the active site.
PDB ID: 1S5BDownload
MMDB ID: 27010
PDB Deposition Date: 2004/1/20
Updated in MMDB: 2007/10
Experimental Method:
x-ray diffraction
Resolution: 2.13  Å
Source Organism:
Similar Structures:
Biological Unit for 1S5B: hexameric; determined by author
Molecular Components in 1S5B
Label Count Molecule
Proteins (6 molecules)
Cholera Enterotoxin, a Chain Precursor(Gene symbol: VC1457)
Molecule annotation
Cholera Toxin B Protein (Ctb)(Gene symbol: VC1456)
Molecule annotation
Chemical (1 molecule)
* Click molecule labels to explore molecular sequence information.

Citing MMDB