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Structural studies of receptor binding by cholera toxin mutants
E A Merritt1, S Sarfaty, M G Jobling
1Department of Biological Structure, University of Washington, Seattle 98195-7742, USA.
Summary
Mutations at Gly 33 of cholera toxin B-pentamer (CTB) surprisingly alter receptor binding. Structural analysis reveals subtle rearrangements explain binding affinity changes, impacting toxin-receptor interactions.
Area of Science:
- Structural biology
- Biochemistry
- Microbiology
Background:
- Cholera toxin B-pentamer (CTB) mutations at Gly 33 yield varied receptor binding affinities, posing a puzzle.
- Aspartate substitution abolishes binding, while arginine substitution retains it despite a larger side chain.
Purpose of the Study:
- To determine the structure of CTB Gly 33-->Arg and LTB Gly 33-->Asp mutants complexed with GM1 oligosaccharide.
- To elucidate the structural basis for altered receptor binding affinities in CTB mutants.
Main Methods:
- X-ray crystallography for structure determination and refinement of CTB and LTB mutants.
- Analysis of protein and oligosaccharide conformations and binding interactions.
Main Results:
- The CTB Gly 33-->Arg mutant shows partial GM1 oligosaccharide binding and toxin:toxin interactions.
- Wild-type CTB and the Gly 33-->Arg mutant exhibit similar saccharide conformations and binding interactions.
- The binding-deficient LTB Gly 33-->Asp mutant shows no substantial conformational difference from toxin:saccharide complexes.
Conclusions:
- The critical role of residue 33 is attributed to limited conformational flexibility in the toxin and saccharide for receptor accommodation.
- Intermolecular interactions in the CTB Gly 33-->Arg mutant suggest its receptor affinity is near self-affinity levels.