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Stoichiometry of anthrax toxin complexes.

Jeremy Mogridge¹, Kristina Cunningham, R John Collier

¹Department of Microbiology and Molecular Genetics, Harvard Medical School, Boston, Massachusetts 02115, USA.

|January 16, 2002

Summary

The protective antigen (PA) heptamer of anthrax toxin binds three protein ligands simultaneously. This binding creates an asymmetric complex, influencing toxin translocation across cell membranes.

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Area of Science:

Biochemistry
Molecular Biology
Toxicology

Background:

Anthrax toxin employs protective antigen (PA) to translocate lethal factor (LF) and edema factor (EF).
PA self-assembles into a heptameric ring structure upon activation.
The PA heptamer facilitates the translocation of EF/LF across the endosomal membrane.

Purpose of the Study:

To determine the stoichiometry of protein ligand binding to the PA heptamer.
To investigate the structural implications of ligand binding on the PA heptamer.

Main Methods:

Isotope ratio mass spectrometry of differentially radiolabeled toxin subunits.
Multiangle laser light scattering to determine molecular masses of unlabeled complexes.

Main Results:

The PA heptamer binds a maximum of three protein ligands under saturating conditions.
Each bound ligand sterically hinders the binding sites of two PA subunits.
A ligand-saturated PA heptamer is asymmetric, with six of seven subunit sites occluded.

Conclusions:

The binding stoichiometry of three ligands per PA heptamer is established.
Ligand-induced asymmetry of the PA heptamer is proposed.
Findings provide insights into the mechanism of anthrax toxin membrane translocation.

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