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pH effects on binding between the anthrax protective antigen and the host cellular receptor CMG2
Maheshinie Rajapaksha1, Scott Lovell, Blythe E Janowiak
1Department of Chemistry, Wichita State University, Wichita, Kansas 67260-0051, USA.
Protein Science : a Publication of the Protein Society
|August 3, 2012
Summary
Lowering pH increases the binding affinity of anthrax protective antigen (PA) to its receptor CMG2. This pH-dependent binding is linked to structural changes in PA, enhancing its interaction with the host cell receptor.
Area of Science:
- Biochemistry
- Structural Biology
- Microbiology
Background:
- Anthrax protective antigen (PA) is a key component of anthrax toxin, mediating entry into host cells.
- PA binds to the host receptor capillary morphogenesis protein 2 (CMG2), a crucial step in anthrax pathogenesis.
- Understanding the factors influencing PA-receptor binding, such as pH, is vital for developing countermeasures.
Purpose of the Study:
- To investigate the effect of pH on the binding kinetics and structural dynamics of anthrax protective antigen (PA).
- To compare the pH-dependent binding and structural behavior of wild-type PA with two variants: 2-fluorohistidine-labeled PA (2-FHisPA) and a W346F mutant.
Main Methods:
- Stopped-flow fluorescence spectroscopy was used to measure the binding rates of PA and its variants to CMG2 across a range of pH values.
- X-ray crystallography was employed to determine the structures of PA and its variants at different pH levels.
- Analysis of B-factor values provided insights into protein flexibility and structural changes.
Main Results:
- The binding rate of PA to CMG2 significantly increased as pH was lowered, with minimal impact on the dissociation rate.
- Structural analysis revealed that while wild-type PA showed limited loop changes between pH 8 and 5.5, the 2-FHisPA and W346F variants exhibited pH-dependent structural alterations.
- Increased protein flexibility, indicated by higher B-factor values, was observed at low pH for all three proteins.
Conclusions:
- Low pH enhances the binding affinity of anthrax protective antigen (PA) to its receptor CMG2, likely by inducing a more favorable 'bound-like' conformation.
- Specific mutations (2-FHisPA, W346F) alter the structural response to pH changes compared to wild-type PA, providing insights into the domain 2β(3)-2β(4) loop's role.
- These findings contribute to a deeper understanding of anthrax toxin-receptor interactions and potential pH-mediated modulation.
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