3q8f
Crystal structure of 2-Fluorohistine labeled Protective Antigen (pH 5.8)
Structural highlights
FunctionPAG_BACAN One of the three proteins composing the anthrax toxin, the agent which infects many mammalian species and that may cause death. PA binds to a receptor (ATR) in sensitive eukaryotic cells, thereby facilitating the translocation of the enzymatic toxin components, edema factor and lethal factor, across the target cell membrane. PA associated with LF causes death when injected, PA associated with EF produces edema. PA induces immunity to infection with anthrax. Publication Abstract from PubMedThe anthrax protective antigen (PA) binds to the host cellular receptor capillary morphogenesis protein 2 (CMG2) with high affinity. In order to gain a better understanding of how pH may affect binding to the receptor, we have investigated the kinetics of binding as a function of pH to the full-length monomeric PA and to two variants: a 2-fluorohistidine-labeled PA (2-FHisPA), which is approximately 1 pH unit more stable to variations in pH than WT, and an approximately 1 pH unit less stable variant in which Trp346 in the domain 2beta(3) -2beta(4) loop is substituted with a Phe (W346F). We show using stopped-flow fluorescence that the binding rate increases as the pH is lowered for all proteins, with little influence on the rate of dissociation. In addition, we have crystallized PA and the two variants and examine the influence of pH on structure. In contrast to previous X-ray studies, the domain 2beta(3) -2beta(4) loop undergoes little change in structure from pH approximately 8 to pH 5.5 for the WT protein, but for the 2-FHis labeled and W346F mutant there are changes in structure consistent with previous X-ray studies. In accord with pH stability studies, we find that the average B-factor values increase by approximately 20-30% for all three proteins at low pH. Our results suggest that for the full-length PA, low pH increases the binding affinity, likely through a change in structure that favors a more "bound-like" conformation. pH effects on binding between the anthrax protective antigen and the host cellular receptor CMG2.,Rajapaksha M, Lovell S, Janowiak BE, Andra KK, Battaile KP, Bann JG Protein Sci. 2012 Jul 31. doi: 10.1002/pro.2136. PMID:22855243[1] From MEDLINE®/PubMed®, a database of the U.S. National Library of Medicine. References
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