title: Exchange characteristics of calcium ions bound to anthrax protective antigen.
abstract: Protective antigen (PA), the receptor-binding moiety of anthrax toxin, contains two calcium atoms buried within domain 1(') (amino acid residues 168-258). We showed that these ions are stably bound and exchange with free 45Ca(2+) only slowly (t(1/2) approximately 4.0 h). Dissociation is the rate-limiting step. PA(63), the heptameric prepore form of PA, showed a slightly higher exchange rate than the monomeric intact protein. Exchange by this form was retarded by binding of the enzymatic moieties of the toxin, but was unaffected by reducing the pH to 5.0, a condition known to trigger conversion of the prepore to the pore form. These results are consistent with the hypothesis that bound Ca(2+) within PA plays primarily a structural role, maintaining domain 1(') in a conformation that allows PA(63) to oligomerize and bind the enzymatic moieties of the toxin.
authors.affiliation: Department of Microbiology and Molecular Genetics, Harvard Medical School, 200 Longwood Avenue, Boston, MA 02115, USA.
authors.forename: Samantha
authors.initials: S
authors.lastname: Gao-Sheridan
authors.forename: Sen
authors.initials: S
authors.lastname: Zhang
authors.forename: R John
authors.initials: RJ
authors.lastname: Collier
mesh_terms: D000942:Antigens, Bacterial; D001408:Bacillus anthracis / Q000737:chemistry; D001427:Bacterial Toxins / Q000737:chemistry* / Q000378:metabolism; D001665:Binding Sites; D002118:Calcium / Q000737:chemistry*; D017136:Ion Transport; D011487:Protein Conformation; D020836:Protein Structure, Quaternary; D011994:Recombinant Proteins / Q000737:chemistry / Q000378:metabolism; D013050:Spectrometry, Fluorescence
chemical_list: D000942:Antigens, Bacterial; D001427:Bacterial Toxins; D011994:Recombinant Proteins; C030325:anthrax toxin; D002118:Calcium