title: Unfolding transitions of Bacillus anthracis protective antigen.
abstract: Protective antigen (PA) is an 83kDa protein which, although essential for toxicity of Bacillus anthracis, is harmless and an effective vaccine component. In vivo it undergoes receptor binding, proteolysis, heptamerisation and membrane insertion. Here we probe the response of PA to denaturants, temperature and pH. We present analyses (including barycentric mean) of the unfolding and refolding behavior of PA and reveal the origin of two critical steps in the denaturant unfolding pathway in which the first step is a calcium and pH dependent rearrangement of domain 1. Thermal unfolding fits a single transition near 50 degrees C. We show for the first time circular dichroism (CD) spectra of the heptameric, furin-cleaved PA63 and the low-pH forms of both PA83 and PA63. Although only PA63 should reach the acidic endosome, both PA83 and PA63 undergo similar acidic transitions and an unusual change from a beta II to a beta I CD spectrum.
authors.affiliation: Institute of Cell and Molecular Biosciences, University of Newcastle upon Tyne, Framlington Place, Newcastle NE2 4HH, UK.
authors.forename: David A
authors.initials: DA
authors.lastname: Chalton
authors.forename: Ian F
authors.initials: IF
authors.lastname: Kelly
authors.forename: Alistair
authors.initials: A
authors.lastname: McGregor
authors.forename: Helen
authors.initials: H
authors.lastname: Ridley
authors.forename: Allan
authors.initials: A
authors.lastname: Watkinson
authors.forename: Julie
authors.initials: J
authors.lastname: Miller
authors.forename: Jeremy H
authors.initials: JH
authors.lastname: Lakey
mesh_terms: D000942:Antigens, Bacterial / Q000737:chemistry* / Q000648:ultrastructure*; D001408:Bacillus anthracis / Q000737:chemistry / Q000276:immunology*; D001427:Bacterial Toxins / Q000737:chemistry*; D002118:Calcium / Q000737:chemistry*; D006863:Hydrogen-Ion Concentration; D008956:Models, Chemical; D008958:Models, Molecular; D044367:Phase Transition; D011487:Protein Conformation; D011489:Protein Denaturation; D017510:Protein Folding
chemical_list: D000942:Antigens, Bacterial; D001427:Bacterial Toxins; C030325:anthrax toxin; D002118:Calcium