Fluorescence studies on anthrax protective antigen pore formation and in the presence of the host receptor, CMG2
The anthrax toxin secreted by Bacillus anthracis is the major virulence factor of anthrax disease. The protective antigen (PA) component of the toxin binds to the von-Willebrand factor A (vWA) domain of capillary morphogenesis protein 2 (CMG2), followed by proteolytic cleavage into two fragments of 20 and 63 kDa. The 63 kDa fragment oligomerizes into heptameric or octameric structures collectively termed “pre-pore”, these are endocytosis, and within an acidified endosome undergo a large conformational change to form a membrane spanning pore, resulting in the entry of edema factor (EF) or lethal factor (LF) into the host cell. Although the general pathway of anthrax toxin entry has been elucidated, many details still remain unknown. The key problem is how the heptameric PA-receptor complex initiates formation of the pore and what happens to the receptor after pore formation; is it still bound or dissociated? Understanding the process of anthrax toxin pore formation and the role of the receptor could provide more clues in the investigation of the mechanism of anthrax toxin action and suggest new strategies for inhibiting toxin action. In my research work, we studied the pH dependent pore formation in presence of receptor, using fluorescence. We show that during the process of pore formation at low pH, a specific tryptophan residue, W346 (located in domain II), comes closer to domain IV, and then receptor dissociates from PA63. Our results show that, even in case of D425A PA63 (as a control, not forming a pore), receptor dissociates at the same pH required for pore formation. This mechanism suggests that movement of domain IV away from domain II must occur quickly, followed by a collapse in which the two domains coalesce, and receptor dissociation is induced by low pH.
Thesis (Ph.D.)--Wichita State University, College of Liberal Arts and Sciences, Dept. of Chemistry
- Dissertations