Conformational fluctuations in anthrax protective antigen: a possible role of calcium in the folding pathway of the protein.

Protective antigen (PA) is the central receptor binding component of anthrax toxin, which translocates catalytic components of the toxin into the cytosol of mammalian cells. Ever since the crystal structure of PA was solved, there have been speculations regarding the possible role of calcium ions present in domain I of the protein. We have carried out a systematic study to elucidate the effect of calcium removal on the structural stability of PA using various optical spectroscopic techniques, limited proteolysis and mutational analysis. Urea denaturation studies clearly suggest that the unfolding pathway of the protein follows a non-two state transition with apo-PA being an intermediate species, whereas the folding pathway shows that calcium ions may be critical for the initial protein assembly.