Association-dissociation processes and supermolecular organisation of the capsule antigen (protein F1) of Yersinia pestis.

The association-dissociation processes involving the capsule antigen of Yersinia pestis were investigated. In aqueous salt solutions the material of the capsule (protein F1) exists in the form of associated species containing identical monomeric protein subunits. Brief heating (100 degrees C, 3 min) of F1 dissolved in buffered salt solutions at pHs between 4.4 and 7.5 leads to dissociation of the antigen into oligomers which reassociate at room temperature into aggregates of high molecular mass. In the presence of 7 M urea and 0.1% SDS such reassociation is suppressed. It is shown that a few seconds after heat treatment the protein exists as the tetramer in phosphate buffer and as the dimer and the monomer in the presence of urea and SDS respectively. Interconversions of these three forms of the antigen were observed on replacement of one buffer by another. The effect of pH, ionic strength, F1 concentration, and temperature on the rate of association of F1 were also investigated. A decrease in pH, an increase in the ionic strength of the solution, and an increase in the concentration of the protein in aqueous buffered salt solutions all accelerate the F1 aggregation process. A model is proposed for the supermolecular structure of the F1 protein associated species in which the subunits are assembled into a single plane containing tens of dimers as a result of lateral interdimer hydrogen bonds. The dimer is stabilised by hydrophobic interactions of the nonpolar sections of the subunits, which in the associated protein form an internal hydrophobic surface analogous to that in lipid bilayer membranes.