Macromolecular organization of the Yersinia pestis capsular F1 antigen: Insights from time-of-flight mass spectrometry

Mass spectrometry has been used to examine the subunit interactions in the capsular F1 antigen from Yersinia pestis, the causative agent of the plague . Introducing the sample using nanoflow electrospray from solution conditio ns in which the protein remains in its native state and applying collisiona l cooling to minimize the internal energy of the ions, multiple subunit int eractions have been maintained. This methodology revealed assemblies of the Fl antigen that correspond in mass to both 7-mers and 14-mers, consistent with interaction of two seven-membered units. The difference between the ca lculated masses and those measured experimentally for these higher-order ol igomers was found to increase proportionately with the size of the complex. This is consistent with a solvent-filled central cavity maintained on asso ciation of the 7-mer to the 14-mer. The charge states of the ions show that an average of one and four surface accessible basic side-chains are involv ed in maintaining the interactions between the 7-mer units and neighboring subunits, respectively. Taken together, these findings provide new informat ion about the stoichiometry and packing of the subunits involved in the ass embly of the capsular antigen structure. More generally, the data show that the symmetry and packing of macromolecular complexes can be determined sol ely from mass spectrometry, without any prior knowledge of higher order str ucture