Localization of calcium on the polyomavirus VP1 capsid protein

Our laboratory has previously shown that the divalent cation Ca2+ is an integral part of the polyomavirus and plays a major role in stabilizing the intact virion structure. In this report, we show that calcium is sequestered on the major capsid protein VP1 of polyomavirus. The virion structural proteins were separated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis before being transferred to nitrocellulose and probed with 45CaCl2. Autoradiography revealed 45Ca binding exclusively to VP1. Increasing the amount of VP1 transferred to the nitrocellulose resulted in a concomitant increase in 45Ca binding. 45Ca binding to VP1 could be reduced by competition with an excess of unlabeled CaCl2. Separation of the species of VP1 by two-dimensional gel electrophoresis before electroblotting and probing with 45CaCl2 revealed that all six species (A to F) bind the radiolabeled calcium. Formic acid cleavage of the 43-kilodalton (kDa) VP1 protein into 29-, 18-, and 16-kDa fragments before 45Ca-binding analysis revealed that only the 18- and 16-kDa carboxyl-terminal fragments of this protein bind 45Ca.