Morphological Study of the Role of Calcium in the Assembly of the Rotavirus Outer Capsid Protein VP7

Maturation of rotavirus occurs in the endoplasmic reticulum (ER), a site of intracellular calcium storage. It was demonstrated previously that calcium plays an important role in the maturation of bovine rotavirus. We used protein A colloidal gold conjugated to an antibody to localize VP7, the outer capsid protein of the simian rotavius SA11, in permeabilized infected cells in the presence and absence of calcium in the culture medium. In medium containing calcium, VP7 was associated with nonenveloped double-shelled particles and membranous structures of the ER. In calcium-free medium, gold particles were not associated with the ER or with virus particles. Gold particles were distributed through the cytoplasm and were mainly associated with granular structures, but did not assemble onto virus particles. Our data suggest that in calcium-free medium, VP7 is synthesized, but does not remain incorporated, in the ER.