Calcium Depletion Blocks the Maturation of Rotavirus by Altering the Oligomerization of Virus-Encoded Proteins in the ER

Maturation of rotavirus occurs in the ER. The virus transiently acquires an ER-derived membrane surrounding the virus particle before the eventual formation of double-shelled particles. The maturation process includes the retention and selective loss of specific viral protein(s) as well as the ER-derived membrane during formation of the outer capsid of the mature virus. When infected cells were depleted of Ca++ by use of the ionophore A23187 in calcium-free medium, membrane-enveloped intermediates were seen to accumulate. When Mn++, an efficient Ca++ competitor, was used to replace Ca++ in the medium, the accumulation of the enveloped intermediate was again observed, pointing to an absolute requirement of Ca++ in the maturation process. It was previously demonstrated in this laboratory that a hetero-oligomeric complex of NS28, VP7, and VP4 exists which may participate in the budding of the single-shelled particle into the ER. The present study demonstrates that either in the absence of Ca++ or in the presence of tunicamycin, a glycosylation inhibitor, VP7 is excluded from these hetero-oligomers. In the presence of Mn++, VP4 was blocked in forming a hetero-oligomeric complex with NS28 and VP7. The electrophoretic mobility of the viral glycoproteins synthesized in the presence of the ionophore were found to be altered. This size difference was attributed to altered N-linked glycosylation and carbohydrate processing of the viral glycoproteins. These results imply a major role for calcium and the state of glycosylation of NS28 in the assembly and acquisition of specific viral protein conformations necessary for the correct association of proteins during virus maturation in the ER.