Rubella virus capsid protein induces apoptosis in transfected RK13 cells

Rubella virus is an enveloped positive-strand RNA virus that can cause mild to severe birth defects or death in an infected fetus. RV induction of programmed cell death, demonstrated in cell culture, has been implicated in the pathogenesis. The timing of apoptosis, 48 h p.i., suggested that accumulation of RV structural proteins might induce cell death in infected cells. Expression of RV structural proteins, capsid, envelope glycoproteins E1 and E2, in transiently transfected RK13 cells was as potent an inducer of cell death as RV infection. Immunofluorescence microscopy revealed that RV structural protein transfected cells exhibited the condensed nuclei typical of apoptotic cell death. Transfection with the capsid protein construct, but not E2 and E1, resulted in as much cell death as joint expression of all three RV structural proteins. Capsid required a membrane-anchoring domain to induce cell death, but a heterologous polypeptide fused to the capsid membrane anchor did not cause apoptosis. Deletion mutants demonstrated that the apoptosis-inducing activity resides in the N-terminal 170 amino acids of capsid. Though apoptosis-inducing capsid constructs appear to have an ER sub-cellular localization, disruption of the ER calcium storage capacity does not correlate with cell death. Mechanisms consistent with these results are discussed.