Transport of envelope proteins of Sendai virus, HN and F0, is blocked at different steps by thapsigargin and other perturbants to intracellular Ca2+.

The effects of thapsigargin (Tg), a specific inhibitor of Ca(2+)-ATPase of the endoplasmic reticulum (ER), on replication of Sendai virus (HVJ) in BALB3T3 cells were examined. In the presence of Tg, the cells infected with HVJ did not release viral particles to the culture medium. Tg inhibited almost completely the expression of viral envelope proteins, HN and F0, on the cell surface, although it did not affect the synthesis of viral proteins. Two other inhibitors of Ca(2+)-ATPase of the ER, 2,5-di(tert-butyl)-1,4-benzohydroquinone (BHQ) and cyclopiazonic acid (CPA), as well as Ca(2+)-ionophores such as A23187 and ionomycin, also inhibited the expression of HN protein on the cell surface. Tg seemed to inhibit the intracellular transport or maturation of the viral membrane proteins by perturbing intracellular distribution of Ca2+ ions. In the presence of Tg, HN protein remained sensitive to endoglycosidaseH (endoH) for 3 h after its synthesis. On the other hand, F0 protein became resistant to endoH and sensitive to neuraminidase even in the presence of Tg. These results indicate that the transport of HN protein is blocked at the ER or the cis-Golgi region, while that of F0 protein is blocked at the post-Golgi stage in the presence of Tg.