Abstract
An earlier report showed that viruses lacking the open reading frames encoding glycoproteins J and D but containing the glycoprotein D in their envelopes (gD-/+ stocks) and viruses lacking both the open reading frames and the glycoproteins in their envelopes (gD-/- stocks) induce apoptosis (G. Zhou, V. Galvan, G. Campadelli-Fiume, and B. Roizman, J. Virol. 74:11782-11791, 2000). Furthermore, apoptosis was blocked by delivery in trans of genes expressing glycoprotein D or J. Whereas gD-/- stocks attach but cannot initiate productive infection, gD-/+ stocks infect cells and produce gD-/- progeny virus. The difference in the infectivity of these two stocks suggested the possibility that the requirements for blocking apoptosis may be different. To test this hypothesis, we cloned into baculoviruses the entire wild-type glycoprotein D (Bac-gD-WT), the ectodomain only (Bac-gD-A), the ectodomain and the transmembrane domain (Bac-gD-B), the ectodomain and the cytoplasmic domain without the transmembrane domain (Bac-gD-C), or the transmembrane domain and the carboxyl-terminal cytoplasmic domain (Bac-gD-D). We report the following. Apoptosis induced by gD-/+ stocks was blocked by delivery in trans of recombinant baculovirus Bac-gD-WT, Bac-gD-A, Bac-gD-B, or Bac-gD-C but not of Bac-gD. Apoptosis induced by gD-/- stocks was blocked by Bac-gD-WT or by a mixture of Bac-gD-B and Bac-gD-D but not by any baculoviruses expressing truncated glycoprotein D alone or by the mixture of Bac-gD-A and Bac-gD-D. We conclude that the requirements to block apoptosis induced by the two virus stocks are different. The gD ectodomain is sufficient to block apoptosis induced by gD, whereas both the ectodomain and the cytoplasmic domain are required to block apoptosis induced by gD-/- stocks. The results indicate that in the case of gD-/- stocks, the transmembrane domain is required either to deliver the ectodomain to the appropriate intracellular compartment or to form multimeric constructs which virtually reconstitute gD through the interaction of transmembrane domains.