Role of pseudorabies virus glycoprotein gI in virus release from infected cells

The Bartha vaccine strain of pseudorabies virus has a deletion in the short unique (U(s)) region of its genome which includes the genes that code for glycoproteins gI and gp63 (E. Petrovskis, J.G. Timmins, T.M. Gierman, and L.E. Post, J. Virol. 60:1166-1169, 1986). Restoration of an intact U(s) to the Bartha strain enhances its ability to be released from infected rabbit kidney cells and increases the size of the plaques formed on these cells (T. Benn-Porat, J.M. DeMarchi, J. Pendrys, R.A. Veach, and A.S. Kaplan, J. Virol. 57:191-196, 1986). To determine which gene function plays a role in virus release from rabbit kidney cels, deletions were introduced into the genomes of both wild-type virus and the 'rescued' Bartha strain (Bartha strain to which an intact U(s) had been restored) that abolish the expression of either the gI gene alone or both gI and gp63 genes. The effect of these deletions on the phenotype of the viruses was studied. Deletion mutants of wild-type virus defective in either gI or gI and gp63 behave like wild-type virus with respect to virus release and plaque size on rabbit kidney cells. Deletion of gI from the rescued Bartha strain, however, strongly affects virus release and causes a decrease in plaque size. We conclude that gI affects virus release but that at least one other viral function also affects this process. This function is defective in the Bartha strain but not in wild-type virus; in its absence gI is essential to efficient release of the virus from rabbit kidney cells.