FUNCTIONAL COMPLEMENTATION OF UL3.5-NEGATIVE PSEUDORABIES VIRUS BY THE BOVINE HERPESVIRUS-1 UL3.5 HOMOLOG

The UL3.5 gene is positionally conserved but highly variable in size a nd sequence in different members of the Alphaherpesvirinae and is abse nt from herpes simplex virus genomes. We have shown previously that th e pseudorabies virus (PrV) UL3.5 gene encodes a nonstructural protein which is required for secondary envelopment of intracytoplasmic virus particles in the trans-Golgi region. In the absence of UL3.5 protein, naked nucleocapsids accumulate in the cytoplasm, release of infectious virions is drastically reduced, and plaque formation in cell culture is inhibited (W. Fuchs, B. G. Klupp, H. Granzow, H.-J. Rziha, and T. C . Mettenleiter, J. Virol, 70:3517-3527, 1996). To assay functional com plementation by a heterologous herpesviral UL3.5 protein, the UL3.5 ge ne of bovine herpesvirus 1 (BHV-1) was inserted at two different sites within the genome of UL3.5-negative PrV, In cells infected with the P rV recombinants the BHV-1 UL3.5 gene product was identified as a 17-kD a protein which was identical in size to the UL3.5 protein detected in BW-l-infected cells, Expression of BHV-1 UL3.5 compensated for the la ck of PrV UL3.5, resulting in a ca. 1,000-fold increase in virus titer and restoration of plaque formation in cell culture. Also, the intrac ellular block in viral egress was resolved by the BHV-1 UL3.5 gene. We conclude that the UL3.5 proteins of PrV and BHV-1 are functionally re lated and are involved in a common step in the egress of alphaherpesvi ruses.