Glycoproteins gIII and gp50 Play Dominant Roles in the Biphasic Attachment of Pseudorabies Virus

Virus infections are initiated by adsorption of virions to target cells. This step is mediated by vital attachment proteins that interact with specific cellular receptors. It has previously been shown that the alphaherpesvirus pseudorabies virus (PrV) attaches to cells by way of a process that involves the viral envelope glycoprotein gIII (gC) and a heparin-like cellular surface moiety (T. C. Mettenleiter, L. Zsak, F. Zuckermann, N. Sugg, H. Kern, and T. Ben-Porat (1990) J. Virol. 64, 278-286). To gain further insight into adsorption of PrV different incubation protocols and an isogenic set of glycoprotein deletion mutants were analyzed. Here we show that attachment of wildtype PrV to target cells can be divided into three distinct stages: After primary adsorption, virus cannot be removed by a thorough wash with PBS but can be displaced by exogenous heparin defining a heparin-sensitive adsorption step. This primary adsorption at 0° converts with a half-time of approximately 20 min into a heparin-resistant binding where attached virus is no longer sensitive to exogenous heparin. The presence of heparin during the adsorption process leads to a heparin-independent basal virus binding to cells. Analysis of null mutants in six PrV glycoproteins confirmed that the nonessential glycoprotein gIII (gC) is a major determinant of primary adsorption. The presence of both gIII and the essential glycoprotein gp50 (gD) was shown to be critical for heparin-resistant binding. The importance of gp50 for this process was demonstrated in two isogenic wildtype PrV and gp50- mutant pairs. Similar results were obtained with a mutant bovine herpesvirus 1 lacking the gD-homologous glycoprotein gIV, indicating a general role for the gD-homologous proteins in stable attachment of alphaherpesviruses. Since the gD-homologs are also involved in penetration they might link the processes of viral attachment and entry.