Role of the cytoplasmic tail of pseudorabies virus glycoprotein E in virion formation

Glycoproteins M (gM), E (gE), and I (gI) of pseudorabies virus (PrV) are required for efficient formation of mature virions. The simultaneous absence of gM and the gE/gI complex results in severe deficiencies in virion morphogenesis and cell-to-cell spread, leading to drastically decreased virus titers and a small-plaque phenotype (A. Brack, J. Dijkstra, H. Granzow, B. G. Klupp, and T. C. Mettenleiter, J. Virol. 73:5364-5372, 1999). Serial passaging in noncomplementing cells of a virus mutant unable to express gM, gE, and gI resulted in a reversion of the small-plaque phenotype and restoration of infectious virus formation to the level of a gM(-) mutant. Genetic analyses showed that reversion of the phenotype was accompanied by a genomic rearrangement which led to the fusion of a portion of the gE gene encoding the cytoplasmic domain to the 3' end of the glycoprotein D gene, resulting in expression of a chimeric gD-gE protein. Since this indicated that the intracytoplasmic domain of gE was responsible for the observed phenotypic alterations, the UL10 (gM) gene was deleted in a PrV mutant, PrV-107, which specifically lacked the cytoplasmic tail of gE. Regarding one-step growth, plaque size, and virion formation as observed under the electron microscope, the mutant lacking gM and the gE cytoplasmic tail proved to be very similar to the gE/I/M triple mutant. Thus, our data indicate that it is the cytoplasmic tail of gE which is responsible for the observed phenotypic effects in conjunction with deletion of gM. We hypothesize that the cytoplasmic domain of gE specifically interacts with components of the capsid and/or tegument, leading to efficient secondary envelopment of intracytoplasmic capsids.