STRUCTURE-FUNCTION ANALYSIS OF THE GE-GI COMPLEX OF FELINE HERPESVIRUS - MAPPING OF GI DOMAINS REQUIRED FOR GE-GI INTERACTION, INTRACELLULAR-TRANSPORT, AND CELL-TO-CELL SPREAD

Alphaherpesvirus glycoproteins gE and gI form a noncovalently associat ed hetero-oligomeric complex, which is involved in cell-to-cell spread . In the absence of gI, feline herpesvirus (FHV) gE is transport incom petent and fully retained in the endoplasmic reticulum. Here, we asses s the effect of progressive C-terminal truncations of FHV gI on the bi osynthesis, intracellular transport, and function of the gE-gI complex . The truncated gI proteins were coexpressed with gE in the vaccinia v irus-based vTF7-3 expression system. The results were corroborated and extended by studying FHV recombinants expressing truncated gI derivat ives. The following conclusions can be drawn. (i) Deletion of the cyto plasmic tail, the transmembrane region plus the C-terminal half of the ectodomain of gI, does not affect intracellular transport of gE. Appa rently, the N-terminal 166 residues of gI constitute a domain involved in gE-gI interaction. (ii) A region mediating stable association with gE is located within the N-terminal 93 residues of gI. (iii) The cyto plasmic domain of gI is not essential for gE-gI-mediated cell-to-cell transmission of FHV, as judged from plaque morphology. Deletion of the cytoplasmic tail of gI reduced plaque size by only 35%. (iv) Recombin ants expressing the N-terminal 166 residues of gI display a small-plaq ue phenotype but produce larger plaques than recombinants with a disru pted gI gene. Thus, a complex consisting of gE and the N-terminal half of the gI ectodomain may retain residual biological activity. The imp lications of these findings for gE-gI interaction and function are dis cussed.