Cell surface expression and fusion by the varicella-zoster virus gH:gL glycoprotein complex: analysis by laser...

Varicella-zoster virus (VZV) open reading frames 37 and 60 encode the glycoproteins gH (gpIII) and gL (gpVI), respectively. The property of gH:gL complex formation is highly conserved among the herpesviruses, even though the VZV gL component diverges greatly from other herpesvirus gL homologs. VZV gL by itself was processed to a mature product within the Golgi. To evaluate the structure:function relationships for VZV gH:gL complex formation, the VZV gL product was modified by site-directed mutagenesis of three cysteine residues. When the transfection products were examined by laser scanning confocal microscopy, expression of the wild-type gH:gL complex was clearly visualized by a uniform distribution of gH molecules across the cell surface. In contrast, transfection with wild-type gH:mutant gL led to a marked change in the trafficking pattern; gH was not processed in the Golgi and not detected at the cell surface. Likewise, replacement of the gL cysteine residues interfered with the fusogenic properties of the gH:gL complex. Whereas coexpression of wild-type VZV gH:gL caused extensive cell-to-cell fusion with polykaryocytosis, no cell fusion occurred following transfection with gH:mutant gL. Whether another VZV glycoprotein could substitute for VZV gL was investigated within the same transfection system, with the discovery that either VZV gE (gpI) or VZV gI (gpIV) facilitated the cell surface expression of VZV gH. The gH:gE or gH:gI interaction led to a capping or patching phenomenon never seen on the surface of a cell expressing gH:gL complexes; furthermore, cell-to-cell fusion was not observed. The fact that VZV gL, unlike other herpesviral glycoproteins, lacked a traditional signal sequence was investigated further by computer-assisted BlockSearch sequence analysis. The BlockSearch program assigned VZV gL to a family of proteins which lack a typical endoplasmic reticulum signal sequence but possess instead an endoplasmic reticulum targeting sequence. Since the latter sequence is common to many chaperone proteins, VZV gL most likely behaves in a similar manner.