Intracellular Transport of the Glycoproteins gE and gI of the Varicella-Zoster Virus

Abstract

The varicella-zoster virus (VZV) is the etiological agent of two different human pathologies, chickenpox (varicella) and shingles (zoster). This alphaherpesvirus is believed to acquire its lipidic envelope in the transGolgi network (TGN). This is consistent with previous data showing that the most abundant VZV envelope glycoprotein gE accumulates at steady-state in this organelle when expressed from cloned cDNA. In the present study, we have investigated the intracellular trafficking of gI, another VZV envelope glycoprotein. In transfected cells, this protein shows a very slow biosynthetic transport to the cell surface where it accumulates. However, upon co-expression of gE, gI experiences a dramatic increase in its exit rate from the endoplasmic reticulum, it accumulates in a sialyltransferase-positive compartment, presumably the TGN, and cycles between this compartment and the cell surface. This differential behavior results from the ability of gE and gI to form a complex in the early stages of the biosynthetic pathway whose intracellular traffic is exclusively determined by the sorting information in the tail of gE. Thus, gI provides the first example of a molecule localized to the TGN by means of its association with another TGN protein. We also show that, during the early stages of VZV infection, both proteins are also found in the TGN of the host cell. This suggests the existence of an intermediate stage during VZV biogenesis in which the envelope glycoproteins, transiently arrested in the TGN, could promote the envelopment of newly synthesized nucleocapsids into this compartment and, therefore, the assembly of infective viruses.

Essential Role Played by the C-Terminal Domain of Glycoprotein I in Envelopment of Varicella-Zoster Virus in the trans -Golgi Network: Interactions of Glycoproteins with Tegument

“…The experiments were designed to test predictions about patterns of trafficking of gI and gE in cells infected with the mutant virions. These predictions were based on the assumptions that gI and gE interact at the N-terminal domains of the molecules and that a complex of these two proteins is formed in the RER (1,13,14,21,24). The sorting of gI has previously been studied at the light microscopic level, both in infected cells (20,30) and in transfected cells expressing only gI (1,31,34).…”

“…These predictions were based on the assumptions that gI and gE interact at the N-terminal domains of the molecules and that a complex of these two proteins is formed in the RER (1,13,14,21,24). The sorting of gI has previously been studied at the light microscopic level, both in infected cells (20,30) and in transfected cells expressing only gI (1,31,34). Nevertheless, the effects of deleting gI, or particular regions of the molecule, on the structure of the TGN, the concentration of other gly- The trafficking of gI in cells infected with intact VZV has previously been described (30) and is thus the expected pattern in cells infected with intact virions.…”

“…This sorting implies that TGN targeting signals must be present in the sequences of at least some VZV glycoproteins. Such signals have indeed been identified in the cytosolic domains of gE (1,2,36,37) and gI (31). Unenveloped nucleocapsids are frequently seen in electron micrographs of the cytosol of cells infected with VZV (10).…”

“…The molecular weight of gE produced in the absence of gI is greater than normal because of an alteration in its glycosylation. gI and gE are known to form a complex with one another in the RER (1,13,14,21,24). The absence of gI would thus be expected to affect post-rather than cotranslational steps in the glycosylation of gE, most of which occur in the Golgi apparatus (18).…”

Membrane Association Domains in Ca2+-dependent Activator Protein for Secretion Mediate Plasma Membrane and Dense-core Vesicle Binding Required for Ca2+-dependent Exocytosis

“…The distal C-terminal region of CAPS contains an acidic cluster consisting of MKDSDEEDEEDD, which resembles recently identified sorting motifs in several proteins. Acidic cluster sorting motifs are present in the cytosolic domains of several proteins with different membrane trafficking itineraries (41), including proprotein convertases furin and PC6B, mannose 6-phosphate receptors, herpes virus envelope glycoproteins, and human immunodeficiency virus type 1 Nef (42)(43)(44)(45)(46)(47). Such clusters often contain consensus sites for phosphorylation by casein kinase II, as does that for CAPS, and phosphorylation by casein kinase II regulates sorting.…”