Trafficking of varicella-zoster virus glycoprotein gI: T-338-dependent retention in the trans-Golgi network, secretion, and mannose 6-phosphate-inhibitable uptake of the ectodomain

The trans-Golgi network (TGN) is putatively the site where varicella zoster virus is enveloped. gE is targeted to the TGN by selective retrieval from the plasmalemma in response to signaling sequences in its endodomain. gI la cks these sequences but forms a complex with gE. We now find that gI is tar geted to the TGN and plasma membrane when expressed in Cos-7 cells; neverth eless, surface labeling revealed that gI is not retrieved from the plasma m embrane. TGN targeting of gI depended on the T-338 of its endodomain and wa s lost when T-338 was deleted or mutated to A, S, or D. The endodomain of g I was sufficient, if it contained T-338, to target a fusion protein contain ing the ectodomain of the human interleukin-2 receptor to the TGN. A trunca ted protein consisting only of the gI ectodomain was secreted and taken up by nontransfected cells. This uptake of the secreted gI ectodomain was bloc ked by mannose 6-phosphate. Following cotransfection, both gI and gE were r etrieved to the TGN from the plasma membrane in 26.7% of cells, neither gI nor gE was internalized in 18.3%, and gE was retrieved to the TGN while gI remained at the plasma membrane in 55%. We suggest that the T-338 of its en dodomain is necessary to retain gI in the TGN; moreover, because gI and gE interact, the signaling sequences of each glycoprotein reinforce one anothe r in ensuring that both glycoproteins are concentrated in the TGN yet remai n on the cell surface.