The human cytomegalovirus ER resident glycoprotein UL148 activates the unfolded protein response

Abstract

Eukaryotic cells are equipped with three sensors that respond to the accumulation of misfolded proteins within the lumen of the endoplasmic reticulum (ER) by activating the unfolded protein response (UPR), which functions to resolve proteotoxic stresses involving the secretory pathway. Here, we identify UL148, a viral ER resident glycoprotein from human cytomegalovirus (HCMV), as an inducer of the UPR. Metabolic labeling results indicate that global mRNA translation is markedly decreased when UL148 expression is induced in uninfected cells. Further, we find evidence suggesting that ectopic expression of UL148 is sufficient to activate at least two UPR sensors: the inositol requiring enzyme-1 (IRE1), as indicated by splicing of Xbp1 mRNA, and the PKR-like ER kinase (PERK), as indicated by phosphorylation of eIF2 and accumulation of ATF4 protein. During wild-type HCMV infection, Xbp-1 splicing, eIF2 phosphorylation and ATF4 accumulation neatly accompanied the onset of UL148 expression. However, the appearance of these UPR indicators was either markedly delayed or absent during UL148-null infections. siRNA depletion of PERK dampened the extent of eIF2 phosphorylation and ATF4 induction observed during wild-type infection, implicating PERK as opposed to other eIF2 kinases. A virus disrupted for UL148 showed statistically significant 2- to 4-fold decreases during infection in the levels of transcripts canonically regulated by PERK/ATF4 and by the ATF6 pathway.Taken together, our results argue that UL148 is sufficient to activate the UPR when expressed ectopically and that UL148 is an important cause of UPR activation in the context of the HCMV infected cell.\n\nIMPORTANCEThe unfolded protein response (UPR) is an ancient cellular response to ER stress of broad importance to viruses. Certain consequences of the UPR, including mRNA degradation and translational shut-off, would presumably be disadvantageous to viruses, while other attributes of the UPR, such as ER expansion and upregulation of protein folding chaperones, might enhance viral replication. Although HCMV is estimated to express at least 200 distinct viral proteins, we show that the HCMV ER resident glycoprotein UL148 contributes substantially to the UPR during infection, and moreover is sufficient to activate the UPR in non-infected cells. Experimental activation of the UPR in mammalian cells is difficult to achieve without the use of toxins. Therefore, UL148 may provide a new tool to investigate fundamental aspects of the UPR. Furthermore, our findings may have implications for understanding the mechanisms underlying the effects of UL148 on HCMV cell tropism and evasion of cell mediated immunity.