Structures and Corresponding Functions of Five Types of Picornaviral 2A Proteins
Inositol-requiring enzyme 1 (IRE1) plays an important role in the endoplasmic reticulum (ER), or unfolded protein, stress response by activating its downstream transcription factor X-box-binding protein 1 (XBP1). We demonstrated previously that enterovirus 71 (EV71) upregulated XBP1 mRNA levels but did not activate spliced XBP1 (XBP1s) mRNA or its downstream target genes, EDEM and chaperones. In this study, we investigated further this regulatory mechanism and found that IRE1 was phosphorylated and activated after EV71 infection, whereas its downstream XBP1s protein level decreased. We also found that XBP1s was not cleaved directly by 2A(pro), but that cleavage of eukaryotic translation initiation factor 4G by the EV71 2A(pro) protein may contribute to the decrease in XBP1s expression. Knockdown of XBP1 increased viral protein expression, and the synthesis of EV71 viral protein and the production of EV71 viral particles were inhibited in XBP1-overexpressing RD cells. When incubated with replication-deficient and UV-irradiated EV71, XBP1-overexpressing RD cells exhibited reduced viral RNA levels, suggesting that the inhibition of XBP1s by viral infection may underlie viral entry, which is required for viral replication. Our findings are the first indication of the ability of XBP1 to inhibit viral entry, possibly via its transcriptional activity in regulating molecules in the endocytic machinery.