Zaire ebolavirus (EBOV) causes a severe hemorrhagic
fever in humans and non-human primates with high morbidity and mortality. EBOV
infection is dependent on its
structural glycoprotein (GP), but high levels of GP expression also trigger cell rounding, detachment, and downregulation of many surface molecules that is thought to contribute to its high pathogenicity. Thus, EBOV has evolved an RNA editing mechanism to reduce its GP expression and increase its fitness. We now report that the GP expression is also suppressed at the
protein level in cells by
protein disulfide isomerases (PDIs). Although PDIs promote oxidative protein folding by catalyzing correct
disulfide formation in the endoplasmic reticulum (ER), PDIA3/ERp57 adversely triggered the GP misfolding by targeting GP
cysteine residues and activated the unfolded protein response (UPR). Abnormally folded GP was targeted by ER-associated protein degradation (ERAD) machinery and, unexpectedly, was degraded via the macroautophagy/autophagy-lysosomal pathway, but not the proteasomal pathway. PDIA3 also decreased the GP expression from other ebolavirus species but increased the GP expression from Marburg virus (MARV), which is consistent with the observation that MARV-GP does not cause cell rounding and detachment, and MARV does not regulate its GP expression via RNA editing during
infection. Furthermore, five other PDIs also had a similar inhibitory activity to EBOV-GP. Thus, PDIs negatively regulate ebolavirus
glycoprotein expression, which balances the viral life cycle by maximizing their
infection but minimizing their cellular effect. We suggest that ebolaviruses hijack the host protein folding and ERAD machinery to increase their fitness via reticulophagy during
infection.Abbreviations: 3-MA: 3-methyladenine; 4-PBA: 4-phenylbutyrate; ACTB: β-actin; ATF: activating
transcription factor; ATG: autophagy-related; BafA1:
bafilomycin A1; BDBV: Bundibugyo ebolavirus; CALR:
calreticulin; CANX:
calnexin; CHX:
cycloheximide; CMA: chaperone-mediated autophagy; ConA:
concanamycin A; CRISPR: clusters of regularly interspaced short palindromic repeats; Cas9:
CRISPR-associated protein 9; dsRNA:
double-stranded RNA; EBOV: Zaire ebolavirus; EDEM: ER degradation enhancing
alpha-mannosidase like
protein; EIF2AK3/PERK: eukaryotic translation
initiation factor 2 alpha
kinase 3; Env: envelope
glycoprotein; ER: endoplasmic reticulum; ERAD: ER-associated protein degradation; ERN1/IRE1: endoplasmic reticulum to nucleus signaling 1; GP:
glycoprotein; HA:
hemagglutinin; HDAC6:
histone deacetylase 6; HMM: high-molecular-mass; HIV-1: human immunodeficiency virus type 1; HSPA5/BiP:
heat shock protein family A (Hsp70) member 5; IAV: influenza A virus; IP: immunoprecipitation; KIF: kifenesine; Lac:
lactacystin;
LAMP: lysosomal associated membrane protein; MAN1B1/ERManI:
mannosidase alpha class 1B member 1; MAP1LC3/LC3:
microtubule associated protein 1 light chain 3; MARV: Marburg virus; MLD:
mucin-like domain; NHK/SERPINA1: alpha1-antitrypsin variant null (Hong Kong);
NTZ:
nitazoxanide; PDI:
protein disulfide isomerase; RAVV: Ravn virus; RESTV:
Reston ebolavirus; SARS-CoV: severe acute respiratory syndrome coronavirus; SBOV: Sudan ebolavirus; sGP: soluble GP; SQSTM1/p62: sequestosome 1; ssGP: small soluble GP; TAFV: Taï Forest ebolavirus; TIZ:
tizoxanide; TGN:
thapsigargin; TLD: TXN (
thioredoxin)-like domain; Ub:
ubiquitin; UPR: unfolded protein response; VLP: virus-like particle; VSV:
vesicular stomatitis virus; WB: Western blotting; WT: wild-type; XBP1:
X-box binding protein 1.