Ebola Virus and Severe Acute Respiratory Syndrome Coronavirus Display Late Cell Entry Kinetics: Evidence that Transport to NPC1 + Endolysosomes Is a Rate-Defining Step

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Abstract
Ebolavirus (EBOV) causes hemorrhagic fevers with high mortality rates. During cellular entry, the virus is internalized by macropinocytosis and trafficked through endosomes until fusion between the viral and an endosomal membrane is triggered, releasing the RNA genome into the cytoplasm. We found that while macropinocytotic uptake of filamentous EBOV virus-like particles (VLPs) expressing the EBOV glycoprotein (GP) occurs relatively quickly, VLPs only begin to enter the cytoplasm after a 30 min lag, considerably later than particles bearing the influenza hemagglutinin or GP from lymphocytic choriomeningitis virus, which enter through late endosomes (LE). For EBOV, the long lag is not due to the large size or unusual shape of EBOV filaments, the need to prime EBOV GP to the 19-kDa receptor-binding species, or a need for unusually low endosomal pH. In contrast, since we observed that EBOV entry occurs upon arrival in NPC1+ endolysosomes (LE/Lys), we propose that trafficking to LE/Lys is a key rate-defining step. Additional experiments revealed, unexpectedly, that SARS S-mediated entry also only begins after a 30 min lag. Furthermore, although SARS does not require NPC1 for entry, SARS entry also begins after colocalization with NPC1. Since the only endosomal requirement for SARS entry is cathepsin L activity, we tested, and provide evidence that NPC1+ LE/Lys have higher cathepsin L activity than LE, with no detectable activity in earlier endosomes. Our findings suggest that both EBOV and SARS traffic deep into the endocytic pathway for entry and that they do so to access higher cathepsin activity.