Appraising the Roles of CBLL1 and the Ubiquitin/Proteasome System for Flavivirus Entry and Replication

Abstract

The ubiquitin ligase CBLL1 (also known as HAKAI) has been proposed to be a critical cellular factor exploited by West Nile virus (WNV) for productive infection. CBLL1 has emerged as a major hit in a recent RNA interference screen designed to identify cellular factors required for the early stages of the WNV life cycle. Follow-up experiments showed that HeLa cells knocked down for CBLL1 by a small interfering RNA (siRNA) failed to internalize WNV particles and resisted infection. Furthermore, depletion of a free-ubiquitin pool by the proteasome inhibitor MG132 abolished WNV endocytosis, suggesting that CBLL1 acts in concert with the ubiquitin proteasome system to mediate virus internalization. Here, we examined the effect of CBLL1 knockdown and proteasome inhibitors on infection by WNV and other flaviviruses. We identified new siRNAs that repress the CBLL1 protein and strongly inhibit the endocytosis of Listeria monocytogenes, a bacterial pathogen known to require CBLL1 to invade host cells. Strikingly, however, we detected efficient WNV, dengue virus, and yellow fever virus infection of human cells, despite potent downregulation of CBLL1 by RNA interference. In addition, we found that the proteasome inhibitors MG132 and lactacystin did not affect WNV internalization but strongly repressed flavivirus RNA translation and replication. Together, these data do not support a requirement for CBLL1 during flavivirus entry and rather suggest an essential role of the ubiquitin/proteasome pathway for flavivirus genome amplification.West Nile virus (WNV), dengue virus (DV), and yellow fever virus (YFV) are mosquito-borne flaviviruses that cause neurological disease and hemorrhagic fever in humans and constitute major public health threats (11,19). They are lipidenveloped viruses that contain a single-stranded, positive-sense RNA genome (12, 16). To initiate their infectious life cycle, flaviviruses interact via their envelope glycoprotein (E protein) with still-uncharacterized cellular receptors (2), enter the host cell by clathrin-mediated endocytosis, and traffic through the endosomal pathway (5,15,27). Endosome acidification triggers major conformational changes in their E proteins that induce fusion of the viral and host cell membranes (4,13,20), resulting in the release of the viral capsid and genomic RNA into the cytosol. The viral genome is translated in the cytoplasm as a single polyprotein, which is co-and posttranslationally processed by both host cell signalases and the virus-encoded protease NS2B/NS3 into three structural and seven nonstructural (NS) proteins (12, 16). Because flaviviruses encode only 10 proteins, they need to subvert and/or counteract host cell functions for viral propagation (8). Understanding the interplay between flaviviruses and the host cell machinery could provide important insights into the molecular mechanisms of viral pathogenesis and could facilitate the development of a novel generation of antiviral drugs targeting host cell factors. A recent genome-wide RNA interference (RNAi) scree...

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