Abstract
ABSTRACTHepatitis C virus (HCV) infection reorganizes cellular membranes to create an active viral replication site named the membranous web (MW). The role that human choline kinase-α (hCKα) plays in HCV replication remains elusive. Here, we first showed that hCKα activity, not the CDP-choline pathway, promoted viral RNA replication. Confocal microscopy and subcellular fractionation of HCV-infected cells revealed that a small fraction of hCKα colocalized with the viral replication complex (RC) on the endoplasmic reticulum (ER) and that HCV infection increased hCKα localization to the ER. In the pTM-NS3-NS5B model, NS3-NS5B expression increased the localization of the wild-type, not the inactive D288A mutant, hCKα on the ER, and hCKα activity was required for effective trafficking of hCKα and NS5A to the ER. Coimmunoprecipitation showed that hCKα was recruited onto the viral RC presumably through its binding to NS5A domain 1 (D1). hCKα silencing or treatment with CK37, an hCKα activity inhibitor, abolished HCV-induced MW formation. In addition, hCKα depletion hindered NS5A localization on the ER, interfered with NS5A and NS5B colocalization, and mitigated NS5A-NS5B interactions but had no apparent effect on NS5A-NS4B and NS4B-NS5B interactions. Nevertheless, hCKα activity was not essential for the binding of NS5A to hCKα or NS5B. These findings demonstrate that hCKα forms a complex with NS5A and that hCKα activity enhances the targeting of the complex to the ER, where hCKα protein, not activity, mediates NS5A binding to NS5B, thereby promoting functional membranous viral RC assembly and viral RNA replication. IMPORTANCE HCV infection reorganizes the cellular membrane to create an active viral replication site named the membranous web (MW). Here, we report that human choline kinase-α (hCKα) acts as an essential host factor for HCV RNA replication. A fraction of hCKα colocalizes with the viral replication complex (RC) on the endoplasmic reticulum (ER) in HCV-infected cells. NS3-NS5B expression increases ER localization of wild-type, but not D288A mutant, hCKα, and hCKα activity facilitates the transport of itself and NS5A to the ER. Silencing or inactivation of hCKα abrogates MW formation. Moreover, hCKα is recruited by NS5A independent of hCKα activity, presumably through binding to NS5A D1. hCKα activity then mediates the ER targeting of the hCKα-NS5A complex. On the ER membrane, hCKα protein, per se, induces NS5A binding to NS5B, thereby promoting membranous RC formation and viral RNA replication. Our study may benefit the development of hCKα-targeted anti-HCV therapeutics.
Highlights
hepatitis C virus (HCV) infection reorganizes the cellular membrane to create an active viral replication site named the membranous web (MW)
This knockdown of hCK␣ expression was concomitant with the reduced expression of HCV proteins, such as NS3, NS5A, and core proteins, compared to that detected in control small interfering RNA (siRNA)-transfected cells (Fig. 1B)
As the PC level was preserved despite impairment of the CDP-choline pathway by hCK␣ or CTP:phosphocholine cytidyltransferase (CCT)␣ knockdown (Fig. 3E, F, and H), we examined whether the alternative route that synthesizes PC from PE by the phosphatidylethanolamine Nmethyltransferase (PEMT) pathway (Fig. 1A) was induced when the CDP-choline pathway was disrupted in Huh7 cells
Summary
HCV infection reorganizes the cellular membrane to create an active viral replication site named the membranous web (MW). On the ER membrane, hCK␣ protein, per se, induces NS5A binding to NS5B, thereby promoting membranous RC formation and viral RNA replication. The viral replication complex (RC), called the replicase, is composed of viral proteins NS3 to NS5B and the replicating viral RNA [4] These viral RCs are housed on altered endoplasmic membranes and form distinct organelle-like structures termed membranous webs (MWs) [5,6,7,8]. NS5A is thought to facilitate the transport of viral RNA from the MW replication site to LDs to interact with core protein, thereby promoting viral RNA encapsidation, nucleocapsid formation, and virus assembly [24,25,26]. The close proximity of LDs, the MW, and the endoplasmic reticulum (ER) membrane effectively couples viral RNA replication to virion assembly
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