The FAT10 Posttranslational Modification Is Involved in Lytic Replication of Kaposi's Sarcoma-Associated Herpesvirus

During Kaposi's sarcoma-associated herpesvirus (KSHV) lytic replication, host cell functions, including protein expression and posttranslational modification pathways, are dysregulated by KSHV to promote virus production. Here, we attempted to identify key proteins for KSHV lytic replication by profiling protein expression in the latent and lytic phases using liquid chromatography-tandem mass spectrometry (LC-MS/MS). Proteomic analysis, immunoblotting, and quantitative PCR demonstrated that antigen F (HLA-F) adjacent transcript 10 (FAT10) and UBE1L2 (also known as ubiquitin-like modifier-activating enzyme 6 [UBA6]) were upregulated during lytic replication. FAT10 is a ubiquitin-like protein (UBL). UBE1L2 is the FAT10-activating enzyme (E1), which is essential for FAT10 modification (FAT10ylation). FAT10ylated proteins were immediately expressed after lytic induction and increased over time during lytic replication. Knockout of UBE1L2 suppressed KSHV production but not KSHV DNA synthesis. In order to isolate FAT10ylated proteins during KSHV lytic replication, we conducted immunoprecipitation using anti-FAT10 antibody and nickel-nitrilotriacetic acid (Ni-NTA) chromatography of exogenously expressed His-tagged FAT10 from cells undergoing latent or lytic replication. LC-MS/MS was performed to identify FAT10ylated proteins. We identified KSHV ORF59 and ORF61 as FAT10ylation substrates. Our study revealed that the UBE1L2-FAT10 system is upregulated during KSHV lytic replication, and it contributes to viral propagation. IMPORTANCE Ubiquitin and UBL posttranslational modifications, including FAT10, are utilized and dysregulated by viruses for achievement of effective infection and virion production. The UBE1L2-FAT10 system catalyzes FAT10ylation, where one or more FAT10 molecules are covalently linked to a substrate. FAT10ylation is catalyzed by the sequential actions of E1 (activation enzyme), E2 (conjugation enzyme), and E3 (ligase) enzymes. The E1 enzyme for FAT10ylation is UBE1L2, which activates FAT10 and transfers it to E2/USE1. FAT10ylation regulates the cell cycle, interferon (IFN) signaling, and protein degradation; however, its primary biological function remains unknown. Here, we revealed that KSHV lytic replication induces UBE1L2 expression and production of FAT10ylated proteins, including KSHV lytic proteins. Moreover, UBE1L2 knockout suppressed virus production during the lytic cycle. This is the first report demonstrating the contribution of the UBE1L2-FAT10 system to KSHV lytic replication. Our findings provide insight into the physiological function(s) of novel posttranslational modifications in KSHV lytic replication.