E2 Enzyme from Chinese White Shrimp Inhibits Replication of White Spot Syndrome Virus and Ubiquitinates its RING Domain Proteins Running title: Shrimp E2 ubiquitinates WSSV RING Domain proteins

Recent studies have shown that the ubiquitin proteasome pathway (UPP) is closely related to immune defense. We have identified a ubiquitin-conjugating enzyme E2 from the Chinese white shrimp Fenneropenaeus chinensis (FcUbc). Injection of recombinant FcUbc protein (rFcUbc) reduced the mortality of shrimp infected with White Spot Syndrome Virus (WSSV) and inhibited replication of WSSV. rFcUbc, but not a mutant FcUbc (mFcUbc), bound to WSSV RING domains (WRDs) from four potential E3 ligase proteins of WSSV in vitro. Importantly, rFcUbc could ubiquitinate the RING domains (named WRD2 and WRD3) of WSSV277 and WSSV304 proteins in vitro and the two proteins in the WSSV infected Drosophila S2 cells. Furthermore, overexpression of FcUbc increased ubiquitination of WSSV277 and WSSV304 during WSSV infection. In summary, our study demonstrates that FcUbc from Chinese white shrimp inhibited WSSV replication and could ubiquitinate WSSV RING domain-containing proteins. This is the first report about antiviral function of Ubc E2 in shrimp.