USP11 及 USP7 對 EB 病毒 Zta 泛素化調控

Abstract

Epstein-Barr Virus (EBV) 是疱疹病毒科的人類致癌病毒，在感染 B 細胞後，常以潛伏狀態存在，而進入溶裂期後才開始增殖，且從潛伏期進入裂解期時，EBV 會表達 Rta 及 Zta 這兩個關鍵轉錄因子，以活化溶裂期基因的表現。目前已知 Zta 會被泛素化修飾，但促進其泛素化的E3泛素連接酶尚不清楚。本實驗室先前研究顯示 E3 泛素連接酶 Ring Finger Protein 4 (RNF4) 能促進 Zta 的泛素化修飾。同時有研究顯示 RNF4 與去泛素化酶 Ubiquitin-specific protease 11 (USP11) 以及 Ubiquitin-specific-processing protease 7 (USP7) 具有拮抗作用，且 USP11 與 USP7 彼此亦可結合。因此本研究的目的是要探討 RNF4、USP11 及 USP7 對於 Zta 泛素化的調控。首先 GST pull-down 及免疫共沉澱結果顯示，RNF4、USP11 及 USP7 皆可與 Zta 結合；以變性蛋白質沈澱分析，結果發現，當過量表達 RNF4 時，Zta 的泛素化會顯著提高。而利用基因敲落 (Knockdown) 抑制細胞中的 USP11 或 USP7表達，Zta 的泛素化則會增加；相反的，過量表達 USP11 或 USP7時，Zta 的泛素化則明顯下降。此外，Cycloheximide (CHX) chase assay 的結果顯示，過量表達 RNF4，Zta 的穩定性明顯下降，而下調 USP11 基因表達時亦有相同的結果；而過量表達 USP7 則提升 Zta 的表現量。綜合上述，本研究發現 RNF4 可透過結合 Zta 的 N 端及 C 端增加其泛素化，進而降低其穩定性，而 USP11 與 USP7 則能降低 Zta 的泛素化修飾，分別提高其穩定性與表現量。本研究對 Zta 如何受到泛素化的調控提出新穎的見解，對於 EBV 的溶裂期進展具有重要的影響。

Epstein-Barr Virus (EBV), is an oncogenic human herpesvirus. After infecting B lymphocytes, the virus is usually maintained under latent conditions. However, the virus has to enter a lytic cycle to proliferate. During the switch from latency to the lytic cycle, EBV expresses two transcription factors, Rta and Zta to activate lytic genes. It is known that Zta is posttranslationally conjugated by ubiquitin, although the ubiquitin E3 ligase that promotes the ubiquitination is unclear. Our previous study shows that an E3 ligase, RING Finger Protein 4 (RNF4) promotes the ubiquitination of Zta. The aims of this study are to elucidate the roles of RNF4 and the deubiquitinases Ubiquitin-Specific Protease 11 (USP11) and Ubiquitin-Specific Protease 7 (USP7) in regulating the ubiquitination of Zta. Firstly, GST pull-down and co-immunoprecipitation assay showed that RNF4, USP11, and USP7 interact with Zta. Denature -immunoprecipitation analysis revealed that RNF4 overexpression enhances Zta’s ubiquitination, while knockdown of the expression of USP11 or USP7 increases the levels of Zta’s ubiquitination; conversely, overexpressing USP11 or USP7 reduced the amounts of Zta’s ubiquitination. Meanwhile, cycloheximide (CHX) chase assays indicated that RNF4 overexpression or USP11 knockdown reduces the stability of Zta, whereas USP7 overexpression increases its expression. These findings suggest that USP11 and USP7 counteract RNF4-mediated Zta’s ubiquitination, and thus enhances its stability. This study provides a novel insight into the post-translational regulation of Zta and its role in EBV lytic progression.