TRIM5α對EB病毒溶裂發展的影響

Abstract

Epstein-Barr Virus (EB病毒) 屬於人類皰疹病毒，其生活史包括潛伏期 (Latent life cycle) 與溶裂期 (Lytic cycle)。在病毒進入溶裂期時，病毒會大量複製其遺傳物質與蛋白質，組裝成完整的病毒顆粒以感染其他細胞。在病毒成熟的過程中，外鞘蛋白質 (Capsid protein) 的產生及組裝是非常重要的一環，卻是溶裂期中最少被研究的部份。皰疹病毒科的外鞘殼體為正二十面體，在EB病毒中，病毒外殼由主要外鞘蛋白質VCA (major capsid protein) 與次要外鞘蛋白質BORF1和BDLF1 (minor capsid protein) 組成。Tripartite-motif 5 alpha (TRIM5α) 是反轉錄病毒的限制因子 (restriction factor)，為具有泛素E3連接酶 (ubiquitin E3 ligase) 活性的細胞蛋白質，先前研究發現TRIM5α會與反轉錄病毒之外鞘蛋白質結合，使其提早解體而影響病毒的感染。本研究首先發現人類的TRIM5α能透過C端的B30.2功能區與EB病毒的外鞘蛋白質BORF1及VCA直接結合，另外TRIM5α在細胞外及細胞內皆會促進BORF1的泛素化修飾，而TRIM5α的SUMO-interacting motifs (SIMs) 對於增進BORF1的泛素化修飾扮演重要之角色。此外，若在細胞中過量表現TRIM5α會導致BORF1的濃度降低，而利用shRNA將細胞內的TRIM5α抑制後發現，BORF1蛋白質的表現量則會增加。最後利用P3HR1細胞收集EB病毒顆粒，發現將TRIM5α抑制後EB病毒產生的量會大幅的增加。綜合以上結果，本研究發現TRIM5α會藉由影響EB病毒的外鞘蛋白質，進而抑制EB病毒溶裂期的發展。

Epstein-Barr virus (EBV) is a human herpesvirus that contains two distinct life cycles, latency and lytic cycle. During the lytic cycle, EBV encodes a series of proteins that are necessary for viral lytic DNA replication, capsid assembly and viral maturation to produce infectious viral particles. The production and assembly of capsid proteins are important to the maturation of EBV virion, but the mechanism is unclear. The capsids of herpesviruses have a common icosahedral structure. In EBV, the capsid contains a major capsid protein, VCA and two minor capsid proteins, BORF1 and BDLF1. Tripartite-motif 5 alpha (TRIM5α) that possesses the ubiquitin E3 ligase activity is a restriction factor of retroviruses. Previous study showed that TRIM5α binds the retroviral capsids and disrupts viral structure to accelerate uncoating of capsids, which influences the replication of retroviruses. This study demonstrates that TRIM5α binds to EBV capsid proteins, BORF1 and VCA, through the C-terminal B30.2 domain. Moreover, TRIM5α promotes the ubiquitination of BORF1 in vitro and in vivo and the SUMO-interaction motifs in TRIM5α are important to the ubiquitination of BORF1. Furthermore, over-expressing of TRIM5α reduces the amount of BORF1, while knocking down of TRIM5α enhances the level of BORF1. Finally, inhibition the expression of TRIM5α enhances EBV lytic progression and promotes virion production. Taken together, this study demonstrates that TRIM5α targets to EBV capsid proteins to inhibit the lytic progression.