Li Lin, Liang Dong, Li Jing-yun, Zhao Richard Y
Department of Pathology, University of Maryland, 10 South Pine Street, MSTF700A, Baltimore, MD 21201, USA.
Retrovirology. 2008 Aug 4;5:72. doi: 10.1186/1742-4690-5-72.
Although APOBEC3G protein is a potent and innate anti-HIV-1 cellular factor, HIV-1 Vif counteracts the effect of APOBEC3G by promoting its degradation through proteasome-mediated proteolysis. Thus, any means that could prevent APOBEC3G degradation could potentially enhance its anti-viral effect. The UBA2 domain has been identified as an intrinsic stabilization signal that protects protein from proteasomal degradation. In this pilot study, we tested whether APOBEC3G, when it is fused with UBA2, can resist Vif-mediated proteasomal degradation and further inhibit HIV-1 infection.
APOBEC3G-UBA2 fusion protein is indeed more resistant to Vif-mediated degradation than APOBEC3G. The ability of UBA2 domain to stabilize APOBEC3G was diminished when polyubiquitin was over-expressed and the APOBEC3G-UBA2 fusion protein was found to bind less polyubiquitin than APOBEC3G, suggesting that UBA2 stabilizes APOBEC3G by preventing ubiquitin chain elongation and proteasome-mediated proteolysis. Consistently, treatment of cells with a proteasome inhibitor MG132 alleviated protein degradation of APOBEC3G and APOBEC3G-UBA2 fusion proteins. Analysis of the effect of APOBEC3G-UBA2 fusion protein on viral infectivity indicated that infection of virus packaged from HEK293 cells expressing APOBEC3G-UBA2 fusion protein is significantly lower than those packaged from HEK293 cells over-producing APOBEC3G or APOBEC3G-UBA2 mutant fusion proteins.
Fusion of UBA2 to APOBEC3G can make it more difficult to be degraded by proteasome. Thus, UBA2 could potentially be used to antagonize Vif-mediated APOBEC3G degradation by preventing polyubiquitination. The stabilized APOBEC3G-UBA2 fusion protein gives stronger inhibitory effect on viral infectivity than APOBEC3G without UBA2.
尽管载脂蛋白B mRNA编辑酶催化多肽样蛋白3G（APOBEC3G）是一种强大的先天性抗HIV-1细胞因子，但HIV-1病毒感染因子（Vif）通过蛋白酶体介导的蛋白水解促进其降解，从而抵消了APOBEC3G的作用。因此，任何能够阻止APOBEC3G降解的方法都可能增强其抗病毒效果。泛素样蛋白激活酶2（UBA2）结构域已被确定为一种内在的稳定信号，可保护蛋白质免受蛋白酶体降解。在这项初步研究中，我们测试了与UBA2融合的APOBEC3G是否能够抵抗Vif介导的蛋白酶体降解，并进一步抑制HIV-1感染。
APOBEC3G-UBA2融合蛋白确实比APOBEC3G更能抵抗Vif介导的降解。当多聚泛素过度表达时，UBA2结构域稳定APOBEC3G的能力减弱，并且发现APOBEC3G-UBA2融合蛋白比APOBEC3G结合的多聚泛素更少，这表明UBA2通过阻止泛素链延长和蛋白酶体介导的蛋白水解来稳定APOBEC3G。一致地，用蛋白酶体抑制剂MG132处理细胞可减轻APOBEC3G和APOBEC3G-UBA2融合蛋白的降解。对APOBEC3G-UBA2融合蛋白对病毒感染性影响的分析表明，从表达APOBEC3G-UBA2融合蛋白的HEK293细胞包装的病毒感染性明显低于从过量产生APOBEC3G或APOBEC3G-UBA2突变融合蛋白的HEK293细胞包装的病毒。
将UBA2与APOBEC3G融合可使其更难被蛋白酶体降解。因此，UBA2可能通过防止多聚泛素化来拮抗Vif介导的APOBEC3G降解。稳定的APOBEC3G-UBA2融合蛋白对病毒感染性的抑制作用比没有UBA2的APOBEC3G更强。
