前沿速递！艾滋病病毒“软肋”终于被找到了，附文献全文

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近日，中国农业科学院哈尔滨兽医研究所基础免疫团队郑永辉研究员和张险峰博士在艾滋病天然免疫研究领域获得新发现，研究成果在线发表在国际病毒学著名专业期刊《病毒学杂志（Journal of Virology）》上。
丝氨酸整合因子（SERINC）是一种膜蛋白，其家族共有5个成员，其中SERINC5和SERINC3是目前最新发现的抗艾滋病病毒的天然免疫分子，可阻断病毒感染新的细胞，从而降低病毒的感染力。然而，对于SERINC家族各成员的抗病毒活性、体内表达以及抗病毒机理，科研人员还知之甚少。


在研究中发现， Ser5-001转录通过实时定量PCR检测至少比其他同种型多10倍。
当Ser5-001及其两个同种型Ser5-005和Ser5-008a从相同的哺乳动物表达载体中表达时，仅Ser5-001稳定表达，而其他同种型由于快速降解而表达差。
此外，与主要位于细胞质中的其他同种型不同，Ser5-001主要定位于质粒膜。为了绘制关键决定因素，产生了带有C末端缺失的Ser5突变体。
发现Ser5稳定表达和质膜定位所需的第10-26跨膜结构域。正如预期的那样，只有Ser5-001强烈地抑制HIV-1感染性，而没有第10-28跨膜的其他Ser5同种型和突变体表现出非常差的活性。
还观察到Nef抗体活性可以容易地被Ser5过表达饱和。因此，研究者们得出结论，Ser5-001是限制HIV-1的主要抗病毒同种型，并且第10,3跨膜结构域通过调节其蛋白质稳定性和质膜靶向在该过程中起关键作用。



最终，张险峰博士通过对SERINC家族5个成员系统性比对，不仅发现SERINC5具有最高的抗病毒活性，还检测到了该分子在人体细胞中表达的5种不同分子亚型。进一步研究发现，5种亚型中只有I亚型能够高效表达，相对于其它亚型，I亚型多了一个跨膜区，从而增强了蛋白稳定性并产生抗病毒效力。同时，I亚型在外周血单核细胞和巨噬细胞中表达丰富，预示其在抵抗病毒感染中的具有重要功能。   

郑永辉研究员近年来致力于针对病毒囊膜糖蛋白的宿主天然免疫分子机制的研究。他表示说，这一最新结果揭示了艾滋病病毒一个“软肋”，为开发新的治疗方法提供了新策略。



ABSTRACT
Among the five serine incorporator (SERINC) family members, SERINC5 (Ser5) was reported to strongly inhibit HIV-1 replication, which is counteracted by Nef. Ser5 produces 5 alternatively spliced isoforms: Ser5-001 has 10 putative transmembrane domains, whereas Ser5-004, -005, -008a, and -008b do not have the last one. Here, we confirmed the strong Ser5 anti-HIV-1 activity and investigated its isoforms' expression and antiviral activity. It was found that Ser5-001 transcripts were detected at least 10-fold more than the other isoforms by real-time quantitative PCR. When Ser5-001 and its two isoforms Ser5-005, and Ser5-008a were expressed from the same mammalian expression vector, only Ser5-001 was stably expressed, whereas the others were poorly expressed due to rapid degradation. In addition, unlike the other isoforms that are located mainly in the cytoplasm, Ser5-001 is localized primarily to the plasma membrane. To map the critical determinant, Ser5 mutants bearing C-terminal deletions were created. It was found that the 10th transmembrane domain is required for Ser5 stable expression and plasma membrane localization. As expected, only Ser5-001 strongly inhibits HIV-1 infectivity, whereas the other Ser5 isoforms and mutants that do not have the 10th transmembrane show a very poor activity. It was also observed that the Nef counteractive activity could be easily saturated by Ser5 overexpression. Thus, we conclude that Ser5-001 is the predominant antiviral isoform that restricts HIV-1, and the 10th transmembrane domain plays a critical role in this process by regulating its protein stability and plasma membrane targeting.

IMPORTANCE
HIV and SIV express a small protein Nef to enhance viral pathogenesis in vivo. Nef has an important in vitro function, which is to make virus particles more infectious, but the mechanism has been unclear. Recently, Nef was reported to counteract a novel anti-HIV host protein SERINC5 (Ser5). Ser5 has five alternatively spliced isoforms including Ser5-001, 004, 005, 008a, and 008b, and only Ser5-001 has an extra C-terminal transmembrane domain. We now show that the Ser5-001 transcripts are produced at least 10-fold more than the others, and only Ser5-001 produces stable proteins that are targeted to plasma membrane. Importantly, only Ser5-001 shows a strong anti-HIV-1 activity. We further demonstrate that the extra transmembrane domain is required for Ser5 stable expression and plasma membrane localization. These results suggest that plasma membrane localization is required for Ser5 antiviral activity, and Ser5-001 is the predominant isoform that contributes to the activity.