Department of Biochemistry, Genetics and Microbiology, University of Pretoria, Pretoria, South Africa.
Department of Biochemistry, Focus Area for Human Metabolomics, North-West University, Potchefstroom, South Africa.
J Gen Virol. 2020 Apr;101(4):366-384. doi: 10.1099/jgv.0.001396. Epub 2020 Feb 28.
African horse sickness virus (AHSV) is the causative agent of the often fatal disease African horse sickness in equids. The non-structural protein NS4 is the only AHSV protein that localizes to the nucleus. Here we report that all AHSV reference and representative field strains express one of the two forms of NS4, i.e. NS4-I or NS4-II. Both forms of NS4 are nucleocytoplasmic proteins, but NS4-I has a stronger nuclear presence whilst NS4-II has a proportionally higher cytoplasmic distribution. A subtype of NS4-II containing a nuclear localization signal (NLS), named NLS-NS4-II, displays distinct punctate foci in the nucleus. We showed that NS4 likely enters the nucleus via passive diffusion as a result of its small size. Colocalization analysis with nuclear compartments revealed that NS4 colocalizes with promyelocytic leukaemia nuclear bodies (PML-NBs), suggesting a role in the antiviral response or interferon signalling. Interestingly, we showed that two other AHSV proteins also interact with nuclear components. A small fraction of the NS1 tubules were present in the nucleus and associated with PML-NBs; this was more pronounced for a virus strain lacking NS4. A component of nuclear speckles, serine and arginine rich splicing factor 2 (SRSF2) was recruited to viral inclusion bodies (VIBs) in the cytoplasm of AHSV-infected cells and colocalized with NS2. Nuclear speckles are important sites for cellular mRNA transcript processing and maturation. Collectively, these results provide data on three AHSV non-structural proteins interacting with host cell nuclear components that could contribute to overcoming antiviral responses and creating conditions that will favour viral replication.
非洲马瘟病毒（AHSV）是马属动物常致命疾病非洲马瘟的病原体。非结构蛋白 NS4 是唯一一种定位于细胞核的 AHSV 蛋白。在这里，我们报告说，所有 AHSV 参考和代表性田间株都表达 NS4 的两种形式之一，即 NS4-I 或 NS4-II。这两种形式的 NS4 都是核质蛋白，但 NS4-I 在核内的存在更强，而 NS4-II 在细胞质中的分布比例更高。一种含有核定位信号（NLS）的 NS4-II 亚型，命名为 NLS-NS4-II，在核内显示出明显的点状焦点。我们表明，NS4 可能由于其体积小而通过被动扩散进入细胞核。与核区室的共定位分析表明，NS4 与早幼粒细胞白血病核体（PML-NBs）共定位，表明其在抗病毒反应或干扰素信号转导中发挥作用。有趣的是，我们表明，另外两种 AHSV 蛋白也与核成分相互作用。一小部分 NS1 小管存在于细胞核中并与 PML-NBs 相关联；对于缺乏 NS4 的病毒株，这种情况更为明显。核斑点的一个组成部分，丝氨酸和精氨酸丰富剪接因子 2（SRSF2）被招募到 AHSV 感染细胞的细胞质中的病毒包涵体（VIBs）中，并与 NS2 共定位。核斑点是细胞 mRNA 转录加工和成熟的重要场所。总的来说，这些结果提供了关于三种 AHSV 非结构蛋白与宿主细胞核成分相互作用的数据，这可能有助于克服抗病毒反应并创造有利于病毒复制的条件。
