Samrat Subodh Kumar, Ha Binh L, Zheng Yi, Gu Haidong
Department of Biological Sciences, Wayne State University, Detroit, Michigan, USA.
Department of Biological Sciences, Wayne State University, Detroit, Michigan, USA
J Virol. 2018 Jan 2;92(2). doi: 10.1128/JVI.01673-17. Print 2018 Jan 15.
Infected cell protein 0 (ICP0) of herpes simplex virus 1 (HSV-1) is an immediate early protein containing a RING-type E3 ubiquitin ligase. It targets several host factors for proteasomal degradation and subsequently activates viral expression. ICP0 has a nuclear localization sequence and functions in the nucleus early during infection. However, later in infection, ICP0 is found solely in the cytoplasm. The molecular mechanism and biological function of the ICP0 nuclear-to-cytoplasmic translocation are not well understood. In this study, we sought to characterize elements important for this translocation. We found that (i) in human embryonic lung fibroblast (HEL) cells, ICP0 C-terminal residues 741 to 775 were necessary but not sufficient for the nuclear-to-cytoplasmic translocation; (ii) the loss of ICP0 E3 ubiquitin ligase activity, which led to defective viral replication in nonpermissive cells, also caused mutant ICP0 to be retained in the nucleus of HEL cells; (iii) in permissive U2OS cells, however, ICP0 lacking E3 ligase activity was translocated to the cytoplasm at a pace faster than that of wild-type ICP0, suggesting that nuclear retention of ICP0 occurs in an ICP0 E3 ligase-dependent manner; and (iv) the ICP0 C terminus and late viral proteins cooperate in order to overcome nuclear retention and stimulate ICP0 cytoplasmic translocation. Taken together, less ICP0 nuclear retention may contribute to the permissiveness of U2OS cells to HSV-1 in the absence of functional ICP0. A distinct characteristic for eukaryotes is the compartmentalization of cell metabolic pathways, which allows greater efficiency and specificity of cellular functions. ICP0 of HSV-1 is a multifunctional viral protein that travels through different compartments as infection progresses. Its main regulatory functions are carried out in the nucleus, but it is translocated to the cytoplasm late during HSV-1 infection. To understand the biological significance of cytoplasmic ICP0 in HSV-1 infection, we investigated the potential players involved in this nuclear-to-cytoplasmic translocation. We found that there is a nuclear retention force in an ICP0 E3 ubiquitin ligase-dependent manner. In addition, we identified the C terminus of ICP0 as a element cooperating with late viral proteins to overcome the nuclear retention and stimulate the nuclear-to-cytoplasmic translocation of ICP0.
单纯疱疹病毒1型（HSV-1）的感染细胞蛋白0（ICP0）是一种即刻早期蛋白，含有一个RING型E3泛素连接酶。它将几种宿主因子靶向蛋白酶体降解，随后激活病毒表达。ICP0具有核定位序列，在感染早期在细胞核中发挥作用。然而，在感染后期，ICP0仅存在于细胞质中。ICP0核转位至细胞质的分子机制和生物学功能尚不清楚。在本研究中，我们试图鉴定对这种转位重要的元件。我们发现：（i）在人胚肺成纤维细胞（HEL）中，ICP0的C末端残基741至775对于核转位至细胞质是必需的，但并不充分；（ii）ICP0的E3泛素连接酶活性丧失导致在非允许细胞中病毒复制缺陷，这也使得突变型ICP0保留在HEL细胞的细胞核中；（iii）然而，在允许性U2OS细胞中，缺乏E3连接酶活性的ICP0转位至细胞质的速度比野生型ICP0更快，这表明ICP0的核保留以依赖ICP0 E3连接酶的方式发生；（iv）ICP0的C末端和晚期病毒蛋白协同作用以克服核保留并刺激ICP0的细胞质转位。综上所述，较少的ICP0核保留可能有助于U2OS细胞在缺乏功能性ICP0的情况下对HSV-1的允许性。真核生物的一个显著特征是细胞代谢途径的区室化，这使得细胞功能具有更高的效率和特异性。HSV-1的ICP0是一种多功能病毒蛋白，随着感染的进展在不同区室中穿梭。其主要调节功能在细胞核中进行，但在HSV-1感染后期转位至细胞质。为了理解细胞质ICP0在HSV-1感染中的生物学意义，我们研究了参与这种核转位至细胞质过程的潜在因素。我们发现存在一种以依赖ICP0 E3泛素连接酶的方式的核保留力。此外，我们鉴定出ICP0的C末端是一个与晚期病毒蛋白协同作用以克服核保留并刺激ICP0核转位至细胞质的元件。
