Human cytomegalovirus (HCMV) glycoprotein gB promotes virus entry in trans acting as the viral fusion protein rather than as a receptor-binding protein.
Human cytomegalovirus (HCMV) glycoprotein gB promotes virus entry in trans acting as the viral fusion protein rather than as a receptor-binding protein.
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DOI：
10.1128/mbio.00332-13
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发表时间：
2013-06-04
期刊：
影响因子：
6.4
通讯作者：
Johnson DC
中科院分区：
文献类型：
--
作者：
Wille PT;Wisner TW;Ryckman B;Johnson DC
人巨细胞病毒 (HCMV) 糖蛋白 gB 和 gH/gL 对于细胞间融合来说是必要且充分的。然而，尚不清楚这些糖蛋白在病毒进入中发挥什么作用，是直接作用于膜融合还是结合受体。对于其他疱疹病毒，gB 似乎是融合蛋白，由 gH/gL 触发，在某些情况下，gH/gL 与受体结合。然而，对于 HCMV，已发表的证据表明 gB 结合促进病毒进入细胞或向细胞发出信号所必需的细胞配体。有关疱疹病毒融合蛋白的大多数机制信息涉及细胞-细胞融合测定，该测定无法确定病毒体包膜中的 gB 或 gH/gL 是否必须面向包含受体的细胞膜。在这里，我们发现缺乏 gB 的 HCMV 病毒体无法进入正常细胞，但可以进入表达 gB 的细胞。对缺乏胞质结构域或在假定的“融合环”中进行取代的 gB 突变体的分析提供了证据，表明这种“反式进入”需要 gB 融合活性。在 gB 介导的反式进入中，gB 面向显然缺乏受体的病毒体包膜，这反对 gB 在结合受体或信号分子中的重要作用。相反，缺乏 gH/gL 的颗粒不会进入表达 gH/gL 的细胞，显然是因为 gH/gL 必须面向细胞膜（具有受体）。结合我们之前的干扰研究，其中细胞中表达的 gH/gL 阻止 HCMV 进入，我们的研究结果支持了这样的假设：HCMV gH/gL 在触发 gB（充当融合蛋白）之前结合细胞受体。人类巨细胞病毒 (HCMV) 在新生儿和免疫抑制的移植患者中产生重大疾病。与其他疱疹病毒一样，HCMV 需要两种膜糖蛋白 gB 和 gH/gL 才能进入宿主细胞。然而，目前尚不清楚 gB 和 gH/gL 在 HCMV 进入途径的两个步骤中如何发挥作用，即 (i) 细胞受体的结合和 (ii) 病毒体包膜与细胞膜的融合。有研究表明 HCMV gB 是受体结合所必需的，其他研究表明 gH/gL 是受体结合蛋白，gB 是融合蛋白。在这里，我们证明缺乏 gB 的 HCMV 病毒粒子可以进入在细胞膜上表达 gB 的细胞。相反，缺乏gH/gL的病毒颗粒不能进入表达gH/gL的细胞。我们的研究支持以下假设：gB 是融合蛋白，gH/gL 作用于 gB 上游，与受体结合，然后激活 gB 进行融合。
Human cytomegalovirus (HCMV) glycoproteins gB and gH/gL are both necessary and sufficient for cell-cell fusion. However, it is not clear what roles these glycoproteins play in virus entry, whether acting directly in membrane fusion or in binding receptors. With other herpesviruses, it appears that gB is the fusion protein and is triggered by gH/gL, which, in some cases, binds receptors. However, for HCMV, there is published evidence that gB binds cellular ligands necessary to promote virus entry into or signaling of cells. Most mechanistic information on herpesvirus fusion proteins involves cell-cell fusion assays, which do not allow a determination of whether gB or gH/gL in the virion envelope must be oriented toward cellular membranes that contain receptors. Here, we showed that HCMV virions lacking gB were unable to enter normal cells but entered cells that expressed gB. Analyses of gB mutants lacking the cytoplasmic domain or with substitutions in putative “fusion loops” provided evidence that gB fusion activity was required for this “entry in trans.” In gB-mediated entry in trans, gB is oriented toward the virion envelope that apparently lacks receptors, arguing against an essential role for gB in binding receptors or signaling molecules. In contrast, particles lacking gH/gL did not enter cells expressing gH/gL, apparently because gH/gL must be oriented toward cellular membranes (which have receptors). Coupled with our previous interference studies, in which gH/gL expressed in cells blocked HCMV entry, our findings here support the hypothesis that HCMV gH/gL binds cellular receptors before triggering gB, which acts as the fusion protein. Human cytomegalovirus (HCMV) produces major disease in neonates and immunosuppressed transplant patients. As with other herpesviruses, HCMV requires two membrane glycoproteins, gB and gH/gL, to enter host cells. However, it has not been clear how gB and gH/gL function in two steps of the HCMV entry pathway, i.e., (i) binding of cellular receptors and (ii) fusion of the virion envelope with cellular membranes. There are studies that suggest that HCMV gB is required for receptor binding and other studies suggesting that gH/gL is the receptor binding protein and gB is the fusion protein. Here, we show that HCMV virions lacking gB can enter cells that express gB in cellular membranes. In contrast, virus particles lacking gH/gL could not enter cells expressing gH/gL. Our study supports the hypothesis that gB is the fusion protein and gH/gL acts upstream of gB to bind receptors and then activate gB for fusion.