Coronaviruses (CoVs) are common human and animal pathogens that can transmit zoonotically and cause severe respiratory disease syndromes. CoV infection requires spike proteins, which bind viruses to host cell receptors and catalyze virus-cell membrane fusion. Several CoV strains have spike proteins with two receptor-binding domains, an S1A that engages host sialic acids and an S1B that recognizes host transmembrane proteins. As this bivalent binding may enable broad zoonotic CoV infection, we aimed to identify roles for each receptor in distinct infection stages. Focusing on two betacoronaviruses, murine JHM-CoV and human Middle East respiratory syndrome coronavirus (MERS-CoV), we found that virus particle binding to cells was mediated by sialic acids; however, the transmembrane protein receptors were required for a subsequent virus infection. These results favored a two-step process in which viruses first adhere to sialic acids and then require subsequent engagement with protein receptors during infectious cell entry. However, sialic acids sufficiently facilitated the later stages of virus spread through cell-cell membrane fusion, without requiring protein receptors. This virus spread in the absence of the prototype protein receptors was increased by adaptive S1A mutations. Overall, these findings reveal roles for sialic acids in virus-cell binding, viral spike protein-directed cell-cell fusion, and resultant spread of CoV infections.IMPORTANCE CoVs can transmit from animals to humans to cause serious disease. This zoonotic transmission uses spike proteins, which bind CoVs to cells with two receptor-binding domains. Here, we identified the roles for the two binding processes in the CoV infection process. Binding to sialic acids promoted infection and also supported the intercellular expansion of CoV infections through syncytial development. Adaptive mutations in the sialic acid-binding spike domains increased the intercellular expansion process. These findings raise the possibility that the lectin-like properties of many CoVs contribute to facile zoonotic transmission and intercellular spread within infected organisms.

中文翻译：

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唾液酸苷和蛋白质受体在冠状病毒感染中的独特作用。

冠状病毒（CoV）是常见的人类和动物病原体，可以人畜共患传播并引起严重的呼吸道疾病综合征。冠状病毒感染需要刺突蛋白，它将病毒与宿主细胞受体结合并催化病毒-细胞膜融合。几种冠状病毒毒株具有带有两个受体结合域的刺突蛋白，一个与宿主唾液酸结合的 S1A 和一个识别宿主跨膜蛋白的 S1B。由于这种二价结合可能会导致广泛的人畜共患 CoV 感染，因此我们的目的是确定每种受体在不同感染阶段的作用。重点关注两种β冠状病毒，即鼠类JHM-CoV和人类中东呼吸综合征冠状病毒（MERS-CoV），我们发现病毒颗粒与细胞的结合是由唾液酸介导的；然而，随后的病毒感染需要跨膜蛋白受体。这些结果支持一个两步过程，其中病毒首先粘附唾液酸，然后在进入感染细胞期间需要与蛋白质受体结合。然而，唾液酸足以促进病毒通过细胞-细胞膜融合传播的后期阶段，而不需要蛋白质受体。这种病毒在缺乏原型蛋白受体的情况下的传播因适应性 S1A 突变而增加。总体而言，这些发现揭示了唾液酸在病毒与细胞结合、病毒刺突蛋白引导的细胞与细胞融合以及由此引起的 CoV 感染传播中的作用。 重要性 冠状病毒可以从动物传播到人类，导致严重疾病。这种人畜共患传播利用刺突蛋白，将冠状病毒与具有两个受体结合域的细胞结合。在这里，我们确定了两个结合过程在 CoV 感染过程中的作用。与唾液酸的结合促进了感染，并通过合胞体发育支持了 CoV 感染的细胞间扩张。唾液酸结合尖峰结构域的适应性突变增加了细胞间扩张过程。这些发现提出了一种可能性，即许多冠状病毒的凝集素样特性有助于在受感染的生物体内轻松传播人畜共患病和细胞间传播。