nsp14 acts as a cap N7 methyltransferase to modify SARS-CoV-2 mRNAs

SARS-CoV-2 nsp14 is a bifunctional enzyme. Apart from its exonuclease activity, which is supported by binding to nsp10, it functions independently as cap N7-methyltransferase (Saramago et al, 2021).<br><br>The genomic and subgenomic (sg) mRNAs of SARS-CoV-1 coronavirus are presumed to be capped at their 5′ end, based on studies of the mouse hepatitis virus (MHV) (Lai and Stohlman 1981) and the equine torovirus (van Vliet et al. 2002). Non-structural protein 14 (nsp14) acts as an RNA guanine-N7-methyltransferase (N7-MTase) that completes the synthesis of the cap-0 on SARS-CoV-1 mRNAs. The cap-0 represents N7-methyl guanosine connected to the 5′ nucleotide through a 5′ to 5′ triphosphate linkage, and is also known as m7G cap or m7Gppp cap. The N7-MTase domain maps to the carboxy-terminal part of nsp14 (Chen et al. 2009). Cap-0 formation requires three sequential reactions catalyzed by RNA triphosphatase (TPase), guanylyltransferase (GTase), and N7-MTase. There is no evidence that nsp14 possesses TPase and GTase activities, and no other SARS-CoV-1 proteins with these activities have been identified, so the identities of the enzymes that mediate these required steps in SARS-CoV-1 remain unknown. Based on the study of the human coronavirus 229E, non-structural protein 13 (nsp13) may have a TPase activity in addition to its established helicase activity (Ivanov and Ziebuhr 2004). In SARS-CoV-2, non-structural protein 12 (nsp12, Rdrp) has been shown to provide the GTase activity (Walker et al. 2021).

SARS-CoV-2的nsp14蛋白是一种双功能酶。除了其与nsp10结合所支持的核酸外切酶活性外，它还能独立地作为N7甲基转移酶（cap N7-methyltransferase）发挥作用（Saramago等，2021年）。基于对小鼠肝炎病毒（MHV）[Lai and Stohlman, 1981]和马肠病毒（equine torovirus）[van Vliet等，2002]的研究，推测SARS-CoV-1冠状病毒的基因组mRNA和亚基因组（sg）mRNA在5'端被加帽。非结构蛋白14（nsp14）作为RNA鸟嘌呤-N7甲基转移酶（N7-MTase），完成SARS-CoV-1 mRNA上帽-0的合成。帽-0是由N7甲基鸟苷通过5'到5'三磷酸键连接到5'核苷酸上，也被称为m7G帽或m7Gppp帽。N7-MTase结构域定位于nsp14的羧基末端（Chen等，2009年）。帽-0的形成需要由RNA三磷酸酶（TPase）、鸟苷酸转移酶（GTase）和N7-MTase催化的三个连续反应。目前没有证据表明nsp14具有TPase和GTase活性，且尚未发现其他具有这些活性的SARS-CoV-1蛋白，因此介导SARS-CoV-1中这些必要步骤的酶的身份尚不明确。根据对人类冠状病毒229E的研究，非结构蛋白13（nsp13）除已知的解旋酶活性外，可能还具有TPase活性（Ivanov and Ziebuhr，2004年）。在SARS-CoV-2中，非结构蛋白12（nsp12，Rdrp）已被证明提供GTase活性（Walker等，2021年）。