在SARS-CoV-2的众多巧合对象中,有一种冠状病毒。这种冠状病毒不是SARS-CoV-2“近亲”,它与SARS-CoV-2进化距离非常遥远,基因组差异非常巨大,它的发现地,也与SARS-CoV-2的发现地远隔重洋、相距万里。然而,这两种天差地远的冠状病毒,偏偏在感染、致病的关键环节、关键位点,存在多项极低概率的,极其反常、极为离奇的结构、能力巧合,如同它们是心有灵犀的跨时空灵魂知己,又如同它们是无比默契的同门师兄弟。这种冠状病毒就是2012年9月现身于沙特的MERS-CoV(Middle East Respiratory Syndrome Coronavirus,中东呼吸综合症冠状病毒)。
The sequence at Spike S1_S2 site enables cleavage by furin and phospho-regulation in SARS-CoV2 but not in SARS-CoV1 or MERS-CoV
S1/S2位点处的序列使SARS-CoV-2 能够被furin切割和磷酸化调节,SARS-CoV-1或MERS-CoV则不然。 https://www.nature.com/articles/s41598-020-74101-0
We performed furin digestion site prediction on the sequence of each type of coronavirus Spike through online software. It was found that all Spike with a SARS-CoV-2 Spike sequence homology greater than 40% did not possess a furin cleavage site (Figure 1, Table 1), including Bat-CoV RaTG13 and SARS-CoV (with sequence identity as 97.4% and 78.6%, respectively). The furin cleavage site “RRAR” of SARS-CoV-2 is unique in its family, rendering by its unique insert of “PRRA.” The furin cleavage site of SARS-CoV-2 is unlikely to have evolved from MERS, HCoV-HKU1, and so on. From the currently available sequences in databases, it is difficult for us to find the source.
We aligned 1,000 Spike sequences and found that all Spikes with sequence homology greater than 40% of SARS-CoV-2 Spike did not have a furin cleavage site, but its possible evolutionary source cannot be found currently, and more novel viruses are needed to be discovered.
论文原文:
From a structural point of view, the SARS-CoV-2 S protein proline residue (P681; Figs. 1d and 3c) in the insertion is eye-catching, because of the special and unique structural properties of this proteinogenic amino (imino) acid. MERS-CoV S protein is one of the other rare CoV spike proteins, that also contains a proline residue at the corresponding position in the S1/S2 protease cleavage site (Fig. 1e). When searching the database FurinDB (Tian et al., 2011) (http://www.nuolan.net/substrates.html), which includes experimentally verified furin cleavage sites, it appears that a proline residue at position P5, i.e., the 5th residue prior to the furin cleavage site, is rare and appears in only 5 out of 132 sequences (three mammalian and two viral sequences).
4)、P5位点的脯氨酸还可能有助于调节Spike蛋白及病毒包膜的融合前构象,降低膜融合的能量门槛,使病毒包膜与细胞膜更容易发生膜融合,使病毒的感染(即细胞进入)更便捷、高效。这一推测基于英法两国科学家1 December 1998发表的以下论文:
A Proline-Rich Motif Downstream of the Receptor Binding Domain Modulates Conformation and Fusogenicity of Murine Retroviral Envelopes
受体结合域下游富含脯氨酸的基序调节鼠逆转录病毒包膜的构象和融合性 https://journals.asm.org/doi/10.1128/jvi.72.12.9955-9965.1998
论文原文:
Further, we describe a surprising finding that the two serines at the edges of the insert SPRRAR↓SV can be efficiently phosphorylated by proline-directed and basophilic protein kinases. Both phosphorylations switch off furin’s ability to cleave the site.
Intriguingly, besides the RxxR motif, the sequence of the loop SPRRAR↓SV contains two serines that match the consensus of proline-directed kinases (SP), and basophilic protein kinases (RxxS), the two largest subfamilies of mammalian kinases21,22.
Spike S680 matches the consensus of proline-directed kinases (SP) and S686 forms a consensus for basophilic kinases (RxxS), two large subfamilies of mammalian kinases21,22.
The SARS-CoV1 S1/S2 segment does not contain a consensus site for proline-directed kinases, and while the MERS-CoV segment contains two potential proline-directed TP sites,