Here, we firstly performed amino acid sequence alignment of ACE2 from different species, including human, five non-human primates (gibbon, green monkey, macaque, orangutan and chimpanzee), two companion animals (cat and dog), six domestic animals (bovine, sheep, goat, swine, horse and chicken), three wild animals (ferret, civet and Chinese horseshoe bat) and two rodents (mouse and rat). The alignment by Clustal W 2.1 shows that they share a high sequence similarity except chicken (data not shown). The result suggests that 2019-nCoV of probable bat origin may not interact with chicken ACE2 and subsequently infect them, which were not considered in the following analyses. In ACE2, the regions at position 30–41, 82–84 and 353–357 are demonstrated to be involved in the interaction with SARS-CoV S protein, where the residues at positions 31, 35, 38, 82 and 353 are critical.9 Therefore, we took a close comparison in these regions and residues. As shown in Fig. 1 , human and non-human primates share the identity sequences in the regions and residues, implying that ACE2 from non-human primates may recognize 2019-nCoV and mediate its infection. As a result, non-human primates may be susceptible to 2019-nCoV and serve as animal models for antiviral research or intermediate hosts for cross-species transmission. In Fig. 1, the residues of most companion, domestic and wild animals are conserved, especially for the critical ones stated above, while certain ones are variable. For example, Lys31, Glu35, Asp/Glu38 and Lys353 are conserved, which probably form salt bridges. Interestingly, the changes at positions 31, 38 and 82 are observed. These changes suggest steric hindrance and electrostatic interference for host-virus interaction. Taking civet ACE2 as an example, the change of Lys31 to Thr31 is likely to form a hydrogen bond instead of a salt bridge. In addition, the polar side chain of Thr82 may influence the hydrophobic interaction of the original Met82. All these changes may result in a lower binding affinity. However, an additional region covering residues 90–93 has been shown to be involved in civet ACE2 binding to SARS-CoV and enhance their interaction.10 Consequently, we can't preclude the existence of other regions to compensate for the residue changes. With most residues in human ACE2, the ones from these compaion, domestic and wild animals may be favorable for 2019-nCoV recognition, which is in consistent with the recent work by Zheng-Li Shi et al. In case cross-species transmission, close contact with sick or asymptomatic companion, domestic and wild animals should be cautious, such as for workers in livestock farming and travellers in the wild.
Fig. 1 Sequence alignment of ACE2 from human (UniProt entry Q9BYF1), Northern white-cheeked gibbon (UniProt entry G1RE79), green monkey (UniProt entry A0A0D9RQZ0), crab-eating macaque (UniProt entry A0A2K5X283), Sumatran orangutan (UniProt entry Q5RFN1), chimpanzee (UniProt entry A0A2J8KU96), cat (UniProt entry Q56H28), dog (UniProt entry J9P7Y2), bovine (UniProt entry Q58DD0), sheep (UniProt entry W5PSB6), goat (UniProt entry A0A452EVJ5), swine (UniProt entry K7GLM4), horse (UniProt entry F6V9L3), ferret (UniProt entry Q2WG88), civet (UniProt entry Q56NL1), Chinese horseshoe bat (UniProt entry E2DHI7), mouse (UniProt entry Q8R0I0) and rat (UniProt entry Q5EGZ1). (A) Region 30–41. (B) Region 82–84. (C) Region 353–357. The conserved residues in the regions are colored in red and the critical residues are marked by asterisks.