SARS-CoV-2 has been sequenced [3]. A phylogenetic analysis [3, 4] found a bat origin for the SARS-CoV-2. There is a diversity of possible intermediate hosts for SARS-CoV-2, including pangolins, but not mice and rats [5].

There are many similarities of SARS-CoV-2 with the original SARS-CoV. Using computer modeling, Xu et al. [6] found that the spike proteins of SARS-CoV-2 and SARS-CoV have almost identical 3-D structures in the receptor-binding domain that maintains van der Waals forces. SARS-CoV spike protein has a strong binding affinity to human ACE2, based on biochemical interaction studies and crystal structure analysis [7]. SARS-CoV-2 and SARS-CoV spike proteins share 76.5% identity in amino acid sequences [6] and, importantly, the SARS-CoV-2 and SARS-CoV spike proteins have a high degree of homology [6, 7].

Wan et al. [4] reported that residue 394 (glutamine) in the SARS-CoV-2 receptor-binding domain (RBD), corresponding to residue 479 in SARS-CoV, can be recognized by the critical lysine 31 on the human ACE2 receptor [8]. Further analysis even suggested that SARS-CoV-2 recognizes human ACE2 more efficiently than SARS-CoV increasing the ability of SARS-CoV-2 to transmit from person to person [4]. Thus, the SARS-CoV-2 spike protein was predicted to also have a strong binding affinity to human ACE2.

This similarity with SARS-CoV is critical because ACE2 is a functional SARS-CoV receptor in vitro [9] and in vivo [10]. It is required for host cell entry and subsequent viral replication. Overexpression of human ACE2 enhanced disease severity in a mouse model of SARS-CoV infection, demonstrating that viral entry into cells is a critical step [11]; injecting SARS-CoV spike into mice worsened lung injury. Critically, this injury was attenuated by blocking the renin-angiotensin pathway and depended on ACE2 expression [12]. Thus, for SARS-CoV pathogenesis, ACE2 is not only the entry receptor of the virus but also protects from lung injury. We therefore previously suggested that in contrast to most other coronaviruses, SARS-CoV became highly lethal because the virus deregulates a lung protective pathway [10, 12].

Zhou et al. [13] demonstrated that overexpressing ACE2 from different species in HeLa cells with human ACE2, pig ACE2, civet ACE2 (but not mouse ACE2) allowed SARS-CoV-2 infection and replication, thereby directly showing that SARS-CoV-2 uses ACE2 as a cellular entry receptor. They further demonstrated that SARS-CoV-2 does not use other coronavirus receptors such as aminopeptidase N and dipeptidyl peptidase 4 [13]. In summary, the SARS-CoV-2 spike protein directly binds with the host cell surface ACE2 receptor facilitating virus entry and replication.