Spike (S) proteins of coronaviruses, including the coronavirus that causes severe acute respiratory syndrome (SARS), associate with cellular receptors to mediate infection of their target cells. Here we identify a metallopeptidase, angiotensin-converting enzyme 2 (ACE2), isolated from SARS coronavirus (SARS-CoV)-permissive Vero E6 cells, that efficiently binds the S1 domain of the SARS-CoV S protein. We found that a soluble form of ACE2, but not of the related enzyme ACE1, blocked association of the S1 domain with Vero E6 cells. 293T cells transfected with ACE2, but not those transfected with human immunodeficiency virus-1 receptors, formed multinucleated syncytia with cells expressing S protein. Furthermore, SARS-CoV replicated efficiently on ACE2-transfected but not mock-transfected 293T cells. Finally, anti-ACE2 but not anti-ACE1 antibody blocked viral replication on Vero E6 cells. Together our data indicate that ACE2 is a functional receptor for SARS-CoV.

Human angiotensin‐converting enzyme 2 (ACE2) is a functional receptor for SARS coronavirus (SARS‐CoV). Here we identify the SARS‐CoV spike (S)‐protein‐binding site on ACE2. We also compare S proteins of SARS‐CoV isolated during the 2002–2003 SARS outbreak and during the much less severe 2003–2004 outbreak, and from palm civets, a possible source of SARS‐CoV found in humans. All three S proteins bound to and utilized palm‐civet ACE2 efficiently, but the latter two S proteins utilized human ACE2 markedly less efficiently than did the S protein obtained during the earlier human outbreak. The lower affinity of these S proteins could be complemented by altering specific residues within the S‐protein‐binding site of human ACE2 to those of civet ACE2, or by altering S‐protein residues 479 and 487 to residues conserved during the 2002–2003 outbreak. Collectively, these data describe molecular interactions important to the adaptation of SARS‐CoV to human cells, and provide insight into the severity of the 2002–2003 SARS epidemic.

Production of amyloid-␤ protein (A␤) is initiated by a ␤-secretase that cleaves the A␤ precursor protein (APP) at the N terminus of A␤ (the ␤ site). A recently identified aspartyl protease, BACE, cleaves the ␤ site and at residue 11 within the A␤ region of APP. Here we show that BACE2, a BACE homolog, cleaves at the ␤ site and more efficiently at a different site within A␤. The Flemish missense mutation of APP, implicated in a form of familial Alzheimer's disease, is adjacent to this latter site and markedly increases A␤ production by BACE2 but not by BACE. BACE and BACE2 respond identically to conservative ␤-site mutations, and alteration of a common active site Arg inhibits ␤-site cleavage but not cleavage within A␤ by both enzymes. These data suggest that BACE2 contributes to A␤ production in individuals bearing the Flemish mutation, and that selective inhibition of these highly similar proteases may be feasible and therapeutically advantageous.T he amyloid-␤ protein (A␤) is the principal component of the senile plaques characteristic of Alzheimer's disease (AD), and multiple lines of evidence have implicated cerebral accumulation of A␤ in AD pathogenesis (1, 2). A␤ is produced from the A␤ precursor protein (APP) by two proteolytic events. A ␤-secretase activity cleaves APP at the N terminus of A␤ (␤ site) between amino acids Met-671 and Asp-672 (using the numbering of the 770-aa isoform of APP). Cleavage at the ␤ site yields a membrane-associated APP fragment of 99 aa (C99). A second site within the transmembrane domain of C99 (␥ site) can then be cleaved by a ␥-secretase to release A␤, a peptide of 39-42 aa. APP can alternatively be cleaved within its A␤ region, predominately at the ␣-secretase cleavage site of APP, to produce a C-terminal APP fragment of 83 aa (C83), which can also be further cleaved by ␥-secretase to produce a small secreted peptide, p3.A number of missense mutations in APP have been implicated in forms of early-onset familial AD. All of these are at or near one of the canonical cleavage sites of APP. Thus, the Swedish double mutation (K670N͞M671L) is immediately adjacent to the ␤-cleavage site and increases the efficiency of ␤-secretase activity, resulting in more total A␤ (3). Any of three mutations at APP residue 717, near the ␥ site, increases the proportion of a more amyloidogenic 42-aa form of A␤ [A␤(1-42)] relative to the more common 40-residue form [A(␤1-40)] (4-7).Two additional mutations of APP have been described which are close but not adjacent to the ␣ site. A mutation (A692G, A␤ residue 21) in a Flemish family and a mutation (E693Q, A␤ residue 22) in a Dutch family each have been implicated in distinct forms of familial AD (8-10). The Flemish mutation, in particular, presents as a syndrome of repetitive intracerebral hemorrhages or as an AD-type dementia. The neuropathological findings include senile plaques in the cortex and hippocampus, and usually multiple amyloid deposits in the walls of cerebral microvessels (8,11,12).Recently, a membrane-associated aspartyl protease, B...