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The 2002–2003 epidemic of SARS(severe acute respiratory syndrome)that killed nearly 10% of the more than

8000 infected people is probably the mostthoroughly studied example of an animalvirus “jumping” into humans. SARS coron-avirus was caught in the act of adapting tohumans, acquiring mutations in several genesthat allowed it to be transmitted from personto person and cause lethal disease. Corona-viruses closely related to the human epidemicstrains of SARS coronavirus were discoveredin several wild animal species, including theHimalayan masked palm civet, in exotic meatmarkets in Southern China (1). By sequenc-ing hundreds of SARS viral RNA genomesfrom humans and animals during and after theepidemic, mutations were identified that dis-tinguish the species-specif ic strains (2).Which of these mutations account for theexplosive and virulent SARS epidemic?Strong evidence implicates the viral spike gly-coprotein as one major determinant of thespecies specificity of coronavirus infection(3). Infection is initiated by trimers of the~200-kD spike glycoprotein on the coron-avirus envelope. The trimers bind SARS virusparticles to their specific receptor glycopro-tein, angiotensin-converting enzyme 2(ACE2), on the surface of host cells (4).

In the spike protein of SARS coronavirus,the ~220–amino acid receptor-bindingdomain was identified by mutational analysisand binding of neutralizing monoclonal anti-bodies (5, 6). Only four amino acids in thereceptor-binding domain differ between thehuman epidemic and civet strains, but theycause more than a 1000-fold difference inbinding affinity to human ACE2 (7). Thelandmark paper by Li et al. on page 1864 ofthis issue characterizes the structure of thereceptor-binding domain of human SARScoronavirus spike protein bound to its recep-tor, human ACE2 (8). Together with previouselegant mutational analyses (7, 9), this struc-tural study identifies critical molecular deter-minants that allow SARS coronavirus to

adapt to humans. The host cell receptor isbound by an extended loop in the spike pro-tein that projects from a compact core withinthe receptor-binding domain. Of the 14residues on the loop that contact 18 residueson human ACE2, only two differ betweenhuman and animal virus strains. The intimateinterface between the loop of a spike protein

from the 2002–2003 SARS coronavirus andits human receptor mediates efficient virusbinding and infection (see the figure, panelA). In particular, a methyl group from a thre-onine residue at position 487 of the spike pro-tein at the interface extends into a hydropho-bic pocket in ACE2 that contains a lysineresidue at position 353. The two amino acidresidues that differ in the spike protein of acivet virus strain would strongly reduce bind-ing to human ACE2 due to absence of themethyl group (a serine residue is present atposition 487) and the introduction of acharged lysine residue at position 479 (see thefigure, panel B). The spike protein from acoronavirus that caused a sporadic and mildSARS case in 2003–2004 (see the figure,panel C) resembles civet virus spike proteinin that it has a serine residue at position 487 aswell. This spike protein also has a proline sub-

stitution for a leucine residue at position 472that reduces the total binding surface tohuman ACE2. These amino acid substitutionsmay account for the reduced virulence andtransmission of the virus in humans.

The ACE2 protein is highly conservedamong mammals and surprisingly few aminoacid substitutions at the virus-binding site canstrongly affect its receptor activity for SARScoronavirus (7–9). Rat ACE2, which does notserve as a receptor for SARS coronavirus, con-tains a large N-linked glycan at an asparagineresidue at position 82 in the binding interfacethat likely inhibits binding to the human SARScoronavirus spike protein. It also lacks thelysine-containing hydrophobic pocket criticalfor binding the key methyl group of threonine487 [see the figure, panel D; (8)].

Many coronaviruses cause disease inmammals and birds, and specific receptorglycoproteins have been identified for coro-naviruses of humans, cats, pigs, and mice(3). In addition to SARS coronavirus, onlythe newly discovered human coronavirusNL63 uses human ACE2 as its receptor (10).As shown by Li et al. (8), the extended loopon the SARS virus spike protein that bindshuman ACE2 has no homolog among spikeproteins of other coronaviruses. Perhaps thelarge (~90 kD) amino-terminal domain ofcoronavirus spike proteins share a conservedstructure from which virus-specific domainsproject that can bind to different host cellreceptors. Will the NL63 spike protein,which lacks a tyrosine-rich receptor-bindingloop like that on the SARS virus spike pro-tein (11), bind to the same site on humanACE2 as does the SARS virus spike? How

S T R U C T U R A L B I O L O G Y

Adaptation of SARS Coronavirus

to HumansKathryn V. Holmes

PERSPECTIVES

The author is at the University of Colorado HealthSciences Center, Mail Stop 8333, Post Office Box 6211,Aurora, CO 80045, USA. E-mail: kathryn.holmes@uchsc.edu

Receptor activity

Good

Human SARS

receptor ACE2

Human SARS

receptor ACE2

Human SARS

receptor ACE2

Rat SARS

receptor ACE2

Human SARS spike

2002–2003Civet SARS spike Human SARS spike

2003–2004

Human SARS spike

2002–2003

N479K

+T487S T487SL472P

M82N

K353H

Poor Poor NoneA B C D

Key amino acids in the SARS coronavirus spike protein and the receptor protein that determinethe host range of the virus. (A) There is a large binding interface between a loop structure in the spikeprotein of the human SARS coronavirus of 2002–2003 and its human receptor ACE2. (B) Two aminoacid substitutions in the spike protein of a civet SARS virus reduce receptor activity of human ACE2 byadding a charge to the binding surface (N479K) and deleting a key methyl group (T487S) that fits intoa hydrophobic pocket in the receptor. (C) In the spike protein from coronavirus of a mild SARS casefrom 2003–2004, the key methyl group is also missing and a proline residue (L472P) reduces the bind-ing surface. (D) Rat ACE2 contains a large glycan at M82N and lacks the hydrophobic pocket (K353H).

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16 SEPTEMBER 2005 VOL 309 SCIENCE www.sciencemag.orgPublished by AAAS

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could a unique receptor-binding domain beintroduced into a spike protein? Coronavirusreplication includes frequent RNA recombi-nation events that can insert or delete longRNA sequences in the genome. Large dele-tions that occur spontaneously in the porcinetransmissible gastroenteritis coronaviruseliminate binding to a carbohydrate moietyand change the tissue tropism and virulenceof the virus (3). Coronaviruses can hijackforeign genes, such as the hemagglutininesterase glycoprotein gene from influenza Cvirus (12). Genes of unknown origin thatencode the virus-specific, nonstructural pro-teins are also acquired and inserted betweenthe essential genes on the coronavirusgenome (3). Thus, coronaviruses mightchange receptor specificity by mutation orby RNA recombination in the genes thatencode their spike glycoproteins.

The rather alarming conclusion from thestructural studies of the SARS virus spike-ACE2 interface (8) is that adaptation of avirus to a homologous receptor of a new hostspecies may require very few amino acidsubstitutions at the large receptor-bindinginterface. This is true not only for SARScoronavirus, but also for other virusesincluding influenza A virus and parvoviruses(13, 14). Why, then, don’t viruses constantlyjump from one host species to another?

Probably because successful adaptation to anew host not only requires mutations to opti-mize receptor binding and entry, but alsomutations in other viral genes that optimizevirus replication and transmission in the newhost. Only when a constellation of mutationsallows a virus to replicate and transmit mod-erately well in the new host can infection in anew species become established.

Can we predict whether another humanSARS epidemic will occur? So far, extensiveepidemiological surveillance has not foundthe 2002–2003 epidemic strains of SARScoronavirus in humans or animals since theepidemic ended in July 2003 (2). However,SARS coronaviruses continue to circulate incivets and perhaps other animals and to causesporadic, mild human cases (2, 15). For-tunately, if new mutants of SARS coronavirusfrom animals do initiate another SARS epi-demic in humans, the disease could promptlybe recognized with new diagnostic tests. Theoutbreak could be stopped by the stringentisolation procedures that controlled the firstSARS epidemic of 2002–2003. This couldperhaps be supplemented with promising newcandidate vaccines and antiviral drugs that arecurrently being developed. The structure ofthe interface between the spike protein andreceptor shown by Li et al. (8) suggests novelstrategies for developing an improved SARS

vaccine and receptor-targeted drugs to blockvirus entry into host cells.

Can the next emerging virus epidemic,other than SARS, be predicted? Probablynot. All viruses mutate, and an unfortunatecombination of mutations could occur andbe selected at any time. The inherent unpre-dictability of emerging viral diseases is thebest reason for further characterization ofviruses in wildlife that could jump tohumans and for global surveillance for newepidemic diseases in humans and animals.

References and Notes1. Y. Guan et al., Science 302, 276 (2003).2. H. D. Song et al., Proc. Natl. Acad. Sci. U.S.A. 102, 2430

(2005).3. M. M. C. Lai, K. V. Holmes, in Fields’ Virology, D. M.

Knipe, P. M. Howley, Eds. (Lippincott Williams andWilkins, Philadelphia, ed. 3, 2001).

4. W. Li et al., Nature 426, 450 (2003).5. G. J. Babcock et al., J. Virol. 78, 4552 (2004).6. Y. He et al., J. Immunol. 174, 4908 (2005).7. W. Li et al., EMBO J. 24, 1634 (2005).8. F. Li et al., Science 309, 1864 (2005).9. X. X. Qu et al., J. Biol. Chem. 280, 29588 (2005)

10. H. Hofmann et al., Proc. Natl. Acad. Sci. U.S.A. 102,7988 (2005).

11. L. van der Hoek et al., Nat. Med. 10, 368 (2004).12. S. L. Smits et al., J. Biol. Chem. 280, 6933 (2005).13. L. Glaser et al., J Virol. 79, 11533 (2005).14. K. Hueffer et al., J Virol. 77, 10099 (2003).15. C.Tu et al., Emerg. Infect. Dis. 10, 2244 (2004).16. This work was supported by NIH grant AI59578.

10.1126/science.1118817

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Understanding the behavior of theactinide elements such as uraniumand plutonium is central to predicting

nuclear weapons performance, advancednuclear fuel cycles, radioactive waste man-agement, and environmental remediation.During much of the past century, knowledgeof the chemical behavior of these elementswas derived principally from investigationsdesigned to develop processes for efficientlarge-scale separation and recovery.Although this has provided models todescribe the coordination and redox behav-ior of the early actinides in acidic aqueousmedia, we still lack a comprehensive pictureof the behavior of elements in this part of theperiodic table. It has been particularly diffi-cult to reconcile descriptions of the fascinat-ing structural and electronic behavior of f-

series metals and compounds in condensed-matter systems [including those displayingf-electron itinerancy (1)] with the solutionmolecular behavior ofthese elements.

Recently, there havebeen suggestions in theliterature that the behav-ior of solid-state actinideoxides has previouslyunappreciated similari-ties to that of molecularsystems (2). The chem-istry of individual metalsites tends to be domi-nated by strong (presum-ably relatively covalent)metal-oxygen multiplebonding; discrete termi-nal metal-oxo units withshort metal-oxo bondsare common structuralelements. One vital

aspect in understanding the electronic struc-ture and thermodynamic stability of thesesystems is assessment of the type and strengthof bonding found in the molecular metal-lig-and bonds (particularly the stability of bridg-ing versus terminal bonds; see the figure).Unfortunately, the molecular chemistry ofAnE moieties (A, actinide; E, first-row ele-ment) has been largely restricted to date tometal-oxo complexes. Evans et al. report onpage 1835 of this issue the first example of amolecular actinide complex containing a

C H E M I S T RY

Bridging a Gap

in Actinide ChemistryCarol J. Burns

The author is in the Chemistry Division at Los AlamosNational Laboratory, MS J514, Los Alamos, NM 87545,USA. E-mail: cjb@lanl.gov

Nitrogen 2p orbitals

U UU UE E

Uranium 6d orbitals Uranium 5f orbitals

A bridge just right. First-row elements (E) such as nitrogen have thecapacity to bridge between two actinide metal centers.The nitrogen 2porbitals are of the appropriate symmetry to overlap with both uranium(U) 6d and 5f orbitals.The bridging mode in the nitride complex reportedby Evans et al. suggests delocalized metal-ligand multiple bonding, asillustrated schematically by the resonance structures (box at top).

www.sciencemag.org SCIENCE VOL 309 16 SEPTEMBER 2005

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DOI: 10.1126/science.1118817, 1822 (2005);309 Science

Kathryn V. HolmesAdaptation of SARS Coronavirus to Humans

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