RESEARCH ARTICLES
`
`Crystal Structure of the Low-pH
`Form of the Vesicular Stomatitis
`Virus Glycoprotein G
`
`Ste´phane Roche,* Ste´ phane Bressanelli,* Fe´lix A. Rey,† Yves Gaudin‡
`
`The vesicular stomatitis virus has an atypical membrane fusion glycoprotein (G) exhibiting a pH-
`dependent equilibrium between two forms at the virus surface. Membrane fusion is triggered
`during the transition from the high- to low-pH form. The structure of G in its low-pH form shows the
`classic hairpin conformation observed in all other fusion proteins in their postfusion conformation,
`in spite of a novel fold combining features of fusion proteins from classes I and II. The structure
`provides a framework for understanding the reversibility of the G conformational change.
`Unexpectedly, G is homologous to gB of herpesviruses, which raises important questions on
`viral evolution.
`
`Entry of enveloped viruses into host
`
`cells requires fusion of the viral enve-
`lope with a cellular membrane. This
`step is mediated by viral glycoproteins that
`undergo a dramatic fusogenic structural re-
`arrangement
`induced by a specific trigger
`(e.g., low pH in the endosome or interactions
`with receptors).
`Two classes of viral fusion proteins have
`been identified so far. The best characterized
`members of class I are the influenza virus hemag-
`glutinin (HA) (1, 2) and the fusion protein (F)
`of the paramyxoviruses (3–5). Class I also
`includes fusion proteins from retroviruses (6),
`filoviruses (7), and coronaviruses (8). The active
`fusogenic form is obtained by proteolytic
`cleavage of a precursor into two fragments and
`bears a hydrophobic fusion peptide at or near
`the amino terminus generated by this cleavage.
`The proteins are organized as trimers and their
`postfusion conformation contains a trimeric
`coiled-coil core, beginning near the carboxy-
`terminal end of the fusion peptide, against which
`are packed, in an antiparallel manner, the seg-
`ments abutting the transmembrane region. The
`protein shape is thus an elongated hairpinlike
`structure bringing together the fusion peptide
`and the C-terminal transmembrane domain.
`Class II contains the E protein of flavivi-
`ruses (9, 10) and E1 of alphaviruses (11). They
`have an internal fusion peptide, located in a
`loop between two b strands, and are synthe-
`
`Institut
`CNRS, Unite´ Mixte de Recherche (UMR) 2472,
`Fe´ de´ ratif de Recherche (IFR) 115, Virologie Mole´ culaire et
`Structurale, 91198, Gif sur Yvette, France; Institut National
`de la Recherche Agronomique (INRA), UMR1157, Virologie
`Mole´ culaire et Structurale, 91198, Gif sur Yvette, France.
`
`*These authors contributed equally to this work.
`Institut
`†Present address: De´ partement de Virologie,
`Pasteur, 25 rue du Docteur Roux, 75724 Paris cedex 15,
`France.
`‡To whom correspondence should be addressed. E-mail:
`gaudin@vms.cnrs-gif.fr
`
`sized within a polyprotein. Folding takes place
`as a complex with a second viral envelope pro-
`tein that plays a chaperone role. Proteolytic
`cleavage of the chaperone primes the fusion
`protein to trigger membrane merger. In their
`native conformation, they form dimers that lie
`flat at the viral surface and are organized with
`icosahedral symmetry (11). On exposure to low
`pH, the dimers dissociate, and the protomers
`reassociate to form trimers. Similarly to class I
`proteins, this transition results in a hairpin struc-
`ture with the fusion loops and the transmembrane
`domains at the same end of an elongated mol-
`ecule that is then perpendicular to the membrane
`(12–14).
`A number of viral fusion proteins, from
`rhabdoviruses and herpesviruses, for instance, do
`not appear to fall within either of these classes.
`Among them, glycoprotein G of the vesicular
`stomatitis virus (VSV), from the Rhabdoviridae
`family, has been the most studied. Rhabdovi-
`ruses are bullet-shaped and are widespread
`among a great variety of organisms (including
`plants, insects, fishes, mammals, reptiles, and
`crustaceans). Their genome is a single RNA
`molecule (about 12 kb) of negative polarity
`encoding five or six proteins in total, among
`which is a single-transmembrane glycoprotein
`(G) that is trimeric and forms the spikes that
`protrude from the viral surface. G is both re-
`sponsible for viral attachment to specific re-
`ceptors and for low pH–induced membrane
`fusion after endocytosis of the virion. Most of
`the mass of G (446 amino acids out of 495 for
`VSV Indiana strain) is located outside the viral
`membrane and constitutes the N-terminal ecto-
`domain, which is the target of neutralizing anti-
`bodies. The two most studied genera of
`E
`rhabdoviruses are the lyssaviruses
`prototype
`^
`virus: rabies virus (RV)
`and the vesiculoviruses
`(prototype virus: VSV).
`Fusion of rhabdoviruses is optimal around
`pH 6 (15–19). Preincubation of the virus at low
`
`pH in the absence of a target membrane leads
`to inhibition of viral fusion. However, this in-
`hibition is reversible, and readjusting the pH to
`above 7 leads to the complete recovery of the
`initial fusion activity (20). Low pH–induced
`conformational changes of G and their relations
`with the fusion activity have been studied by
`different biophysical and biochemical
`tech-
`niques (16, 21–26). G can adopt at least three
`different conformational states (15, 16): the na-
`tive state detected at the viral surface above
`pH 7; the activated hydrophobic state, which
`interacts with the target membrane as a first
`step of the fusion process (24); and the fusion
`inactive, postfusion conformation that is anti-
`genically distinct from both the native and
`activated states (27). There is a pH-dependent
`equilibrium between the different states of G
`that is shifted toward the inactive state at low
`pH (27). Thus, unlike fusogenic glycoproteins
`from other viral families, the native, prefusion
`conformation is not metastable. Furthermore,
`no a-helical coiled-coil motif characteristic of
`class I viral fusion proteins (28) is predicted
`from the amino acid sequence (29). Finally,
`although G contains an internal fusion domain
`(24), it is not cleaved from a polyprotein pre-
`cursor or associated with a second envelope
`protein, as is the case for class II viral fusion
`proteins (9, 11). All these characteristics sug-
`gest that the structure of G is distinct from the
`structure of any fusion protein described so far.
`Here we describe the structure of the VSV-
`G ectodomain (residues 1 to 410), generated by
`limited proteolysis with thermolysin (Gth),
`under its postfusion conformation at 2.4 )
`resolution.
`Molecular architecture. The structure of the
`Gth trimer is depicted in Fig. 1. The overall
`shape of the molecule resembles an inverted
`cone (Fig. 1B). The length of the molecule is
`125 ), and the diameter at the head is 60 ).
`The crystals that allowed the structural deter-
`mination were grown at pH 7.0, but the same
`conformation was found in crystals grown at
`pH 6.0 (see Materials and Methods). The
`dimensions of the molecule—identical to those
`measured for RV G ectodomain low-pH form
`by electron microscopy (16)—and its hairpin-
`like organization (see below) indicate that this
`structure corresponds to the low-pH, postfusion
`conformation (i.e., the fusion inactive confor-
`mation). Thus, during crystal growth at pH 7.0,
`the minor fraction of G in the low-pH con-
`formation was sequestered in the crystals, dis-
`placing the equilibrium between the different
`states of G (27).
`Gth has an altogether different structural or-
`ganization from those of both class I and class
`II viral fusion proteins described so far. The
`polypeptide chain of Gth folds into four distinct
`domains (Fig. 1, A and D, and fig. S1). A b
`sheet–rich lateral domain at the top of the mol-
`ecule (domain I), a central, mostly a-helical
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`RESEARCH ARTICLES
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`domain that is involved in the trimerization of
`the top of the molecule (domain II), a neck
`domain that has the characteristic fold of
`pleckstrin homology (PH) domains (domain
`III), and the elongated fusion domain that
`makes the trimeric stem of the molecule (do-
`main IV). Three of these compact domains are
`made from noncontiguous segments of the
`polypeptide chain (Fig. 1D). The single seg-
`ment making the fusion domain is inserted into
`the PH domain, and the PH domain is inserted
`into domain II. The C-terminal part of Gth (411
`to 422) is not ordered in either crystal form, and
`the polypeptide chain can be drawn up to res-
`idue 410 on one protomer and only to residue
`408 on the other two. Nevertheless, the ori-
`entation of the chain after the end of domain
`II [residues 406 to 410 in magenta on Fig. 1A
`(right) and 1B] indicates that
`it
`is pointing
`toward the tip of the fusion domain. Thus, as in
`the postfusion conformation of other fusion
`proteins, the transmembrane domain and the
`fusion domains are located at the same end of
`the molecule.
`Alignment of five G protein sequences from
`animal rhabdoviruses belonging to different
`genera is shown in fig. S2. Although the overall
`amino acid identity is very low, it remains sig-
`nificant, and all of them are predicted to display
`the same fold except possibly the C-terminal
`part of ephemeroviruses G.
`Surprisingly, the structural organization of
`G is the same as that of herpesvirus gB that is
`described in this issue (30). This similarity ex-
`tends from the N-terminal part to at least the
`end of helix G of domain II. It includes both the
`PH domain and the fusion domain [Dali score,
`0
`5.2 for 109 residues of the fusion domain
`Z
`(31)], as well as part of the trimerization do-
`main (fig. S3), and reveals a clear and unex-
`pected homology between the two proteins.
`Description of the domains. The top lateral
`domain I (Fig. 2A) contains about 90 residues
`in two segments (1 to 17 and 310 to 383). It is
`made of three antiparallel b sheets (astu, rsa¶,
`and vwxys¶) that are wrapped around the N-
`terminal b strands a to a¶. b Strand s is also
`involved in the formation of b sheet rsa¶ with
`the glycosylation site at position 320 on loop rs
`that is located at the very top of the molecule.
`Four other loops of this domain (vw, xy, s¶t, and
`tu) are exposed at the surface of the molecule.
`It is noteworthy that an antigenic site has been
`reported in this domain, on loop s¶t on the
`native conformation (32).
`Domain II (Fig. 3A) is made of three seg-
`ments (18 to 35, 259 to 309, and 384 to 405)
`and contains four helices (A, F, G, and H). In
`the trimer, the two longest helices F and H
`make a six-helix bundle, reminiscent of the
`structure found in class I fusion proteins in their
`postfusion conformation (28), in which helix F
`forms the trimerization region (Fig. 1C, Fig.
`3C). As in class I proteins, the fusion domain is
`N-terminal to the central helix F, and the trans-
`
`membrane domain is located at the C terminus
`of the antiparallel outer helix H. Helices F and
`H are zipped together by a small antiparallel b
`sheet formed by strands q and z. Helix F is
`linked by a disulfide bond (bridging cysteines
`C24 and C284) (33) to the long extended N-
`terminal part of this domain, which is wound
`around the top of the molecule. The FG loop is
`exposed at the top of the domain. The segment
`delimited by P296 and P310, which contains helix
`G, and the top of helix H (residues 384 to 391)
`are hydrophobically packed against domain I,
`interacting with strands s, r, v, w, and x.
`
`Domain III is inserted within domain II. It is
`made of two segments (36 to 50 and 181 to
`258) and has the fold of a PH domain (Fig. 2B).
`It contains two four-stranded b sheets (bjkl and
`pmno) and two helices D and E that are re-
`spectively located between strands j and k and
`strands o and p. The domain contains a disul-
`fide bridge that stabilizes sheet pmno by bridg-
`ing C219 and C253. Numerous epitopes of the
`native prefusion conformation have been re-
`ported in this domain (32).
`Domain IV (51 to 180) is inserted in a loop
`of the PH domain (Figs. 1D and 2B). It is an
`
`Fig. 1. Overall structure of glycoprotein G. (A) Ribbon diagram of the G protomer (residues 1 to
`410). (Left) The chain is colored by residue number in a gradient from blue (N terminus) to red (C
`terminus); (right) the chain is colored by domain. The triangles indicate the glycosylation sites (on
`N163 and N320). (B) (Left) Ribbon diagram of the G trimer, colored by domain; (right) surface
`representation of the G trimer, colored by domain. The arrows indicate the plane of the section
`shown in (C), bottom. (C) (Top) Top view of the trimer (ribbon diagram), colored by domain;
`(middle) top view of the trimer (surface representation); and (bottom) plane section of the G trimer,
`at the level of the C-terminal segment, showing the cavity inside the molecule. (D) Domain
`architecture of G. Domains observed in the crystal structure are colored as in (A), right. The C
`terminus, not observed in the structure, is in gray with the transmembrane segment hatched. All
`structural figures were generated with PYMOL (38).
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`extended b-sheet structure organized around
`two long antiparallel b strands [c and e in (Fig.
`4A)] that contribute to the formation of a small
`six-stranded b barrel (cefghi) at one end, and at
`the other, to a three-stranded b sheet (dce) (Fig.
`4A). The six-stranded b barrel exposes the
`glycosylation site at position 163 on loop hi. It
`is stabilized by the disulfide bridge linking C153
`and C158. The segments of the protein making
`the b barrel are the most conserved elements
`of the G amino acid sequence (fig. S2). Just
`before the junction of domain IV and the
`second segment of domain III is a helix C,
`which is covalently linked by a disulfide bond
`(bridging C177 and C226) to loop lm of domain
`III (Fig. 4A).
`Separating the top b barrel (cefghi) from the
`bottom sheet (dce), the horizontal helix B packs
`against strands c and e via hydrophobic inter-
`actions. Helix B is linked to strand c by the
`disulfide bond between C59 and C92. After helix
`B, the chain adopts an elongated conformation
`through a P107GFPP111 motif that has a poly-
`proline helix conformation. The very tip of the
`protomer is made of two loops (cd and Pe)
`containing aromatic residues and kept together
`by the disulfide bridge between C68 and C114
`(Fig. 4A). As discussed below,
`these loops
`constitute the membrane-interacting motif of
`the G ectodomain.
`The overall structural organization of this
`fusion domain, particularly the base of the
`stem, is strikingly similar to the one of class II
`fusion proteins (Fig. 4A). Nevertheless, these
`domains are not homologous: The topology of
`the strands making the sheet that exposes the
`fusion loops in VSV G is unrelated to the one
`in class II fusion proteins. Thus, this structural
`similarity appears as the result of convergent
`evolution.
`The fusion loops. The tip of the trimeric
`stem has a bowllike concave shape similar to
`that already described for E protein of flavi-
`viruses (Fig. 4B). Nevertheless, unlike these
`fusion proteins, the fusogenic motif of VSV G
`
`is made of both loops cd and Pe and, thus, is
`bipartite as already suggested for the viral
`hemorrhagic septicemia virus, another rhabdo-
`virus (19). Indeed, four hydrophobic residues
`W72, Y73 (both located on loop cd), Y116, and
`A117 (both located on loop Pe) are fully ex-
`posed at the tip of VSV G (Fig. 4, A and C).
`Not surprisingly, replacement of A117 by lysine
`abolishes G fusion properties (18).
`This ‘‘fusion patch’’ cannot penetrate deep
`into the membrane: Even if the hydroxyl of
`Y116 participates in a hydrogen bond with the
`carbonyl of W72, the nitrogen of the indole ring
`of W72 and several carbonyls (e.g., those of Y73
`and A117) remain exposed and hinder penetra-
`tion into the hydrophobic moiety of
`the
`membrane. R71 and D69, located on the rim of
`the bowl, and K76, which points toward the
`threefold axis, set an upper limit of about 8.5 A˚
`for membrane insertion (Fig. 4C).
`the
`that all
`Sequence alignments reveal
`rhabdoviral G proteins have at least one polar
`aromatic residue in their fusion loops (fig. S2).
`Tyrosines and tryptophans are residues typi-
`cally found at the interface between the fatty
`acid chains and head-group layers of lipids
`(34). Such an interfacial interaction involv-
`ing a large number of aromatic residues per
`trimer is probably sufficient to destabilize the
`membrane.
`The trimeric interface. The buried interface
`between two subunits in the trimer is roughly
`3860 )2 per protomer; the main part of it
`(2620 )2) is located in the top of the molecule
`(domain II) and the rest in the stem (domain
`IV). The core of the trimer in domain II is the
`six-helix bundle (Fig. 3C). The stabilizing in-
`teractions are mostly hydrophobic but also
`involve a salt bridge between E286 from one
`protomer and K290 from another. In domain IV,
`the trimer is stabilized by lateral interactions
`between the polyproline segment and strand d
`of the neighboring protomer. These interactions
`involve conserved residues P111, I78, and I82
`(Fig. 4D), which keep together the fusion loops
`
`Fig. 2. Structural organization of domains I and III (PH domain). (A) Structure of domain I. The
`triangle indicates the glycosylation site. (B) Structure of domain III (left) and structure of the PH
`0
`domain of the insulin receptor substrate 1, which is the best homolog found using Dali [Dali score Z
`4.8 (31)]. The loop of the PH domain in which G fusion domain is inserted is colored red. The disulfide
`bridge is indicated in green. Also shown is C226, which makes a disulfide bond with C177 of domain IV.
`
`RESEARCH ARTICLES
`
`of the three protomers and thus ensure their
`correct positioning at the tip of the molecule.
`The other contacts between domains IV in the
`trimer involve the C terminus of helix B, which
`makes two hydrogen bonds (through the
`carboxyl group of K100 and the side chain of
`Q101, respectively) with the relatively con-
`served H132 (on strand e) and H162 (on loop
`hi) of the neighboring protomer (Fig. 4D).
`Inside the trimeric structure, there is a cavity
`(Fig. 1C, bottom) that is limited at the base of
`the stem by Q112. The bottom of the cavity is a
`narrow channel flanked by the polyproline
`motif. It enlarges at the level of L106 into a
`chamber 5 nm long and up to 2 nm wide that
`ends at the base of helix F in domain II. This
`chamber has three large apertures delimited by
`the tops of domain IV of two protomers. These
`openings may be occluded by the C-terminal
`part of the ectodomain in the full-length
`protein.
`This cavity, a relatively rare feature in
`proteins, is also found in the postfusion con-
`formation of the flavivirus protein E (12, 14).
`For both E and G, it certainly limits the stability
`of the trimeric organization of the fusion
`domains. Only a few bonds have to be broken
`(Fig. 4D) to allow the structure to adopt an
`open conformation such as the one that has
`been crystallized in the case of the low-pH
`form of the Semliki Forest Virus E1 protein
`(13). The versatility of the association of the
`fusion domains is probably necessary during
`the fusion process. Indeed, mutations in the
`polyproline motif (at positions 108, 109, and
`111) result in a decrease of the fusion efficiency
`or a drastic shift of the pH threshold for fusion
`toward lower values (17, 18).
`Molecular basis for conformational
`change reversibility. It has been proposed that
`the reversibility of the low pH–induced confor-
`mational change is essential to allow G to be
`transported through the acidic compartments of
`the Golgi apparatus and to recover its native
`structure at the viral surface (35). The structure
`gives the clues to the molecular basis of this
`unusual property. Indeed, although one crystal
`form was grown at pH 7.0, it is clear that the
`structure of Gth that we have determined cannot
`be stable at high pH in solution.
`Indeed, a large number of acidic amino
`acids are brought close together in the six-helix
`bundle (Fig. 3, A and D). These residues are
`clearly protonated and form hydrogen bonds
`(Fig. 3D), and therefore, the negative logarithm
`of acid constant (pKa) for them is abnormally
`high. Their deprotonation at higher pH will
`induce strong repulsive forces that destabilize
`the trimer this step initiates the transition back
`toward the prefusion form. The regions that
`will thus be pushed apart include the bottom
`part of helix F from each protomer (through
`D268), helices H and F from neighboring
`protomers (through D274 and D395), and helices
`H and F within the same protomer (through
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`E276 and D393) (Fig. 3D). Although these amino
`acids are not conserved among the rhab-
`dovirus family, helices F and H of all rhab-
`doviruses G contain numerous acidic residues
`(fig. S2). These residues are certainly involved
`in the destabilization of the low-pH form
`above pH 7.
`The trimeric state of domain IV, as it is in
`the Gth low-pH structure, should be also affected
`at pH above 7, because, in its deprotonated
`form, H132 cannot maintain its interactions with
`both the carboxyl group of K100 of the neigh-
`boring protomer and the side chain of D145
`(Fig. 4D).
`
`Finally, D137, Y139, and the dipeptide
`H407P408, which are conserved among the rhab-
`dovirus family, cluster together to interact with
`the tip of domain II qz zipper. They direct the
`C-terminal part of the ectodomain toward the
`base of the molecule (Fig. 3B). The salt bridge
`between D137 and H407 cannot exist after depro-
`tonation of the histidine residue. Thus, a pH
`increase should also have a destabilizing effect
`on this small structural motif.
`It is worth noting that the large number of
`protonated residues involved in the stability of
`the postfusion conformation explains the high
`cooperativity of the structural back transition
`
`Fig. 3. Structural organization of domain II (trimerization domain). (A) Ribbon diagram of domain
`II. The disulfide bridge is indicated in green. Acidic residues E276 and D393, which destabilize the
`domain at pH above 7, are also represented. (B) Close-up view of the qz sheet (in blue) and how it
`is stabilized by residues D137 and Y139 of domain IV and by the dipeptide H407P408. Hydrogen
`bonds are indicated as dotted lines in magenta. D137 makes a salt bridge with the imidazole ring of
`H407, which is stacked against the aromatic group of Y139. (C) Top view of the six-helix bundle (the
`helices labeled H and F and the residue labeled W32, involved in lateral interactions, are from the
`same protomer). Aliphatic residues involved in interactions between helices F (I272, V275, L279, L283)
`are indicated in yellow; those involved in lateral interactions between two protomers (L271 and L278
`in helix F and L392 in helix H) are indicated in orange; and E286 and K290, which make a salt bridge,
`are indicated in cyan. (D) Top view of the six-helix bundle showing the cluster of acidic residues.
`Hydrogen bonds are indicated as dotted lines in magenta.
`
`upon deprotonation (27): An initial destabiliza-
`tion of the low-pH form at the qz zipper results
`in an increased solvent accessibility of acidic
`residues in domain II, a drop in their pKa, and
`their concomitant deprotonation.
`Mutations affecting the structural transi-
`tion of G. For both VSV and rabies virus, mu-
`tant viruses have been selected for their ability
`to escape neutralization by antibodies directed
`against G in its low-pH conformation (36) or
`for their ability to infect cells at low pH (37).
`Seven mutations have been described that result
`in stabilization of the prefusion conformation
`(either kinetically or thermodynamically). Two
`of them (Q285 Y
`R for VSV and E282 Y
`K for
`RV) take place in the long helix F. Two others
`(V392 Y
`G and M396 Y
`T for RV) take place
`in the neighborhood of the small qz sheet and
`the conserved dipeptide H397P398 (residue num-
`bering of RV G, fig. S2). These observations
`confirm that helix F and the cluster made by
`D137, Y139, and the dipeptide H407P408 are in-
`volved in the structural rearrangements of G.
`The phenotype linked to the last three muta-
`tions (M44 Y
`V/I for RV and F2 Y
`L for VSV)
`cannot be explained and may affect only other
`conformations of G.
`Final remarks. In spite of having a novel
`fold, the low pH of G displays the classic hair-
`pin conformation expected for the postfusion
`form of a fusogenic protein. It combines fea-
`tures of both class I and class II proteins. To-
`gether with gB of herpesviruses, it defines a
`new family of fusion proteins having a new
`fusion module: an elongated b structure inserted
`in a PH domain and carrying two fusion loops.
`This homology between G and gB invites us to
`reconsider the evolution of the Mononegavirales
`order: It suggests that Mononegavirales are able
`to steal genes, probably from their cellular host,
`likely by copying exogenous mRNA during ge-
`nome synthesis.
`
`1.
`
`References and Notes
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`RESEARCH ARTICLES
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`Fig. 4. Structural organization of domain IV (fusion domain). (A) Ribbon
`diagram of VSV G (left) and Dengue virus E (right) fusion domains
`showing their structural similarity. The disulfide bridges are indicated in
`green. Also shown is C177, which makes a disulfide bond with C226 of
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`representation of the tip of VSV G fusion domain. Hydrophobic residues
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`38. W. Delano, the PyMOL Molecular Graphics System (2002).
`39. We thank A. Flamand for her constant support of this
`project; J. Lepault, R. Ruigrok, M. Knossow, A. Benmansour,
`C. Tuffereau, and D. Blondel for helpful discussions at
`different stages of this work; C. Maheu for virus
`purification; and S. Harrison and K. Heldwein for sharing
`information before publication. Data collection was
`performed in part at the Swiss Light Source, Paul Scherrer
`Institut, Villigen, Switzerland, and at the European
`
`Synchrotron Radiation Facility, Grenoble, France. We
`gratefully acknowledge the help in data collection of
`T. Tomizaki (beamline X06SA, SLS); C. Petosa (beamline
`ID14-1, ESRF); and S. Duquerroy, F. Ternois, and
`G. Squires. We thank B. Gigant and I. Gallay for the use
`of rotating anode sources for crystal testing. We
`acknowledge support from the CNRS and INRA, the
`CNRS program ‘‘Physique et Chimie du Vivant,’’ the
`INRA Animal Health Department program ‘‘Les virus
`des animaux et leurs interactions avec la cellule,’’ the
`Ministe` re de l’e´ ducation nationale, de la recherche
`et de la technologie (MENRT) program ‘‘ACI blanche,’’
`and the Agence National de la Recherche (ANR)
`program.
`
`Supporting Online Material
`www.sciencemag.org/cgi/content/full/313/5784/187/DC1
`Materials and Methods
`SOM Text
`Figs. S1 to S3
`References and Notes
`
`21 March 2006; accepted 17 May 2006
`10.1126/science.1127683
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`Crystal Structure of the Low-pH Form of the Vesicular Stomatitis Virus
`Glycoprotein G
`Stéphane Roche, Stéphane Bressanelli, Félix A. Rey, and Yves Gaudin
`
`Science 313 (5784), . DOI: 10.1126/science.1127683
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