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Hexameric structures of the archaeal secretion ATPase GspE and implications for a universal secretion mechanism

2007; Springer Nature; Volume: 26; Issue: 3 Linguagem: Inglês

10.1038/sj.emboj.7601544

1460-2075

Atsushi Yamagata, John A. Tainer,

Bacterial Genetics and Biotechnology

Article25 January 2007free access Hexameric structures of the archaeal secretion ATPase GspE and implications for a universal secretion mechanism Atsushi Yamagata Atsushi Yamagata Department of Molecular Biology, The Skaggs Institute for Chemical Biology, The Scripps Research Institute, La Jolla, CA, USA Search for more papers by this author John A Tainer Corresponding Author John A Tainer Department of Molecular Biology, The Skaggs Institute for Chemical Biology, The Scripps Research Institute, La Jolla, CA, USA Search for more papers by this author Atsushi Yamagata Atsushi Yamagata Department of Molecular Biology, The Skaggs Institute for Chemical Biology, The Scripps Research Institute, La Jolla, CA, USA Search for more papers by this author John A Tainer Corresponding Author John A Tainer Department of Molecular Biology, The Skaggs Institute for Chemical Biology, The Scripps Research Institute, La Jolla, CA, USA Search for more papers by this author Author Information Atsushi Yamagata1 and John A Tainer 1 1Department of Molecular Biology, The Skaggs Institute for Chemical Biology, The Scripps Research Institute, La Jolla, CA, USA *Corresponding author. Department of Molecular Biology, MB 4, The Skaggs Institute for Chemical Biology, The Scripps Research Institute, 10550 Torrey Pines Road, La Jolla, CA 92037, USA. Tel.: +1 858 784 8119; Fax: +1 858 784 2277; E-mail: [email protected] The EMBO Journal (2007)26:878-890https://doi.org/10.1038/sj.emboj.7601544 PDFDownload PDF of article text and main figures. ToolsAdd to favoritesDownload CitationsTrack CitationsPermissions ShareFacebookTwitterLinked InMendeleyWechatReddit Figures & Info The secretion superfamily ATPases are conserved motors in key microbial membrane transport and filament assembly machineries, including bacterial type II and IV secretion, type IV pilus assembly, natural competence, and archaeal flagellae assembly. We report here crystal structures and small angle X-ray scattering (SAXS) solution analyses of the Archaeoglobus fulgidus secretion superfamily ATPase, afGspE. AfGspE structures in complex with ATP analogue AMP-PNP and Mg2+ reveal for the first time, alternating open and closed subunit conformations within a hexameric ring. The closed-form active site with bound Mg2+ evidently reveals the catalytically active conformation. Furthermore, nucleotide binding results and SAXS analyses of ADP, ATPγS, ADP-Vi, and AMP-PNP-bound states in solution showed that asymmetric assembly involves ADP binding, but clamped closed conformations depend on both ATP γ-phosphate and Mg2+ plus the conserved motifs, arginine fingers, and subdomains of the secretion ATPase superfamily. Moreover, protruding N-terminal domain shifts caused by the closed conformation suggest a unified piston-like, push–pull mechanism for ATP hydrolysis-dependent conformational changes, suitable to drive diverse microbial secretion and assembly processes by a universal mechanism. Introduction Bacterial and archaeal microbes use multi-protein translocation assembles to control the traffic out of cells by proteins and toxins (Remaut and Waksman, 2004). Type II secretion (T2S) is a major translocation mode, in which the secreted protein has a signal sequence and is transported from the cytoplasm to periplasmic space by Sec machinery that is called a general secretory pathway (GSP). So T2S is also termed the main terminal branch of GSP. Its protein components are designated in alphabetical order, for example, GspA-O in Escherichia coli (Sandkvist, 2001). Among them, the cytoplasmic ATPase, generally termed GspE, is especially important as the only protein able to utilize the energy of ATP hydrolysis and as belonging to the large superfamily of 'type II/IV secretion NTPases', or 'secretion superfamily ATPases'. This superfamily involves the ATPases, which function in multiple macromolecular transport systems in bacteria and archaea, such as bacterial T2S and type IV secretion (T4S), type IV pilus (T4 pilus) assembly, DNA uptake, and archaeal flagellae assembly systems (Planet et al, 2001; Peabody et al, 2003; Craig et al, 2004). The secretion ATPase superfamily proteins share similar amino-acids sequences along with the characteristic conserved motifs of the Walker box A, containing the P-loop GX4GK(S/T) of an NTP binding motif, an atypical Walker box B, plus Asp and His Boxes (Possot and Pugsley, 1994). They are subdivided into T2S and T4S classes by sequence similarity. Although both T2S and T4S are powered by secretion ATPases, their basic mechanisms are likely fundamentally different. The T2S system forms the pilin-like or 'pseudopilin' filament, and secretion ATPase assembles this filament. For T4S, the secretion ATPase may be the protein translocator, which interacts with the secreted protein folded in the cytoplasm. The archaeal secretion NTPase FlaI acts in the archaeal flagellar assembly system that resembles the better characterized bacterial T4 pilus and T2S systems (Patenge et al, 2001; Thomas et al, 2002). The characteristic N-terminal signal and conserved hydrophobic sequences of archaeal flagellins are remarkably similar to bacterial T4 pilin and T2S 'pseudopilin' (Bardy et al, 2003; Peabody et al, 2003). The best characterized of these, the T4 pilus, is assembled from pilin subunits into a filament on the pathogenic bacteria cell surface, where it acts in attachment to human cells, host cell signaling, biofilm formation, and DNA uptake (Craig et al, 2004). The T2S 'pseudopilus' is composed of subunits similar to T4P and may act as a molecular piston designed to push the secreted proteins (Sauvonnet et al, 2000; Sandkvist, 2001). Full-length T4P crystal structures show that the conserved hydrophobic sequence forms an extended α-helix (Parge et al, 1995; Craig et al, 2003), which assembles as an inner scaffold for filament assembly (Craig et al, 2006). Thus, the assembly utilizing conserved hydrophobic sequences may be universal to the archaeal flagellar, T2S pseudopilins, and T4 pilus (Kohler et al, 2004; Trachtenberg et al, 2005). These filament biogenesis mechanisms also resemble each other. Their filament subunits are transported by the Sec system, followed by signal sequence cleavage by a specific membrane-bound signal peptidase (Bardy and Jarrell, 2003). The matured subunits are then anchored in the inner membrane and assembled into a filament by the secretion ATPases (Turner et al, 1993). Notably, amino-acid sequences of FlaI and other archaeal secretion ATPases resemble bacterial T2S and T4S type ATPases, suggesting that the archaeal secretion ATPases may be a progenitor of the secretion ATPase superfamily. Archaeal genomes encode multiple FlaI-like secretion superfamily NTPase genes. The flaI gene of the fla gene cluster is immediately followed by the flaJ gene that encodes a multi-transmembrane protein (Patenge et al, 2001; Thomas et al, 2002). Interestingly, most other archaeal secretion superfamily ATPase genes are also associated with FlaJ-like membrane protein genes (Albers and Driessen, 2005; Falb et al, 2005). These protein pairs may be functionally coupled and act in unique archaeal secretion systems that resemble T2S (Albers and Driessen, 2005; Albers et al, 2006). The Sulfolobus solfataricus sugar binding protein secreted from cytoplasm to periplasm harbors the archaeal flagellin-like hydrophobic sequence and signal sequence (Albers and Driessen, 2002). Secretion of these sugar binding proteins requires signal sequence cleavage by the membrane-bound signal peptidase, which also cleaves the flagellin signal sequence (Albers et al, 2003). Putative secreted protein genes with flagellin-like signal sequences also occur in other archaea (Albers and Driessen, 2005; Falb et al, 2005). So the secreted protein may be transported from cytoplasm to the inner membrane by Sec, and then extracted from the membrane by FlaI-like ATPase, coupled with the FlaJ-like membrane proteins (Albers and Driessen, 2005; Albers et al, 2006). These homologous secretion superfamily ATPase and membrane protein pairs also occur in bacteria. Secretion ATPase family phylogenetic analysis shows that TadA ATPase needed for the bacterial Flp T4 pilus assembly (Planet et al, 2001) resembles the archaeal secretion ATPases. This tadA gene cluster includes the tadB and tadC genes, that resemble FlaJ (Peabody et al, 2003; Planet et al, 2003). Thus, these NTPase/membrane protein pairs appear evolutionally related across bacteria and archaea. Although the secretion superfamily ATPase is the critical conserved component responsible for archaeal flagellae assembly, bacterial T2S and T4S, and T4 pilus assembly, as well as twitching motility and biofilm formation, a unified molecular understanding of the mechanism and function of this ATPase superfamily resulting from defined conformation and assembly states has been elusive. At least some secretion superfamily NTPase family proteins assemble into hexameric rings, as seen by electron microscopy of T4S ATPase VirB11 (Krause et al, 2000) and of BfpD ATPase, which assembles the bundle-forming T4 pilin (Crowther et al, 2005). The crystal structure of the HP0525 VirB11 ATPase from Helicobactor pylori also revealed hexameric rings (Yeo et al, 2000; Savvides et al, 2003), and that of EpsE from Vibrio cholerae has a related six-fold helical assembly (Robien et al, 2003). As hexameric association can precede ATP hydrolysis, a key functional role is implied for ATP binding to GspE (Shiue et al, 2006). However, it has been unclear if other secretion superfamily NTPase proteins also form functionally-important rings, and furthermore, how their individual domain movements and overall quarternary assembly are coupled to ATP binding, hydrolysis, and biological function. This lack of unified mechanistic understanding reflects the fact that no hexameric ring structures in distinct functional conformations have been available for any secretion ATPase that functions by a T2S-like mechanism. To help elucidate the secretion superfamily ATPase structure, conformational state, and dynamic assembly and their functional implications, we solved the crystal structure of the Archaeoglobus fulgidus GspE (afGspE) secretion ATPase with and without nucleotide and furthermore examined structures of the ADP, ATPγS, ADP plus vanadate (ADP-Vi), and AMP-PNP-bound states in solution by small angle X-ray scattering (SAXS). The crystal and solution structures reveal distinct open and closed subunit conformations and a new hexameric assembly. The resulting alternating conformational states and hexameric assembly have specific and general implications for domain movement and molecular rotation mechanisms upon ATP hydrolysis. Taken together, the results support a unified molecular push–pull model, whereby ATP hydrolysis-coupled rotation and domain translocation may be transmitted to distinct partner proteins as a universal molecular mechanism for assembling microbial filamentous pilin, pseudopilin, and archaeal flagilin subunits, and for promoting secretion by the microbial T2S and T4S systems. Results and discussion GspE protein from A. fulgidus To identify and purify a prototypic archaeal secretion ATPase, we searched the A. fulgidus genome and found four potential secretion ATPase genes, afgspE1–4. All translated AfGspE1–4 protein sequences are homologous to FlaI from Methanococcus voltae and Halobacterium salinarum, with sequence identities of ∼35–41% and similarities of ∼51–58% within coding sequences of similar length (see Figure 2), and belong to the archaeal secretion ATPase family (Supplementary Figure 1). As afGspE4 belongs to the fla gene cluster, it is classified as FlaI. In afgspE2, three genes are immediately followed by multi-transmembrane proteins, with marginal similarity to FlaJ/TadB/TadC. Recent analyses determined that S. solfataricus, Natronomonas pharaonis, and H. salinuram genomes also harbor these secretion NTPase and membrane protein couples (Albers and Driessen, 2005; Falb et al, 2005). Although putative secreted protein genes near the secretion NTPase gene were reported in S. solfataricus, we could not identify any candidate genes near afgspE1–3 genes (Albers and Driessen, 2005). The recombinant expression of the S. solfataricus secretion NTPases in E. coli was unsuccessful owing to inclusion body formation (Albers and Driessen, 2005). However, we successfully expressed and purified the afGspE2 ATPase, which provides a prototypic archaeal secretion superfamily ATPase for structure–function characterizations. As afGspE1–4 are highly homologous to each other, we designate afGspE2 as afGspE in the following sections. Figure 1.AfGspE subunit structure. (A) Experimental electron density (contoured at 1σ) overlaid with the final refined model. (B) AfGspE fold as ribbons (molecule B) with subdomains N1 (yellow), N2 (green), C1 (blue), and C2 (magenta) plus bound AMP-PNP (red sticks), and P-loop (orange, center). (C) Topology schematic, (D) Cα stereo diagram of two superimposed afGspE subunit structures with bound AMP-PNP (red, CPK spheres). The open form (molecule A, orange) needs NTD rotations to yield the closed form (molecule B, blue). Download figure Download PowerPoint Subunit structure and conformations To reveal the atomic details for subunit structure and assembly, we determined the X-ray crystal structure of the afGspE ATPase. Crystals of selenomethionine (Se-Met)-substituted afGspE were grown in the presence of 10 mM AMP-PNP, a non-hydrolyzable ATP analog, and 10 mM Mg2+. Interpretable electron density maps generated by multi-wavelength anomalous dispersive phasing and subsequent solvent flipping (Figure 1A) allowed us to build the final models to the electron density for two molecules in the asymmetric unit. Both models are full-length, missing only four residues at the N terminus and a few disordered residues in the flexible regions (Figure 1). The resulting 2.95 Å resolution refined structure has R and Rfree factors of 22.6 and 23.3%, respectively (Supplementary Table I). Figure 2.Sequence conservation, secondary structure and residue function. AfGspE sequence aligned with H. salinuram FlaI, and M. voltae FlaI reveals identical (white letters in red box) and homologous residues (red letters in blue box) with secondary structure helices (bars) and strands (arrows) colored as in Figure 1B. Conserved Walker motifs A (orange) and B (green) along with Asp (pink) and His (yellow) box motifs are labeled in boxes. Dual arginine fingers (Arg208 and 227) are highlighted by magenta boxes. Also shown as largely conserved are the residues binding AMP-PNP (red reverse triangles) and in subunit-subunit interactions (blue triangles). Download figure Download PowerPoint Each afGspE subunit has two overall domains: the N-terminal domain (NTD) and the C-terminal domain (CTD) (Figure 1B and C). The NTD is divided into two subdomains. The N1 subdomain consists of a 4-α-helix bundle (α1–4) and 3-stranded β-sheet (β1–3). Compared with other regions, the N1 electron density was less ordered, suggesting flexibility; in particular, molecule A N1 residues 105–113 were missing in the electron density maps and side chains for the α1–β2 loop and the α2–α3 loop in both molecules A and B also had poor electron density. These weak-electron density regions imply flexibility within N1, which may be stabilized through disorder–order transitions induced by binding to partner proteins. The evidently more ordered and rigid N2 subdomain is comprised of six antiparallel β-strands (β4–9), forming a flat face against the CTD, and two helices (α5–6) located on the opposite side (Figure 1B). CTD also has two subdomains: C1 and C2 (Figure 1B and C). C1 is functionally critical with the Walker A and B motifs plus Asp box and His box motifs (Figures 1 and 2). C1 consists of a central β-strand core (β10–16) flanked on both sides by a total of seven α-helices (α7–13), with a topology similar to Walker type ATPases (Figure 1C), such as the RecA-like ATPases, including Rad51, F1-ATPase, the Holliday junction branch migration motor RuvB AAA+ ATPase, and the ABC ATPases, according to a DALI database search (Story and Steitz, 1992; Holm and Sander, 1993; Abrahams et al, 1994; Hopfner et al, 2000; Putnam et al, 2001; Shin et al, 2003). The molecule B β12–α10 loop was missing, although that in molecule A was well observed. One feature of afGspE C1 is that the two β-strands, β15–16, are extended from the central β-strand core toward the NTD (Figure 1B and C). C2 is composed of two β-strands (β17–18) and the four α-helices (α14–17). Significantly, the AMP-PNP nucleotide bound by Walker A motif in C1, lies in the interface between the NTD N2 and CTD C1 (Figure 1B–D), which contain the most conserved sequence blocks (Figure 2). Significantly, the two afGspE subunits within the asymmetric unit have surprisingly distinct conformations, involving a large rigid body movement of the NTDs about the bound nucleotide (Figure 1D). Molecule A reveals the open form (Figure 1D), in which NTD (light orange) twists and splays out from CTD (orange), while molecule B reveals the closed form (blue), in which NTD (light blue) closes against the CTD (blue). The range of the rotation between the open–closed forms is ∼16 Å, generating an overall ∼10-Å N1 shift. Conformational changes between the NTD and CTD also occur in HP0525 (Savvides et al, 2003). The Apo-HP0525 structure, which has six molecules with different conformations in the asymmetric unit, showed that the NTD is capable of 2° to 15° rotations. The open form of afGspE resembles the most open form of the apo-HP0525 (molecule F), whereas the afGspE's closed form resembles the apo-HP0525 closed form (molecule A). Also, the structure of a truncated GspE ATPase, V. cholerae EpsE, (Robien et al, 2003) has a single molecule in the asymmetric unit that adopts an 'half-open' form, compared with the afGspE subunit structures. These results suggest the afGspE open to closed conformational changes and range of movement is a conserved functional feature for the secretion ATPase superfamily. Secretion ATPase hexamer structure Two afGspE subunits in the asymmetric unit form a hexameric ring structure via a crystallographic three-fold symmetry axes. The external ring diameter is ∼135 Å, with a height of ∼75 Å (Figure 3A). The inner diameter is ∼35 Å between the NTDs, which narrows to ∼25 Å at bottom side between the CTDs (Figure 3A). Similar appearing ring structures of afGspE in the presence of AMP-PNP were also observed by electron microscopy, with somewhat smaller diameters of about 100 Å, consistent with some dehydration (Figure 3B). Electron density for the upper side of the ring is sparse, as it is composed of the N1 helical region (Figure 3A). The core of the ring is composed of the N2 and C1 subdomains, which line the ∼50 Å height of the cylindrical core hole (Figure 3A). The bottom of the cylindrical hole is composed of C2 β17–18. The outside of the ring is surrounded by N1 β-strands and C2 α-helices. At the top of the afGspE hexameric ring, N1 changes promote distinct open and closed subunit conformations because the N1 α-helices show large nucleotide-dependent conformational changes relative to the ring's core (Figure 3A). Figure 3.Hexameric ring structure, assembly, AMP-PNP binding, alternating subunit conformations, and subunit interactions. (A) AfGspE hexamer assembly and fold shown as ribbons and as schematic shapes viewed from top and side. Bound AMP-PNP (red, CPK spheres) in closed form molecules (blue, light blue NTD) and alternate open form molecules (orange, light orange NTD). (B) Electron micrograph of negatively stained AfGspE proteins with AMP-PNP. Ring structures are indicated by black arrows. (C) Subunit-subunit interactions with domain colors as in A, and encircled residues for N2:C1 (red), for C1:C1 (green), and N1:C2 (magenta) interactions. Download figure Download PowerPoint Subunit–subunit interactions The afGspE hexamer has three distinct subunit interfaces (Figure 3C). The dominant inter-subunit interaction is between the N2 of one molecule and the C1 of the adjacent molecule (Figure 3C, red elipse). Six N2 β-strands (β4–9) and their associated loops form a large interface with the C1 β12-loop-α10 and the N-terminal end of β11. Analogous interactions are formed within the N:C domain interface within the HP0525 ring and the N2:C1 domain interface of the EpsE helical assembly (Yeo et al, 2000; Robien et al, 2003). The ∼2000-Å2 buried surface of the afGspE major N2:C1 interface resembles the ∼1800-Å2 EpsE N2:C1 interface (Robien et al, 2003). Also FlaI family conserved residue pairs, such as Thr225:Lys289, Asp200:Arg331, and Glu216:Arg312, contribute key salt bridges and hydrogen bonds for the N2:C1 interface (Figure 2). A second major but variable interface occurs between adjacent C1 domains and is conserved within the HP0525 and EpsE structures (Figure 3C, green circle). The C1:C1 interface buried surface area of ∼900 Å2 is half the size of the N2:C1 interface, but the conserved histidine residues (His 358 and 365) in His box are involved in C1:C1 interactions. Also, although the interacting residues pairs are consistent between both N2(molecule A):C1(molecule B) and N2(molecule B): C1(molecule A) pairs, the C1:C1 residue interactions are less conserved between the two different sets of intermolecular interactions. This, together with the smaller interface, suggests that the C1:C1 interactions alter when converting between the open and closed subunit forms. A third small interface forms between extended loop from the N1 domain β2 and the end of α16 from the C2 domain (Figure 3C, magenta elipse). This is the first observation of this interaction, as the N1 domain is truncated in the EpsE crystal structure and HP0525 lacks an N1 domain (Yeo et al, 2000; Robien et al, 2003). The nucleotide binding site Bound AMP-PNP has clear electron density in both molecules' active site; however, Mg2+ density was only observed in the closed form (molecule B) (Figure 4A and B). In molecule A, AMP-PNP is bound within the CTD by Walker A and P-loop residues (Figure 4A). The AMP-PNP triphosphates are hydrogen bonded by P-loop backbone nitrogens (Ala270 to Thr274) and Thr275, and Lys273 Nζ interacts with the β-phosphate. Side chains of Thr274 and 275 are also involved in interactions with the phosphates. Also, the conserved Glu298 and 341 interact with the γ-phosphate, although they are suitable for Mg2+ binding (see below). Notably, Glu298 is the conserved acidic residue in Asp box and Glu341 is the Walker motif B residue. The AMP-PNP adenine ring is stabilized by base stacking-like interactions with Arg408, which also interacts with ribose O4. The adenine N6 amide group interacts with Thr244 Oγ1, suggesting a preference for binding ATP rather than GTP. There are no interactions between NTD and AMP-PNP in molecule A. Figure 4.The nucleotide binding site. Left: Fo–Fc difference Fourier map (contoured at 3σ) was calculated with the protein residues and superimposed onto the refined structure with AMP-PNP (purple), P-loop residues (red), Glu298 and 341 (green) in Walker B motif. The active site residues are shown in sticks. Right: nucleotide binding site showing stick models (blue nitrogens and red oxygens) and Mg2+ (gray sphere). (A) The nucleotide binding site in molecule A (open form). (B) The nucleotide binding site in molecule B (closed form). Dual arginine fingers (Arg208 and 227, red) interact with AMP-PNP γ-phosphate (dotted lines) and Glu298/341 face Mg2+ ion. (C) Phosphate binding site in closed form (molecule B). Arg227 and 408 conformational changes from the AMP-PNP bound form (light magenta) to phosphate bound form are indicated by arrows. Download figure Download PowerPoint In contrast, molecule B NTD makes additional contacts with AMP-PNP (Figure 4B). Dual arginine fingers formed by Arg208 and 227 interact with the AMP-PNP γ-phosphate. Also, the Thr230 Oγ1 and Lys228 main-chain amide contacts AMP-PNP 2′-O (not shown in Figure 4B). Dual arginine fingers are conserved in the FlaI family and most other secretion ATPase superfamily proteins (Figure 2). The bound nucleotide interactions with the dual arginine fingers also occur in the HP0525 structure, in which Arg103 and 113 interact with γ-phosphate in the ATPγS complex (Savvides et al, 2003). In the EpsE structure, the arginine fingers (Arg 210 and 227) do not contact the AMP-PNP phosphates as it is in the half-open form. Importantly, these dual arginine fingers may be distinguished as arginine clamps from the single arginine finger in other hexameric ATPases: all these secretion ATPase's dual arginine fingers:phosphate interactions occur within the same subunit, and not from the adjacent molecule, as do the distinct single arginine fingers in other hexameric ATPases, such as the F1-ATPase (Abrahams et al, 1994), the T7 gene 4 helicase (Singleton et al, 2000), and RuvB AAA+ ATPase (Putnam et al, 2001). No nucleotide contacts from adjacent molecules occur in either afGspE or HP0525. The NTD residues interacting with AMP-PNP also form major contacts with CTD in the closed form. There are only a few NTD:CTD interactions except for these nucleotide-mediated interactions, suggesting that the nucleotide absence destabilizes the closed conformation. In the adenine ring moiety, Arg408 makes extensive adenine ring contacts and also can bind Glu145 from NTD ∼3.3 Å away (Figure 4B). In addition, Arg406 interacts with the NTD residues, Glu145, Asp146, and Tyr167 (not shown). Glu145 and Asp146 are conserved in the FlaI family (Figure 2). Molecule B has clear electron density for Mg2+, liganded by the Walker A motif Thr 274 Oγ1 and two AMP-PNP phosphates. The side chains of Glu 298 (Asp box) and Glu 341 (Walker motif B) face the Mg2+ at distances of 3.6 and 3.8 Å, respectively, which are long for direct metal ion coordination. These Glu298/341 residues are equivalent to Glu209/248 in HP0525, and Glu296/334 in EpsE and analogous to RecA Glu96/Asp144 (Story and Steitz, 1992; Robien et al, 2003; Savvides et al, 2003). Taken together, these data suggest that afGspE Glu341 is a water activator for the in-line nucleophilic attack of the nucleotide γ-phosphate, and Glu298 stabilizes the hydrolytic intermediate by binding the water molecule liganded to Mg2+ ion. Phosphate-bound afGspE hexamer structure Surprisingly, apo afGspE crystallized in the same conditions as the AMP-PNP bound form, except without nucleotide and Mg2+ ion, still forming a hexameric ring with alternating subunit conformations. Both 2Fo–Fc and Fo–Fc difference Fourier maps showed an unexpected strong positive peak within the P-loop residues, which we interpret as phosphate ion scavenged from the crystallization conditions (Figure 4C). In the open form (molecule A), a single phosphate ion fits the density peak, whereas in the closed form (molecule B), the larger electron density peak fits three phosphate ions (see Supplementary data). As described below, the phosphate in molecule B interacts with the arginine fingers, which should stabilize the closed conformation and form the same hexameric structure as the AMP-PNP-bound form. The overall structure of afGspE:phosphate matches afGspE:AMP-PNP, but Arg227 and Arg408 showed significant conformational changes. In the AMP-PNP-bound structure, Arg227 protrudes toward the γ-phosphate in the closed form (Figure 4B), whereas in the open form, it protrudes in the opposite direction (Figure 4A) to make contact with the main-chain Arg331 O (not shown in Figure 4A) in the adjacent molecule. In the afGspE:phosphate structure (closed form), the Arg227 side chain moves into a middle position between those of the open and closed forms in the AMP-PNP-bound structure, as it contacts both the bound phosphate and Arg331 from the adjacent molecule (Figure 4C). This suggests that Arg227 has a sufficiently flexible conformation to be a sensor for different phosphate states of the bound nucleotide. Phosphate absence may promote the open form Arg227 conformation, which then destabilizes the closed form NTD:CTD interactions. In the phosphate-bound structure, Arg408 moves to the adenine ring position in the AMP-PNP-bound structure, which breaks the Glu145 interaction seen in the AMP-PNP-bound form. Thus, nucleotide absence destabilizes closed conformations by breaking NTD:CTD interactions between phosphates and arginine fingers, and between Glu145 and Arg408. ATP and Mg2+ binding and the catalytically active conformation The remarkable differences in the nucleotide binding site structures between the open and closed forms led us to hypothesize that they have different ATP binding affinities. Indeed, the HP0525:ATPγS hexamer structure obtained by soaking ATPγS into HP0525:sulfate crystals revealed two three-fold symmetric related alternative occupancies of ATPγS, although all subunits have the same closed form (Savvides et al, 2003). To estimate afGspE affinity for AMP-PNP, we performed a crystallographic titration experiment for afGspE structures co-crystallized with 0.2, 0.5, 1 mM AMP-PNP and 10 mM Mg2+ (Supplementary Figure 2). Bound AMP-PNP was similar in the open and closed forms at about 0.5 mM AMP-PNP, but at 0.2 mM AMP-PNP (∼1:1 protein:nucleotide ratio), we measured a partial occupancy for nucleotide binding in the open form, whereas the closed form showed full occupancy. Thus, open and closed afGspE have surprisingly similar nucleotide occupancy at ∼mM levels of ATP. Notably, afGspE structures with different AMP-PNP concentrations consistently showed that Mg2+ was bound only in the closed form. Even for Mg2+ concentration 10 times or greater than that of AM