Structure of the regulatory subunit of acetohydroxyacid synthase isozyme III from Escherichia coli

The enzyme acetohydroxyacid synthase (AHAS) catalyses the first common step in the biosynthesis of the three branched-chain amino acids. Enzymes in the AHAS family generally consist of regulatory and catalytic subunits. Here, we describe the first crystal structure of an AHAS regulatory subunit, the ilvH polypeptide, determined at a resolution of 1.75 A. IlvH is the regulatory subunit of one of three AHAS isozymes expressed in Escherichia coli, AHAS III. The protein is a dimer, with two beta alpha beta beta alpha beta ferredoxin domains in each monomer. The two N-terminal domains assemble to form an ACT domain structure remarkably close to the one predicted by us on the basis of the regulatory domain of 3-phosphoglycerate dehydrogenase (3PGDH). The two C-terminal domains combine so that their beta-sheets are roughly positioned back-to-back and perpendicular to the extended beta-sheet of the N-terminal ACT domain. On the basis of the properties of mutants and a comparison with 3PGDH, the effector (valine) binding sites can be located tentatively in two symmetrically related positions in the interface between a pair of N-terminal domains. The properties of mutants of the ilvH polypeptide outside the putative effector-binding site provide further insight into the functioning of the holoenzyme. The results of this study open avenues for further studies aimed at understanding the mechanism of regulation of AHAS by small-molecule effectors.     

Kinetic, mutagenic, and structural homology analysis of L-serine dehydratase from Legionella pneumophila

A structural database search has revealed that the same fold found in the allosteric substrate binding (ASB) domain of Mycobacterium tuberculosis D-3-phosphoglycerate dehydrogenase (PGDH) is found in l-serine dehydratase from Legionella pneumophila. The M. tuberculosis PGDH ASB domain functions in the control of catalytic activity. Bacterial l-serine dehydratases are 4Fe-4S proteins that convert l-serine to pyruvate and ammonia. Sequence homology reveals two types depending on whether their α and β domains are on the same (Type 2) or separate (Type 1) polypeptides. The α domains contain the catalytic iron-sulfur center while the β domains do not yet have a described function, but the structural homology with PGDH suggests a regulatory role. Type 1 β domains also contain additional sequence homologous to PGDH ACT domains. A continuous assay for l-serine dehydratase is used to demonstrate homotropic cooperativity, a broad pH range, and essential irreversibility. Product inhibition analysis reveals a Uni-Bi ordered mechanism with ammonia dissociating before pyruvate. l-Threonine is a poor substrate and l-cysteine and d-serine are competitive inhibitors with K(i) values that differ by almost 10-fold from those reported for Escherichia colil-serine dehydratase. Mutagenesis identifies the three cysteine residues at the active site that anchor the iron-sulfur complex.     

Identification of amino acid residues contributing to the mechanism of cooperativity in Escherichia coli D-3-phosphoglycerate dehydrogenase

L-Serine inhibits the catalytic activity of Escherichia coli D-3-phosphoglycerate dehydrogenase (PGDH) by binding to its regulatory domain. This domain is a member of the ACT domain family of regulatory domains that are modulated by small molecules. A comparison of the phi and psi torsional angle differences between the crystal structures of PGDH solved in the presence and in the absence of L-serine demonstrated a clustering of significant angle deviations in the regulatory domain. A similar clustering was not observed in either of the other two structural domains of PGDH. In addition, significant differences were also observed at the active site and in the Trp-139 loop. To determine if these residues were functionally significant and not just due to other factors such as crystal packing, mutagenic analysis of these residues was performed. Not unexpectedly, this analysis showed that residues that affected the kcat/Km were grouped around the active site and those that affected the serine sensitivity were grouped in the regulatory domain. However, more significantly, residues that affected the cooperativity of inhibition of activity were identified at both locations. These latter residues represent structural elements that participate in both the initial and the ultimate events of the transfer of cooperative behavior from the regulatory domain to the active site. As such, their identification will assist in the elucidation of the pathway of cooperative interaction in this enzyme as well as in the elucidation of the regulatory mechanism of the ACT domain in general.     

Multiconformational states in phosphoglycerate dehydrogenase

Phosphoglycerate dehydrogenase (PGDH) catalyzes the first step in the serine biosynthetic pathway. In lower plants and bacteria, the PGDH reaction is regulated by the end-product of the pathway, serine. The regulation occurs through a V(max) mechanism with serine binding and inhibition occurring in a cooperative manner. The three-dimensional structure of the serine inhibited enzyme, determined by previous work, showed a tetrameric enzyme with 222 symmetry and an unusual overall toroidal appearance. To characterize the allosteric, cooperative effects of serine, we identified W139G PGDH as an enzymatically active mutant responsive to serine but not in a cooperative manner. The position of W139 near a subunit interface and the active site cleft suggested that this residue is a key player in relaying allosteric effects. The 2.09 A crystal structure of W139G-PGDH, determined in the absence of serine, revealed major quaternary and tertiary structural changes. Contrary to the wildtype enzyme where residues encompassing residue 139 formed extensive intersubunit contacts, the corresponding residues in the mutant were conformationally flexible. Within each of the three-domain subunits, one domain has rotated approximately 42 degrees relative to the other two. The resulting quaternary structure is now in a novel conformation creating new subunit-to-subunit contacts and illustrates the unusual flexibility in this V(max) regulated enzyme. Although changes at the regulatory domain interface have implications in other enzymes containing a similar regulatory or ACT domain, the serine binding site in W139G PGDH is essentially unchanged from the wildtype enzyme. The structural and previous biochemical characterization of W139G PGDH suggests that the allosteric regulation of PGDH is mediated not only by changes occurring at the ACT domain interface but also by conformational changes at the interface encompassing residue W139.     

Allosteric mechanisms in ACT domain containing enzymes involved in amino acid metabolism

Summary. An important sequence motif identified by sequence analysis is shared by the ACT domain family, which has been found in a number of diverse proteins. Most of the proteins containing the ACT domain seem to be involved in amino acid and purine synthesis and are in many cases allosteric enzymes with complex regulation enforced by the binding of ligands. Here we explore the current understanding of the ACT domain function including its role as an allosteric module in a selected group of enzymes. We will further describe in more detail three of the proteins where some understanding is available on function and structure: i) the archetypical ACT domain protein E. coli 3PGDH, which catalyzes the first step in the biosynthesis of L-Ser, ii) the bifunctional chorismate mutase/prephenate dehydratase (P-protein) from E. coli, which catalyzes the first two steps in the biosynthesis of L-Phe, and iii) the mammalian aromatic amino acid hydroxylases, with special emphasis on phenylalanine hydroxylase, which catalyzes the first step in the catabolic degradation of L-phenylalanine (L-Phe). The ACT domain is commonly involved in the binding of a small regulatory molecule, such as the amino acids L-Ser and L-Phe in the case of 3PGDH and P-protein, respectively. On the other hand, for PAH, and probably for other enzymes, this domain appears to have been incorporated as a handy, flexible small module with the potential to provide allosteric regulation via transmission of finely tuned conformational changes, not necessarily initiated by regulatory ligand binding at the domain itself.Current address: Protein Biophysics & Delivery, Novo Nordisk A/S, Novo Allé, 2880 Bagsværd, Denmark.     

De-regulation of D-3-phosphoglycerate dehydrogenase by domain removal.

Escherichia coli 3-phosphoglycerate dehydrogenase (PGDH) catalyzes the first step in serine biosynthesis, and is allosterically inhibited by serine. Structural studies revealed a homotetramer in which the quaternary arrangement of subunits formed an elongated ellipsoid. Each subunit consisted of three domains: nucleotide, substrate and regulatory. In PGDH, extensive interactions are formed between nucleotide binding domains. A second subunit-subunit interaction occurs between regulatory domains creating an extended beta sheet. The serine-binding sites overlap this interface. In these studies, the nucleotide and substrate domains (NSDs) were subcloned to identify changes in both catalytic and physical properties upon removal of a subunit-subunit interface. The NSDs did not vary significantly from PGDH with respect to kinetic parameters with the exception that serine no longer had an effect on catalysis. Temperature dependent dynamic light scattering (DLS) revealed the NSDs aggregated > 5 degrees C before PGDH, indicating decreased stability. DLS and gel filtration studies showed that the truncated enzyme formed a tetramer. This result negated the hypothesis that the removal of the regulatory domain would create an enzyme mimic of the unregulated, closely related dimeric enzymes. Expression of the regulatory domain, to study conformational changes induced by serine binding, yielded a product that by CD spectra contained stable secondary structure. DLS and pulsed field gradient NMR studies of the regulatory domain showed the presence of higher oligomers instead of the predicted dimer. We have concluded that the removal of the regulatory domain is sufficient to eliminate serine inhibition but does not have the expected effect on the quaternary structure.     

Adaptations of the helix-grip fold for ligand binding and catalysis in the START domain superfamily

With a protein structure comparison, an iterative database search with sequence profiles, and a multiple-alignment analysis, we show that two domains with the helix-grip fold, the star-related lipid-transfer (START) domain of the MLN64 protein and the birch allergen, are homologous. They define a large, previously underappreciated superfamily that we call the START superfamily. In addition to the classical START domains that are primarily involved in eukaryotic signaling mediated by lipid binding and the birch antigen family that consists of plant proteins implicated in stress/pathogen response, the START superfamily includes bacterial polyketide cyclases/aromatases (e.g., TcmN and WhiE VI) and two families of previously uncharacterized proteins. The identification of this domain provides a structural prediction of an important class of enzymes involved in polyketide antibiotic synthesis and allows the prediction of their active site. It is predicted that all START domains contain a similar ligand-binding pocket. Modifications of this pocket determine the ligand-binding specificity and may also be the basis for at least two distinct enzymatic activities, those of a cyclase/aromatase and an RNase. Thus, the START domain superfamily is a rare case of the adaptation of a protein fold with a conserved ligand-binding mode for both a broad variety of catalytic activities and noncatalytic regulatory functions. Proteins 2001;43:134-144.     

Crystal structures of TM0549 and NE1324--two orthologs of E. coli AHAS isozyme III small regulatory subunit

Crystal structures of two orthologs of the regulatory subunit of acetohydroxyacid synthase III (AHAS, EC 2.2.1.6) from Thermotoga maritima (TM0549) and Nitrosomonas europea (NE1324) were determined by single-wavelength anomalous diffraction methods with the use of selenomethionine derivatives at 2.3 A and 2.5 A, respectively. TM0549 and NE1324 share the same fold, and in both proteins the polypeptide chain contains two separate domains of a similar size. Each protein contains a C-terminal domain with ferredoxin-type fold and an N-terminal ACT domain, of which the latter is characteristic for several proteins involved in amino acid metabolism. The ferredoxin domain is stabilized by a calcium ion in the crystal structure of NE1324 and by a Mg(H2O)(6)2+ ion in TM0549. Both TM0549 and NE1324 form dimeric assemblies in the crystal lattice.     

Solution structure and folding characteristics of the C-terminal SH3 domain of c-Crk-II

Crk-II is a signaling adaptor protein that is involved in many cellular processes including apoptosis, proliferation, and differentiation. It has a modular domain architecture consisting of an Src homology 2 domain (SH2) followed by two Src homology 3 (SH3) domains. The structures and ligand-binding properties of the SH2 and the middle SH3 domains are well-characterized. Several studies suggest that the C-terminal SH3 domain plays an important regulatory role in the protein; however, no structural information is available on this domain, and relatively little is known about its binding partners. In the current work, we have solved the solution NMR structure of the C-terminal SH3 domain. The domain adopts the standard SH3 fold comprising a five-stranded beta barrel. In agreement with alignment and modeling studies, the structure indicates that the canonical-binding surface of the SH3 domain is unusually polar and suggests that this domain may not bind typical PXXP ligands or that it may bind them with reduced affinity. Thermodynamic and kinetic studies show that the domain folds in a reversible two-state manner and that the stability of the fold is similar to that observed for other SH3 domains. These studies offer some insight into the likely structural and thermodynamic consequences of point mutations in the cSH3 domain that are known to deregulate Crk-II function. Our results set the stage for a better understanding the role of the cSH3 domain in the context of the full-length protein.     

A novel organization of ACT domains in allosteric enzymes revealed by the crystal structure of Arabidopsis aspartate kinase

Asp kinase catalyzes the first step of the Asp-derived essential amino acid pathway in plants and microorganisms. Depending on the source organism, this enzyme contains up to four regulatory ACT domains and exhibits several isoforms under the control of a great variety of allosteric effectors. We report here the dimeric structure of a Lys and S-adenosylmethionine-sensitive Asp kinase isoform from Arabidopsis thaliana in complex with its two inhibitors. This work reveals the structure of an Asp kinase and an enzyme containing two ACT domains cocrystallized with its effectors. Only one ACT domain (ACT1) is implicated in effector binding. A loop involved in the binding of Lys and S-adenosylmethionine provides an explanation for the synergistic inhibition by these effectors. The presence of S-adenosylmethionine in the regulatory domain indicates that ACT domains are also able to bind nucleotides. The organization of ACT domains in the present structure is different from that observed in Thr deaminase and in the regulatory subunit of acetohydroxyacid synthase III.     

Crystal structure and anion binding in the prokaryotic hydrogenase maturation factor HypF acylphosphatase-like domain

[NiFe]-hydrogenases require a set of complementary and regulatory proteins for correct folding and maturation processes. One of the essential regulatory proteins, HypF (82kDa) contains a N-terminal acylphosphatase (ACT)-like domain, a sequence motif shared with enzymes catalyzing O-carbamoylation, and two zinc finger motifs similar to those found in the DnaJ chaperone. The HypF acylphosphatase domain is thought to support the conversion of carbamoylphosphate into CO and CN(-), promoting coordination of these ligands to the hydrogenase metal cluster. It has been shown recently that the HypF N-terminal domain can aggregate in vitro to yield fibrils matching those formed by proteins linked to amyloid diseases. The 1.27A resolution HypF acylphosphatase domain crystal structure (residues 1-91; R-factor 13.1%) shows a domain fold of betaalphabetabetaalphabeta topology, as observed in mammalian acylphosphatases specifically catalyzing the hydrolysis of the carboxyl-phosphate bonds in acylphosphates. The HypF N-terminal domain can be assigned to the ferredoxin structural superfamily, to which RNA-binding domains of small nuclear ribonucleoproteins and some metallochaperone proteins belong. Additionally, the HypF N-terminal domain displays an intriguing structural relationship to the recently discovered ACT domains. The structures of different HypF acylphosphatase domain complexes show a phosphate binding cradle comparable to the P-loop observed in unrelated phosphatase families. On the basis of the catalytic mechanism proposed for acylphosphatases, whereby residues Arg23 and Asn41 would support substrate orientation and the nucleophilic attack of a water molecule on the phosphate group, fine structural features of the HypF N-terminal domain putative active site region may account for the lack of acylphosphatase activity observed for the expressed domain. The crystallographic analyses here reported were undertaken to shed light on the molecular bases of inactivity, folding, misfolding and aggregation of the HypF N-terminal acylphosphatase domain.     

Sequence annotation of nuclear receptor ligand-binding domains by automated homology modeling

The quality of three-dimensional homology models derived from protein sequences provides an independent measure of the suitability of a protein sequence for a certain fold. We have used automated homology modeling and model assessment tools to identify putative nuclear hormone receptor ligand-binding domains in the genome of Caenorhabditis elegans. Our results indicate that the availability of multiple crystal structures is crucial to obtaining useful models in this receptor family. The majority of annotated mammalian nuclear hormone receptors could be assigned to a ligand-binding domain fold by using the best model derived from any of four template structures. This strategy also assigned the ligand-binding domain fold to a number of C.elegans. sequences without prior annotation. Interestingly, the retinoic acid receptor crystal structure contributed most to the number of sequences that could be assigned to a ligand-binding domain fold. Several causes for this can be suggested, including the high quality of this protein structure in terms of our assessment tools, similarity between the biological function or ligand of this receptor and the modeled genes and gene duplication in C.elegans.     

The lineage and diversity of putative amino acid sensor ACR proteins in plants

Amino acid metabolic enzymes often contain a regulatory ACT domain, named for aspartate kinase, chorismate mutase, and TyrA (prephenate dehydrogenase). Arabidopsis encodes 12 putative amino acid sensor ACT repeat (ACR) proteins, all containing ACT repeats but no identifiable catalytic domain. Arabidopsis ACRs comprise three groups based on domain composition and sequence: group I and II ACRs contain four ACTs each, and group III ACRs contain two ACTs. Previously, all three groups had been documented only in Arabidopsis. Here, we extended this to algae and land plants, showing that all three groups of ACRs are present in most, if not all, land plants, whereas among algal ACRs, although quite diverse, only group III is conserved. The appearance of canonical group I and II ACRs thus accompanied the evolution of plants from living in water to living on land. Alignment of ACTs from plant ACRs revealed a conserved motif, DRPGLL, at the putative ligand-binding site. Notably, the unique features of the DRPGLL motifs in each ACT domain are conserved in ACRs from algae to land plants. The conservation of plant ACRs is reminiscent of that of human cellular arginine sensor for mTORC1 (CASTOR1), a member of a small protein family highly conserved in animals. CASTOR proteins also have four ACT domains, although the sequence identities between ACRs and CASTORs are very low. Thus, plant ACRs and animal CASTORs may have adapted the regulatory ACT domains from a more ancient metabolic enzyme, and then evolved independently.     

Searching distant homologs of the regulatory ACT domain in phenylalanine hydroxylase

High sequence divergence, evolutionary mobility, and superfold topology characterize the ACT domain. Frequently found in multidomain proteins, these domains induce allosteric effects by binding a regulatory ligand usually to an ACT domain dimer interface. In mammalian phenylalanine hydroxylase (PAH), no contacts are formed between ACT domains, and the domain promotes an allosteric effect despite the apparent lack of ligand binding. The increased functional scenario of this abundant domain encouraged us to search for distant homologs, aiming to enhance the understanding of the ACT domain in general and the ACT domain of PAH in particular. The PDB was searched using the FATCAT server with the ACT domain of PAH as a query. The hits that were confirmed by the SSAP algorithm were divided into known ACT domains (KADs) and potential ACT domains (PADs). The FATCAT/SSAP procedure recognized most of the established KADs, as well 18 so far unrecognized non-redundant PADs with extremely low sequence identities and high divergence in functionality and oligomerization. However, analysis of the structural similarity provides remarkable clustering of the proteins according to similarities in ligand binding. Despite enormous sequence divergence and high functional variability, there is a common regulatory theme among these domains. The results reveal the close relationships of the ACT domain of PAH with amino acid binding and metallobinding ACT domains and with acylphosphatase.     

Crystal structure of a gamma-butyrolactone autoregulator receptor protein in Streptomyces coelicolor A3(2)

The gamma-butyrolactone-type autoregulator/receptor systems in the Gram-positive bacterial genus Streptomyces regulate morphological differentiation or antibiotic production, or both. The autoregulator receptors act as DNA-binding proteins, and on binding their cognate ligands (gamma-butyrolactones) they are released from the DNA, thus serving as repressors. The crystal structure of CprB in Streptomyces coelicolor A3(2), a homologue of the A-factor-receptor protein, ArpA, in Streptomyces griseus, was determined. The overall structure of CprB shows that the gamma-butyrolactone receptors belong to the TetR family. CprB is composed of two domains, a DNA-binding domain and a regulatory domain. The regulatory domain contains a hydrophobic cavity, which probably serves as a ligand-binding pocket. On the basis of the crystal structure of CprB and on the analogy of the characteristics of ligand-TetR binding, the binding of gamma-butyrolactones to the regulatory domain of the receptors is supposed to induce the relocation of the DNA-binding domain through conformational changes of residues located between the ligand-binding site and the DNA-binding domain, which would result in the dissociation of the receptors from their target DNA.     

Crystal structures of the Tie2 receptor ectodomain and the angiopoietin-2-Tie2 complex

The Tie receptor tyrosine kinases and their angiopoietin (Ang) ligands play central roles in developmental and tumor-induced angiogenesis. Here we present the crystal structures of the Tie2 ligand-binding region alone and in complex with Ang2. In contrast to prediction, Tie2 contains not two but three immunoglobulin (Ig) domains, which fold together with the three epidermal growth factor domains into a compact, arrowhead-shaped structure. Ang2 binds at the tip of the arrowhead utilizing a lock-and-key mode of ligand recognition-unique for a receptor kinase-where two complementary surfaces interact with each other with no domain rearrangements and little conformational change in either molecule. Ang2-Tie2 recognition is similar to antibody-protein antigen recognition, including the location of the ligand-binding site within the Ig fold. Analysis of the structures and structure-based mutagenesis provide insight into the mechanism of receptor activation and support the hypothesis that all angiopoietins interact with Tie2 in a structurally similar manner.     

Kinetic Evidence of a Noncatalytic Substrate Binding Site That Regulates Activity in Legionella pneumophilal-Serine Dehydratase

The l-serine dehydratase from Legionella pneumophila (lpLSD) has recently been shown to contain a domain (β domain) that has a high degree of structural homology with the ASB domain of d-3-phosphoglycerate dehydrogenase (PGDH) from Mycobacterium tuberculosis. Furthermore, this domain has been shown by sequence homology to be present in all bacterial l-serine dehydratases that utilize an Fe-S catalytic center. In PGDH, l-serine binds to the ACT domain to inhibit catalytic activity. However, substrate must be bound to the ASB domain for serine to exert its effect. As such, the ASB domain acts as a codomain for the action of l-serine. Pre-steady-state kinetic analysis of l-serine binding to lpLSD demonstrates that l-serine binds to a second noncatalytic site and produces a conformational change in the enzyme. The rate of this conformational change is too slow for its participation in the catalytic cycle but rather occurs prior to catalysis to produce an activated form of the enzyme. That the conformational change must occur prior to catalysis is shown by a lag in the production of product that exhibits essentially the same rate constant as the conformational change. The second, noncatalytic site for l-serine is likely to be the ASB domain (β domain) of lpLSD that functions in a manner similar to that in PGDH. A mechanism whose overall effect is to keep l-serine levels from accumulating to high levels while not completely depleting the l-serine pool in the bacterial cell is proposed.