Crystal structure of the rac activator, Asef, reveals its autoinhibitory mechanism

The Rac-specific guanine nucleotide exchange factor (GEF) Asef is activated by binding to the tumor suppressor adenomatous polyposis coli mutant, which is found in sporadic and familial colorectal tumors. This activated Asef is involved in the migration of colorectal tumor cells. The GEFs for Rho family GTPases contain the Dbl homology (DH) domain and the pleckstrin homology (PH) domain. When Asef is in the resting state, the GEF activity of the DH-PH module is intramolecularly inhibited by an unidentified mechanism. Asef has a Src homology 3 (SH3) domain in addition to the DH-PH module. In the present study, the three-dimensional structure of Asef was solved in its autoinhibited state. The crystal structure revealed that the SH3 domain binds intramolecularly to the DH domain, thus blocking the Rac-binding site. Furthermore, the RT-loop and the C-terminal region of the SH3 domain interact with the DH domain in a manner completely different from those for the canonical binding to a polyproline-peptide motif. These results demonstrate that the blocking of the Rac-binding site by the SH3 domain is essential for Asef autoinhibition. This may be a common mechanism in other proteins that possess an SH3 domain adjacent to a DH-PH module.     

Crystal structures of histone and p53 methyltransferase SmyD2 reveal a conformational flexibility of the autoinhibitory C-terminal domain

SmyD2 belongs to a new class of chromatin regulators that control gene expression in heart development and tumorigenesis. Besides methylation of histone H3 K4, SmyD2 can methylate non-histone targets including p53 and the retinoblastoma tumor suppressor. The methyltransferase activity of SmyD proteins has been proposed to be regulated by autoinhibition via the intra- and interdomain bending of the conserved C-terminal domain (CTD). However, there has been no direct evidence of a conformational change in the CTD. Here, we report two crystal structures of SmyD2 bound either to the cofactor product S-adenosylhomocysteine or to the inhibitor sinefungin. SmyD2 has a two-lobed structure with the active site located at the bottom of a deep crevice formed between the CTD and the catalytic domain. By extensive engagement with the methyltransferase domain, the CTD stabilizes the autoinhibited conformation of SmyD2 and restricts access to the catalytic site. Unexpectedly, despite that the two SmyD2 structures are highly superimposable, significant differences are observed in the first two helices of the CTDs: the two helices bend outwards and move away from the catalytic domain to generate a less closed conformation in the sinefungin-bound structure. Although the overall fold of the individual domains is structurally conserved among SmyD proteins, SmyD2 appear to be a conformational "intermediate" between a close form of SmyD3 and an open form of SmyD1. In addition, the structures reveal that the CTD is structurally similar to tetratricopeptide repeats (TPR), a motif through which many cochaperones bind to the heat shock protein Hsp90. Our results thus provide the first evidence for the intradomain flexibility of the TPR-like CTD, which may be important for the activation of SmyD proteins by Hsp90.     

Crystal structures of the HslVU peptidase-ATPase complex reveal an ATP-dependent proteolysis mechanism

BACKGROUND: The bacterial heat shock locus HslU ATPase and HslV peptidase together form an ATP-dependent HslVU protease. Bacterial HslVU is a homolog of the eukaryotic 26S proteasome. Crystallographic studies of HslVU should provide an understanding of ATP-dependent protein unfolding, translocation, and proteolysis by this and other ATP-dependent proteases. RESULTS: We present a 3.0 A resolution crystal structure of HslVU with an HslU hexamer bound at one end of an HslV dodecamer. The structure shows that the central pores of the ATPase and peptidase are next to each other and aligned. The central pore of HslU consists of a GYVG motif, which is conserved among protease-associated ATPases. The binding of one HslU hexamer to one end of an HslV dodecamer in the 3.0 A resolution structure opens both HslV central pores and induces asymmetric changes in HslV. CONCLUSIONS: Analysis of nucleotide binding induced conformational changes in the current and previous HslU structures suggests a protein unfolding-coupled translocation mechanism. In this mechanism, unfolded polypeptides are threaded through the aligned pores of the ATPase and peptidase and translocated into the peptidase central chamber.     

Crystal structures of human caspase 6 reveal a new mechanism for intramolecular cleavage self-activation

Dimeric effectors caspase 3 and caspase 7 are activated by initiator caspase processing. In this study, we report the crystal structures of effector caspase 6 (CASP6) zymogen and N-Acetyl-Val-Glu-Ile-Asp-al-inhibited CASP6. Both of these forms of CASP6 have a dimeric structure, and in CASP6 zymogen the intersubunit cleavage site (190)TEVD(193) is well structured and inserts into the active site. This positions residue Asp 193 to be easily attacked by the catalytic residue Cys 163. We demonstrate biochemically that intramolecular cleavage at Asp 193 is a prerequisite for CASP6 self-activation and that this activation mechanism is dependent on the length of the L2 loop. Our results indicate that CASP6 can be activated and regulated through intramolecular self-cleavage.     

Crystal structures of Fms1 and its complex with spermine reveal substrate specificity

Fms1 is a rate-limiting enzyme for the biosynthesis of pantothenic acid in yeast. Fms1 has polyamine oxidase (PAO) activity, which converts spermine into spermidine and 3-aminopropanal. The 3-aminopropanal is further oxidized to produce beta-alanine, which is necessary for the biosynthesis of pantothenic acid. The crystal structures of Fms1 and its complex with the substrate spermine have been determined using the single-wavelength anomalous diffraction (SAD) phasing method. Fms1 consists of an FAD-binding domain, with Rossmann fold topology, and a substrate-binding domain. The active site is a tunnel located at the interface of the two domains. The substrate spermine binds to the active site mainly via hydrogen bonds and hydrophobic interactions. In the complex, C11 but not C9 of spermine is close enough to the catalytic site (N5 of FAD) to be oxidized. Therefore, the products are spermidine and 3-aminopropanal, rather than 3-(aminopropyl) 4-aminobutyraldehyde and 1,3-diaminoprone.     

Crystal structures of Toxoplasma gondii adenosine kinase reveal a novel catalytic mechanism and prodrug binding

Adenosine kinase (AK) is a key purine metabolic enzyme from the opportunistic parasitic protozoan Toxoplasma gondii and belongs to the family of carbohydrate kinases that includes ribokinase. To understand the catalytic mechanism of AK, we determined the structures of the T. gondii apo AK, AK:adenosine complex and the AK:adenosine:AMP-PCP complex to 2.55 A, 2.50 A and 1.71 A resolution, respectively. These structures reveal a novel catalytic mechanism that involves an adenosine-induced domain rotation of 30 degrees and a newly described anion hole (DTXGAGD), requiring a helix-to-coil conformational change that is induced by ATP binding. Nucleotide binding also evokes a coil-to-helix transition that completes the formation of the ATP binding pocket. A conserved dipeptide, Gly68-Gly69, which is located at the bottom of the adenosine-binding site, functions as the switch for domain rotation. The synergistic structural changes that occur upon substrate binding sequester the adenosine and the ATP gamma phosphate from solvent and optimally position the substrates for catalysis. Finally, the 1.84 A resolution structure of an AK:7-iodotubercidin:AMP-PCP complex reveals the basis for the higher affinity binding of this prodrug over adenosine and thus provides a scaffold for the design of new inhibitors and subversive substrates that target the T. gondii AK.     

Crystal structures of Toxoplasma gondii adenosine kinase reveal a novel catalytic mechanism and prodrug binding

Adenosine kinase (AK) is a key purine metabolic enzyme from the opportunistic parasitic protozoan Toxoplasma gondii and belongs to the family of carbohydrate kinases that includes ribokinase. To understand the catalytic mechanism of AK, we determined the structures of the T. gondii apo AK, AK:adenosine complex and the AK:adenosine:AMP-PCP complex to 2.55 A, 2.50 A and 1.71 A resolution, respectively. These structures reveal a novel catalytic mechanism that involves an adenosine-induced domain rotation of 30 degrees and a newly described anion hole (DTXGAGD), requiring a helix-to-coil conformational change that is induced by ATP binding. Nucleotide binding also evokes a coil-to-helix transition that completes the formation of the ATP binding pocket. A conserved dipeptide, Gly68-Gly69, which is located at the bottom of the adenosine-binding site, functions as the switch for domain rotation. The synergistic structural changes that occur upon substrate binding sequester the adenosine and the ATP gi phosphate from solvent and optimally position the substrates for catalysis. Finally, the 1.84 A resolution structure of an AK:7-iodotubercidin:AMP-PCP complex reveals the basis for the higher affinity binding of this prodrug over adenosine and thus provides a scaffold for the design of new inhibitors and subversive substrates that target the T. gondii AK.     

Crystal structures of archaemetzincin reveal a moldable substrate-binding site

Background

Archaemetzincins are metalloproteases occurring in archaea and some mammalia. They are distinct from all the other metzincins by their extended active site consensus sequence HEXXHXXGXXHCX₄CXMX₁₇CXXC featuring four conserved cysteine residues. Very little is known about their biological importance and structure-function relationships.

Principal Findings

Here we present three crystal structures of the archaemetzincin AfAmzA (Uniprot {"type":"entrez-protein","attrs":{"text":"O29917","term_id":"74514303","term_text":"O29917"}}O29917) from Archaeoglobus fulgidus, revealing a metzincin architecture featuring a zinc finger-like structural element involving the conserved cysteines of the consensus motif. The active sites in all three structures are occluded to different extents rendering the enzymes proteolytically inactive against a large variety of tested substrates. Owing to the different ligand binding there are significant differences in active site architecture, revealing a large flexibility of the loops covering the active site cleft.

Conclusions

The crystal structures of AfAmzA provide the structural basis for the lack of activity in standard proteolytic assays and imply a triggered activity onset upon opening of the active site cleft.     

Crystal structures of Xanthomonas campestris OleA reveal features that promote head-to-head condensation of two long-chain fatty acids

OleA is a thiolase superfamily enzyme that has been shown to catalyze the condensation of two long-chain fatty acyl-coenzyme A (CoA) substrates. The enzyme is part of a larger gene cluster responsible for generating long-chain olefin products, a potential biofuel precursor. In thiolase superfamily enzymes, catalysis is achieved via a ping-pong mechanism. The first substrate forms a covalent intermediate with an active site cysteine that is followed by reaction with the second substrate. For OleA, this conjugation proceeds by a nondecarboxylative Claisen condensation. The OleA from Xanthomonas campestris has been crystallized and its structure determined, along with inhibitor-bound and xenon-derivatized structures, to improve our understanding of substrate positioning in the context of enzyme turnover. OleA is the first characterized thiolase superfamily member that has two long-chain alkyl substrates that need to be bound simultaneously and therefore uniquely requires an additional alkyl binding channel. The location of the fatty acid biosynthesis inhibitor, cerulenin, that possesses an alkyl chain length in the range of known OleA substrates, in conjunction with a single xenon binding site, leads to the putative assignment of this novel alkyl binding channel. Structural overlays between the OleA homologues, 3-hydroxy-3-methylglutaryl-CoA (HMG-CoA) synthase and the fatty acid biosynthesis enzyme FabH, allow assignment of the two remaining channels: one for the thioester-containing pantetheinate arm and the second for the alkyl group of one substrate. A short β-hairpin region is ordered in only one of the crystal forms, and that may suggest open and closed states relevant for substrate binding. Cys143 is the conserved catalytic cysteine within the superfamily, and the site of alkylation by cerulenin. The alkylated structure suggests that a glutamic acid residue (Glu117β) likely promotes Claisen condensation by acting as the catalytic base. Unexpectedly, Glu117β comes from the other monomer of the physiological dimer.     

Crystal structures of VAP1 reveal ADAMs' MDC domain architecture and its unique C-shaped scaffold

ADAMs (a disintegrin and metalloproteinase) are sheddases possessing extracellular metalloproteinase/disintegrin/cysteine-rich (MDC) domains. ADAMs uniquely display both proteolytic and adhesive activities on the cell surface, however, most of their physiological targets and adhesion mechanisms remain unclear. Here for the first time, we reveal the ADAMs' MDC architecture and a potential target-binding site by solving crystal structures of VAP1, a snake venom homolog of mammalian ADAMs. The D-domain protrudes from the M-domain opposing the catalytic site and constituting a C-shaped arm with cores of Ca2+ ions. The disintegrin-loop, supposed to interact with integrins, is packed by the C-domain and inaccessible for protein binding. Instead, the hyper-variable region (HVR) in the C-domain, which has a novel fold stabilized by the strictly conserved disulfide bridges, constitutes a potential protein-protein adhesive interface. The HVR is located at the distal end of the arm and faces toward the catalytic site. The C-shaped structure implies interplay between the ADAMs' proteolytic and adhesive domains and suggests a molecular mechanism for ADAMs' target recognition for shedding.     

Crystal structures of creatininase reveal the substrate binding site and provide an insight into the catalytic mechanism

Creatininase from Pseudomonas putida is a member of the urease-related amidohydrolase superfamily. The crystal structure of the Mn-activated enzyme has been solved by the single isomorphous replacement method at 1.8A resolution. The structures of the native creatininase and the Mn-activated creatininase-creatine complex have been determined by a difference Fourier method at 1.85 A and 1.6 A resolution, respectively. We found the disc-shaped hexamer to be roughly 100 A in diameter and 50 A in thickness and arranged as a trimer of dimers with 32 (D3) point group symmetry. The enzyme is a typical Zn2+ enzyme with a binuclear metal center (metal1 and metal2). Atomic absorption spectrometry and X-ray crystallography revealed that Zn2+ at metal1 (Zn1) was easily replaced with Mn2+ (Mn1). In the case of the Mn-activated enzyme, metal1 (Mn1) has a square-pyramidal geometry bound to three protein ligands of Glu34, Asp45, and His120 and two water molecules. Metal2 (Zn2) has a well-ordered tetrahedral geometry bound to the three protein ligands of His36, Asp45, and Glu183 and a water molecule. The crystal structure of the Mn-activated creatininase-creatine complex, which is the first structure as the enzyme-substrate/inhibitor complex of creatininase, reveals that significant conformation changes occur at the flap (between the alpha5 helix and the alpha6 helix) of the active site and the creatine is accommodated in a hydrophobic pocket consisting of Trp174, Trp154, Tyr121, Phe182, Tyr153, and Gly119. The high-resolution crystal structure of the creatininase-creatine complex enables us to identify two water molecules (Wat1 and Wat2) that are possibly essential for the catalytic mechanism of the enzyme. The structure and proposed catalytic mechanism of the creatininase are different from those of urease-related amidohydrolase superfamily enzymes. We propose a new two-step catalytic mechanism possibly common to creatininases in which the Wat1 acts as the attacking nucleophile in the water-adding step and the Wat2 acts as the catalytic acid in the ring-opening step.     

Crystal structures of two archaeal Pelotas reveal inter-domain structural plasticity

Dom34 from Saccharomyces cerevisiae is one of the key players in no-go mRNA decay, a surveillance pathway by which an abnormal mRNA stalled during translation is degraded by an endonucleolytic cleavage. Its homologs called Pelota are found in other species. We showed previously that S. cerevisiae Dom34 (domain 1) has an endoribonuclease activity, which suggests its direct catalytic role in no-go decay. Pelota from Thermoplasma acidophilum and Dom34 from S. cerevisiae have been structurally characterized, revealing a tripartite architecture with a significant difference in their overall conformations. To gain further insights into structural plasticity of the Pelota proteins, we have determined the crystal structures of two archaeal Pelotas from Archaeoglobus fulgidus and Sulfolobus solfataricus. Despite the structural similarity of their individual domains to those of T. acidophilum Pelota and S. cerevisiae Dom34, their overall conformations are distinct from those of T. acidophilum Pelota and S. cerevisiae Dom34. Different overall conformations are due to conformational flexibility of the two linker regions between domains 1 and 2 and between domains 2 and 3. The observed inter-domain structural plasticity of Pelota proteins suggests that large conformational changes are essential for their functions.     

Crystal structures of the Cid1 poly (U) polymerase reveal the mechanism for UTP selectivity

Polyuridylation is emerging as a ubiquitous post-translational modification with important roles in multiple aspects of RNA metabolism. These poly (U) tails are added by poly (U) polymerases with homology to poly (A) polymerases; nevertheless, the selection for UTP over ATP remains enigmatic. We report the structures of poly (U) polymerase Cid1 from Schizoscaccharomyces pombe alone and in complex with UTP, CTP, GTP and 3′-dATP. These structures reveal that each of the 4 nt can be accommodated at the active site; however, differences exist that suggest how the polymerase selects UTP over the other nucleotides. Furthermore, we find that Cid1 shares a number of common UTP recognition features with the kinetoplastid terminal uridyltransferases. Kinetic analysis of Cid1’s activity for its preferred substrates, UTP and ATP, reveal a clear preference for UTP over ATP. Ultimately, we show that a single histidine in the active site plays a pivotal role for poly (U) activity. Notably, this residue is typically replaced by an asparagine residue in Cid1-family poly (A) polymerases. By mutating this histidine to an asparagine residue in Cid1, we diminished Cid1’s activity for UTP addition and improved ATP incorporation, supporting that this residue is important for UTP selectivity.     

Crystal structures of human CtBP in complex with substrate MTOB reveal active site features useful for inhibitor design

The oncogenic corepressors C-terminal Binding Protein (CtBP) 1 and 2 harbor regulatory d-isomer specific 2-hydroxyacid dehydrogenase (d2-HDH) domains. 4-Methylthio 2-oxobutyric acid (MTOB) exhibits substrate inhibition and can interfere with CtBP oncogenic activity in cell culture and mice. Crystal structures of human CtBP1 and CtBP2 in complex with MTOB and NAD⁺ revealed two key features: a conserved tryptophan that likely contributes to substrate specificity and a hydrophilic cavity that links MTOB with an NAD⁺ phosphate. Neither feature is present in other d2-HDH enzymes. These structures thus offer key opportunities for the development of highly selective anti-neoplastic CtBP inhibitors.     

Crystal and electron microscopy structures of sticholysin II actinoporin reveal insights into the mechanism of membrane pore formation

Sticholysin II (StnII) is a pore-forming protein (PFP) produced by the sea anemone Stichodactyla helianthus. We found out that StnII exists in a monomeric soluble state but forms tetramers in the presence of a lipidic interface. Both structures have been independently determined at 1.7 A and 18 A resolution, respectively, by using X-ray crystallography and electron microscopy of two-dimensional crystals. Besides, the structure of soluble StnII complexed with phosphocholine, determined at 2.4 A resolution, reveals a phospholipid headgroup binding site, which is located in a region with an unusually high abundance of aromatic residues. Fitting of the atomic model into the electron microscopy density envelope suggests that while the beta sandwich structure of the protein remains intact upon oligomerization, the N-terminal region and a flexible and highly basic loop undergo significant conformational changes. These results provide the structural basis for the membrane recognition step of actinoporins and unexpected insights into the oligomerization step.     

Crystal structures of prethrombin-2 reveal alternative conformations under identical solution conditions and the mechanism of zymogen activation

Prethrombin-2 is the immediate zymogen precursor of the clotting enzyme thrombin, which is generated upon cleavage at R15 and separation of the A chain and catalytic B chain. The X-ray structure of prethrombin-2 determined in the free form at 1.9 ? resolution shows the 215-217 segment collapsed into the active site and occluding 49% of the volume available for substrate binding. Remarkably, some of the crystals harvested from the same crystallization well, under identical solution conditions, diffract to 2.2 ? resolution in the same space group but produce a structure in which the 215-217 segment moves >5 ? and occludes 24% of the volume available for substrate binding. The two alternative conformations of prethrombin-2 have the side chain of W215 relocating >9 ? within the active site and are relevant to the allosteric E*-E equilibrium of the mature enzyme. Another unanticipated feature of prethrombin-2 bears on the mechanism of prothrombin activation. R15 is found buried within the protein in ionic interactions with E14e, D14l, and E18, thereby making its exposure to solvent necessary for proteolytic attack and conversion to thrombin. On the basis of this structural observation, we constructed the E14eA/D14lA/E18A triple mutant to reduce the level of electrostatic coupling with R15 and promote zymogen activation. The mutation causes prethrombin-2 to spontaneously convert to thrombin, without the need for the snake venom ecarin or the physiological prothrombinase complex.     

Crystal structures of autoinhibitory PDZ domain of Tamalin: implications for metabotropic glutamate receptor trafficking regulation

Metabotropic glutamate receptors (mGluRs) function as neuronal G-protein-coupled receptors and this requires efficient membrane targeting through associations with cytoplasmic proteins. However, the molecular mechanism regulating mGluR cell-surface trafficking remains unknown. We report here that mGluR trafficking is controlled by the autoregulatory assembly of a scaffold protein Tamalin. In the absence of mGluR, Tamalin self-assembles into autoinhibited conformations, through its PDZ domain and C-terminal intrinsic ligand motif. X-ray crystallographic analyses visualized integral parts of the oligomeric self-assemblies of Tamalin, which require not only the novel hydrophobic dimerization interface but also canonical and noncanonical PDZ/ligand autoinhibitory interactions. The mGluR cytoplasmic region can competitively bind to Tamalin at a higher concentration, disrupting weak inhibitory interactions. The atomic view of mGluR association suggests that this rearrangement is dominated by electrostatic attraction and repulsion. We also observed in mammalian cells that the association liberates the intrinsic ligand toward a motor protein receptor, thereby facilitating mGluR cell-surface trafficking. Our study suggests a novel regulatory mechanism of the PDZ domain, by which Tamalin switches between the trafficking-inhibited and -active forms depending on mGluR association.     

Crystal structures of Bbp from Staphylococcus aureus reveal the ligand binding mechanism with Fibrinogen α

Bone sialoprotein-binding protein (Bbp), a MSCRAMMs (Microbial Surface Components Recognizing Adhesive Matrix Molecules) family protein expressed on the surface of Staphylococcus aureus (S. aureus), mediates adherence to fibrinogen α (Fg α), a component in the extracellular matrix of the host cell and is important for infection and pathogenesis. In this study, we solved the crystal structures of apo-Bbp^273−598 and Bbp^273−598-Fg α^561−575 complex at a resolution of 2.03 Å and 1.45 Å, respectively. Apo-Bbp^273−598 contained the ligand binding region N2 and N3 domains, both of which followed a DE variant IgG fold characterized by an additional D1 strand in N2 domain and D1′ and D2′ strands in N3 domain. The peptide mapped to the Fg α^561−575 bond to Bbp^273−598 on the open groove between the N2 and N3 domains. Strikingly, the disordered C-terminus in the apo-form reorganized into a highly-ordered loop and a β-strand G′′ covering the ligand upon ligand binding. Bbp^{Ala298−Gly301} in the N2 domain of the Bbp^273−598-Fg α^561−575 complex, which is a loop in the apo-form, formed a short α-helix to interact tightly with the peptide. In addition, Bbp^{Ser547−Gln561} in the N3 domain moved toward the binding groove to make contact directly with the peptide, while Bbp^{Asp338−Gly355} and Bbp^{Thr365−Tyr387} in N2 domain shifted their configurations to stabilize the reorganized C-terminus mainly through strong hydrogen bonds. Altogether, our results revealed the molecular basis for Bbp-ligand interaction and advanced our understanding of S. aureus infection process.