Characterization of the aluminum and beryllium fluoride species which activate transducin. Analysis of the binding and dissociation kinetics

Automated Radiosynthesis, Preliminary In Vitro/In Vivo Characterization of OncoFAP-Based Radiopharmaceuticals for Cancer Imaging and Therapy

FAP-targeted radiopharmaceuticals represent a breakthrough in cancer imaging and a viable option for therapeutic applications. OncoFAP is an ultra-high-affinity ligand of FAP with a dissociation constant of 680 pM. OncoFAP has been recently discovered and clinically validated for PET imaging procedures in patients with solid malignancies. While more and more clinical validation is becoming available, the need for scalable and robust procedures for the preparation of this new class of radiopharmaceuticals continues to increase. In this article, we present the development of automated radiolabeling procedures for the preparation of OncoFAP-based radiopharmaceuticals for cancer imaging and therapy. A new series of [⁶⁸Ga]Ga-OncoFAP, [¹⁷⁷Lu]Lu-OncoFAP and [¹⁸F]AlF-OncoFAP was produced with high radiochemical yields. Chemical and biochemical characterization after radiolabeling confirmed its excellent stability, retention of high affinity for FAP and absence of radiolysis by-products. The in vivo biodistribution of [¹⁸F]AlF-NOTA-OncoFAP, a candidate for PET imaging procedures in patients, was assessed in mice bearing FAP-positive solid tumors. The product showed rapid accumulation in solid tumors, with an average of 6.6% ID/g one hour after systemic administration and excellent tumor-to-healthy organs ratio. We have developed simple, quick, safe and robust synthetic procedures for the preparation of theranostic OncoFAP-compounds based on Gallium-68, Lutetium-177 and Fluorine-18 using the commercially available FASTlab synthesis module.

Direct Interaction of Avian Cryptochrome 4 with a Cone Specific G-Protein

Background: Night-migratory birds sense the Earth’s magnetic field by an unknown molecular mechanism. Theoretical and experimental evidence support the hypothesis that the light-induced formation of a radical-pair in European robin cryptochrome 4a (ErCry4a) is the primary signaling step in the retina of the bird. In the present work, we investigated a possible route of cryptochrome signaling involving the α-subunit of the cone-secific heterotrimeric G protein from European robin. Methods: Protein–protein interaction studies include surface plasmon resonance, pulldown affinity binding and Förster resonance energy transfer. Results: Surface plasmon resonance studies showed direct interaction, revealing high to moderate affinity for binding of non-myristoylated and myristoylated G protein to ErCry4a, respectively. Pulldown affinity experiments confirmed this complex formation in solution. We validated these in vitro data by monitoring the interaction between ErCry4a and G protein in a transiently transfected neuroretinal cell line using Förster resonance energy transfer. Conclusions: Our results suggest that ErCry4a and the G protein also interact in living cells and might constitute the first biochemical signaling step in radical-pair-based magnetoreception.

Optimal 18F-fluorination conditions for the high radiochemical yield of [18F]AlF-NOTA-NHS complexes

18F-fluorination using aluminum-fluoride ([18F]AlF) chelate technique has been reported to give a low-to-moderate radiochemical yield, between 5 and 20%. Therefore, the work described here outlines the optimum 18F-fluorination condition for the formation of [18F]AlF2+ and [18F]AlF-NOTA-NHS complex with the radiochemical yield (RCY) and purity (RCP) of more than 90% as a prerequisite step before proceeding with the radiopharmaceutical preparation using the [18F]AlF-bifunctional chelator technique. As well as being simple, the suggested method is practical and relevant for beginners interested in 18F-fluorination with [18F]AlF-chelate complex technique or also for a researcher who aims to proceed on an extensive scale.

Principles of fluoride toxicity and the cellular response: a review

Fluoride is ubiquitously present throughout the world. It is released from minerals, magmatic gas, and industrial processing, and travels in the atmosphere and water. Exposure to low concentrations of fluoride increases overall oral health. Consequently, many countries add fluoride to their public water supply at 0.7-1.5 ppm. Exposure to high concentrations of fluoride, such as in a laboratory setting often exceeding 100 ppm, results in a wide array of toxicity phenotypes. This includes oxidative stress, organelle damage, and apoptosis in single cells, and skeletal and soft tissue damage in multicellular organisms. The mechanism of fluoride toxicity can be broadly attributed to four mechanisms: inhibition of proteins, organelle disruption, altered pH, and electrolyte imbalance. Recently, there has been renewed concern in the public sector as to whether fluoride is safe at the current exposure levels. In this review, we will focus on the impact of fluoride at the chemical, cellular, and multisystem level, as well as how organisms defend against fluoride. We also address public concerns about fluoride toxicity, including whether fluoride has a significant effect on neurodegeneration, diabetes, and the endocrine system.

Structural state recognition facilitates tip tracking of EB1 at growing microtubule ends

The microtubule binding protein EB1 specifically targets the growing ends of microtubules in cells, where EB1 facilitates the interactions of cellular proteins with microtubule plus-ends. Microtubule end targeting of EB1 has been attributed to high-affinity binding of EB1 to GTP-tubulin that is present at growing microtubule ends. However, our 3D single-molecule diffusion simulations predicted a ~ 6000% increase in EB1 arrivals to open, tapered microtubule tip structures relative to closed lattice conformations. Using quantitative fluorescence, single-molecule, and electron microscopy experiments, we found that the binding of EB1 onto opened, structurally disrupted microtubules was dramatically increased relative to closed, intact microtubules, regardless of hydrolysis state. Correspondingly, in cells, the blunting of growing microtubule plus-ends by Vinblastine was correlated with reduced EB1 targeting. Together, our results suggest that microtubule structural recognition, based on a fundamental diffusion-limited binding model, facilitates the tip tracking of EB1 at growing microtubule ends.

A Comprehensive Review of Non-Covalent Radiofluorination Approaches Using Aluminum [18F]fluoride: Will [18F]AlF Replace 68Ga for Metal Chelate Labeling?

Due to its ideal physical properties, fluorine-18 turns out to be a key radionuclide for positron emission tomography (PET) imaging, for both preclinical and clinical applications. However, usual biomolecules radiofluorination procedures require the formation of covalent bonds with fluorinated prosthetic groups. This drawback makes radiofluorination impractical for routine radiolabeling, gallium-68 appearing to be much more convenient for the labeling of chelator-bearing PET probes. In response to this limitation, a recent expansion of the 18F chemical toolbox gave aluminum [18F]fluoride chemistry a real prominence since the late 2000s. This approach is based on the formation of an [18F][AlF]2+ cation, complexed with a 9-membered cyclic chelator such as NOTA, NODA or their analogs. Allowing a one-step radiofluorination in an aqueous medium, this technique combines fluorine-18 and non-covalent radiolabeling with the advantage of being very easy to implement. Since its first reports, [18F]AlF radiolabeling approach has been applied to a wide variety of potential PET imaging vectors, whether of peptidic, proteic, or small molecule structure. Most of these [18F]AlF-labeled tracers showed promising preclinical results and have reached the clinical evaluation stage for some of them. The aim of this report is to provide a comprehensive overview of [18F]AlF labeling applications through a description of the various [18F]AlF-labeled conjugates, from their radiosynthesis to their evaluation as PET imaging agents.

Fluoride Exposure Induces Inhibition of Sodium-and Potassium-Activated Adenosine Triphosphatase (Na+, K+-ATPase) Enzyme Activity: Molecular Mechanisms and Implications for Public Health

In this study, several lines of evidence are provided to show that Na + , K + -ATPase activity exerts vital roles in normal brain development and function and that loss of enzyme activity is implicated in neurodevelopmental, neuropsychiatric and neurodegenerative disorders, as well as increased risk of cancer, metabolic, pulmonary and cardiovascular disease. Evidence is presented to show that fluoride (F) inhibits Na + , K + -ATPase activity by altering biological pathways through modifying the expression of genes and the activity of glycolytic enzymes, metalloenzymes, hormones, proteins, neuropeptides and cytokines, as well as biological interface interactions that rely on the bioavailability of chemical elements magnesium and manganese to modulate ATP and Na + , K + -ATPase enzyme activity. Taken together, the findings of this study provide unprecedented insights into the molecular mechanisms and biological pathways by which F inhibits Na + , K + -ATPase activity and contributes to the etiology and pathophysiology of diseases associated with impairment of this essential enzyme. Moreover, the findings of this study further suggest that there are windows of susceptibility over the life course where chronic F exposure in pregnancy and early infancy may impair Na + , K + -ATPase activity with both short- and long-term implications for disease and inequalities in health. These findings would warrant considerable attention and potential intervention, not to mention additional research on the potential effects of F intake in contributing to chronic disease.

Al18F-NODA Benzothiazole Derivatives as Imaging Agents for Cerebrovascular Amyloid in Cerebral Amyloid Angiopathy

In this study, we synthesized four novel Al^18/19F-labeled 2-phenylbenzothiazole derivatives conjugated to 1,4,7-triazacyclononane-1,4-diacetic acid via alkyl linkers and evaluated them as imaging agent targets to amyloid-β (Aβ) plaques deposited in the blood vessels of cerebral amyloid angiopathy (CAA) brain. The four ligands exhibited moderate-to-high binding ability to Aβ_1-42 aggregates, of which complex 17 possessing the most potent affinity (K _i = 11.3 nM) was selected for further biological evaluations. In vitro fluorescent staining and in vitro autoradiography studies on brain sections from CAA patients proved that this ligand could label Aβ deposits in blood vessels selectively. In biodistribution study, [¹⁸F]17 can hardly penetrate the blood-brain barrier (brain_2 min = 0.3% ID/g) and displayed a rapid blood washout rate (blood_2 min/blood_60 min = 25.2), which is favorable as CAA imaging agents. In conclusion, this Al¹⁸F-labeled 2-phenylbenzothiazole complex was developed and proved to be a promising CAA positron emission tomography agent.

18F-AlF Labeled Peptide and Protein Conjugates as Positron Emission Tomography Imaging Pharmaceuticals

The clinical applications of positron emission tomography (PET) imaging pharmaceuticals have increased tremendously over the past several years since the approval of ¹⁸fluorine-fluorodeoxyglucose (¹⁸F-FDG) by the Food and Drug Administration (FDA). Numerous ¹⁸F-labeled target-specific potential imaging pharmaceuticals, based on small and large molecules, have been evaluated in preclinical and clinical settings. ¹⁸F-labeling of organic moieties involves the introduction of the radioisotope by C-¹⁸F bond formation via a nucleophilic or an electrophilic substitution reaction. However, biomolecules, such as peptides, proteins, and oligonucleotides, cannot be radiolabeled via a C-¹⁸F bond formation as these reactions involve harsh conditions, including organic solvents, high temperature, and nonphysiological conditions. Several approaches, including ¹⁸F-labeled prosthetic groups, silicon, boron, and aluminum fluoride acceptor chemistry, and click chemistry have been developed, in the past, for ¹⁸F labeling of biomolecules. Linear and macrocyclic polyaminocarboxylates and their analogs and derivatives form thermodynamically stable and kinetically inert aluminum chelates. Hence, macrocyclic polyaminocarboxylates have been used for conjugation with biomolecules, such as folate, peptides, affibodies, and protein fragments, followed by ¹⁸F-AlF chelation, and evaluation of their targeting abilities in preclinical and clinical environments. The goal of this report is to provide an overview of the ¹⁸F radiochemistry and ¹⁸F-labeling methodologies for small molecules and target-specific biomolecules, a comprehensive review of coordination chemistry of Al³⁺, ¹⁸F-AlF labeling of peptide and protein conjugates, and evaluation of ¹⁸F-labeled biomolecule conjugates as potential imaging pharmaceuticals.

Validation of a [Al18F]PSMA-11 preparation for clinical applications

Imaging prostate-specific membrane antigen (PSMA) using positron emission tomography (PET) has been presented so far as the most sensitive and specific with regard to prostate cancer detection, in particular in high-risk prostate cancer patients. Currently, it mainly features Gallium-68 (⁶⁸Ga) labeled PSMA ligands, notably [⁶⁸Ga]Glu-urea-Lys(Ahx)-HBED-CC ([⁶⁸Ga]-PSMA-11) and [⁶⁸Ga]DOTAGA-FFK (Sub-KuE termed ([⁶⁸Ga]PSMA-I&T). However, ⁶⁸Ga has several shortcomings as radionuclide including a short half-life and non-ideal energies. This has motivated consideration of ¹⁸F-labeled analogues for PET imaging of prostate cancer. Here, we describe a simple synthesis and validation of a fluorine-18 labeled Glu-urea-Lys(Ahx)-HBED-CC ([Al¹⁸F]PSMA-11) for nuclear medicine applications. An efficient method for preparation of [Al¹⁸F]PSMA-11 was developed and validated (according to Pharm Eur) for routinely clinical applications. [Al¹⁸F]PSMA-11 was reproducibly obtained in radiochemical yields of 84 ± 6% (n = 15) and > 98% radiochemical purity using an improved one-step radiofluorination in aqueous solution. The total (production/preparation) time, including purification, pharmacological formulation of the isolated product and the quality control of the injectable solution was less than 60min. The [Al¹⁸F]PSMA-11 was stable over 4h in 1% EtOH/saline selected as injection solution. The solution was sterile, non-pyrogenic and ready for clinical applications after sterile filtration through a 0.22µm membrane filter under sterile conditions. In addition, [Al¹⁸F]PSMA-11 exhibited higher uptake and retention in PMSA-expressing LNCap prostate cells as compared to its clinically established ⁶⁸Ga-labeled analogues [⁶⁸Ga]PSMA-11 and [⁶⁸Ga]PSMA-I&T as well as to [⁶⁸Ga]NOTA-Bn-PSMA. The simple and fast preparation of [Al¹⁸F]PSMA-11 combined with its favorable pharmacological properties warrant its translation to a clinical setting.

CONCLUSION

The facile and high-yielding radiosynthesis of [Al¹⁸F]PSMA-11as well as its promising in vitro and in-vivo characteristics makes it worthy of clinical development for PET imaging of prostate cancer.

Invited review: Activation of G proteins by GTP and the mechanism of Gα-catalyzed GTP hydrolysis

This review addresses the regulatory consequences of the binding of GTP to the alpha subunits (Gα) of heterotrimeric G proteins, the reaction mechanism of GTP hydrolysis catalyzed by Gα and the means by which GTPase activating proteins (GAPs) stimulate the GTPase activity of Gα. The high energy of GTP binding is used to restrain and stabilize the conformation of the Gα switch segments, particularly switch II, to afford stable complementary to the surfaces of Gα effectors, while excluding interaction with Gβγ, the regulatory binding partner of GDP-bound Gα. Upon GTP hydrolysis, the energy of these conformational restraints is dissipated and the two switch segments, particularly switch II, become flexible and are able to adopt a conformation suitable for tight binding to Gβγ. Catalytic site pre-organization presents a significant activation energy barrier to Gα GTPase activity. The glutamine residue near the N-terminus of switch II (Glncat ) must adopt a conformation in which it orients and stabilizes the γ phosphate and the water nucleophile for an in-line attack. The transition state is probably loose with dissociative character; phosphoryl transfer may be concerted. The catalytic arginine in switch I (Argcat ), together with amide hydrogen bonds from the phosphate binding loop, stabilize charge at the β-γ bridge oxygen of the leaving group. GAPs that harbor "regulator of protein signaling" (RGS) domains, or structurally unrelated domains within G protein effectors that function as GAPs, accelerate catalysis by stabilizing the pre-transition state for Gα-catalyzed GTP hydrolysis, primarily by restraining Argcat and Glncat to their catalytic conformations. © 2016 Wiley Periodicals, Inc. Biopolymers 105: 449-462, 2016.

EF-G Activation by Phosphate Analogs

Elongation factor G (EF-G) is a universally conserved translational GTPase that promotes the translocation of tRNA and mRNA through the ribosome. EF-G binds to the ribosome in a GTP-bound form and subsequently catalyzes GTP hydrolysis. The contribution of the ribosome-stimulated GTP hydrolysis by EF-G to tRNA/mRNA translocation remains debated. Here, we show that while EF-G•GDP does not stably bind to the ribosome and induce translocation, EF-G•GDP in complex with phosphate group analogs BeF3(-) and AlF4(-) promotes the translocation of tRNA and mRNA. Furthermore, the rates of mRNA translocation induced by EF-G in the presence of GTP and a non-hydrolyzable analog of GTP, GDP•BeF3(-) are similar. Our results are consistent with the model suggesting that GTP hydrolysis is not directly coupled to mRNA/tRNA translocation. Hence, GTP binding is required to induce the activated, translocation-competent conformation of EF-G while GTP hydrolysis triggers EF-G release from the ribosome.

Contribution of each Trp residue toward the intrinsic fluorescence of the Giα1 protein

Giα1 is the inhibitory G-protein that, upon activation, reduces the activity of adenylyl cyclase. Comparison of the crystal structures of Giα1 bound to GDP•AMF or GTPγS with that of the inactive, GPD-bound protein indicates that a conformational change occurs in the activation step centered on three switch regions. The contribution of each tryptophan residue (W211 in the switch II region, W131 in the α-helical domain, and W258 in the GTPase domain) toward the intrinsic protein fluorescence was evaluated by using W211F, W131F, and W258F mutants. All three tryptophan residues contributed significantly toward the emission spectra regardless of the conformation. When activated by either GDP•AMF or GTPγS, the observed maximal-fluorescence scaled according to the solvent accessibilities of the tryptophan residues, calculated from molecular dynamics simulations. In the GDP•AMF and GTPγS, but not in the GDP, conformations, the residues W211 and R208 are in close proximity and form a π-cation interaction that results in a red shift in the emission spectra of WT, and W131F and W258F mutants, but a blue shift for the W211F mutant. The observed shifts did not show a relationship with the span of the W211-R208 bridge, but rather with changes in the total interaction energies. Trypsin digestion of the active conformations only occurred for the W211F mutant indicating that the electrostatic π-cation interaction blocks access to R208, which was consistent with the molecular dynamics simulations. We conclude that solvent accessibility and interaction energies account for the fluorescence features of Giα1 .

The sarcoplasmic Ca2+-ATPase: design of a perfect chemi-osmotic pump

The sarcoplasmic (SERCA 1a) Ca2+-ATPase is a membrane protein abundantly present in skeletal muscles where it functions as an indispensable component of the excitation-contraction coupling, being at the expense of ATP hydrolysis involved in Ca2+/H+ exchange with a high thermodynamic efficiency across the sarcoplasmic reticulum membrane. The transporter serves as a prototype of a whole family of cation transporters, the P-type ATPases, which in addition to Ca2+ transporting proteins count Na+, K+-ATPase and H+, K+-, proton- and heavy metal transporting ATPases as prominent members. The ability in recent years to produce and analyze at atomic (2·3-3 Å) resolution 3D-crystals of Ca2+-transport intermediates of SERCA 1a has meant a breakthrough in our understanding of the structural aspects of the transport mechanism. We describe here the detailed construction of the ATPase in terms of one membraneous and three cytosolic domains held together by a central core that mediates coupling between Ca2+-transport and ATP hydrolysis. During turnover, the pump is present in two different conformational states, E1 and E2, with a preference for the binding of Ca2+ and H+, respectively. We discuss how phosphorylated and non-phosphorylated forms of these conformational states with cytosolic, occluded or luminally exposed cation-binding sites are able to convert the chemical energy derived from ATP hydrolysis into an electrochemical gradient of Ca2+ across the sarcoplasmic reticulum membrane. In conjunction with these basic reactions which serve as a structural framework for the transport function of other P-type ATPases as well, we also review the role of the lipid phase and the regulatory and thermodynamic aspects of the transport mechanism.

What can be learned about the function of a single protein from its various X-ray structures: the example of the sarcoplasmic calcium pump

Improvements in the handling of membrane proteins for crystallization, combined with better synchrotron sources for X-ray diffraction analysis, are leading to clarification of the structural details of an ever increasing number of membrane transporters and receptors. Here we describe how this development has resulted in the elucidation at atomic resolution of a large number of structures of the sarcoplasmic Ca(2+)-ATPase (SERCA1a) present in skeletal muscle. The structures corresponding to the various intermediary states have been obtained after stabilization with structural analogues of ATP and of metal fluorides as mimicks of inorganic phosphate. From these results it is possible, in accordance with previous biochemical and molecular biology data, to give a detailed structural description of both ATP hydrolysis and Ca(2+) transport through the membrane, to serve as the starting point for a fuller understanding of the pump mechanism and, in future studies, on the regulatory role of this ubiquitous intracellular Ca(2+)-ATPase in cellular Ca(2+) metabolism in normal and pathological conditions.

Effect of concurrent chronic exposure of fluoride and aluminum on rat brain

The present in vivo study was designed to investigate the toxic potential of fluoride alone and in conjugation with aluminum on the rat brain. The region-specific response of both elements was studied in different regions of brain, namely the cerebrum, cerebellum, and medulla oblongata. Following fluoride exposure, oxidative stress increased significantly, estimated by increased lipid peroxidation and a decrease in the activity of the antioxidant enzyme, superoxide dismutase. The neurotransmitter (e.g., dopamine, norepinephrine, and serotonin) content was also altered. However, these aspects were more pronounced in animals given fluoride and aluminum together. Histological evidence showed deprivation of neuronal integrity with higher magnitude in concurrent fluoride and aluminum exposure, as compared to fluoride alone. Thus, it can be concluded that aluminum appears to enhance the neurotoxic hazards caused by fluoride.

Kinetic analysis of beta-phosphoglucomutase and its inhibition by magnesium fluoride

The isomerization of beta-glucose-1-phosphate (betaG1P) to beta-glucose-6-phosphate (G6P) catalyzed by beta-phosphoglucomutase (betaPGM) has been examined using steady- and presteady-state kinetic analysis. In the presence of low concentrations of beta-glucose-1,6-bisphosphate (betaG16BP), the reaction proceeds through a Ping Pong Bi Bi mechanism with substrate inhibition (kcat = 65 s(-1), K(betaG1P) = 15 microM, K(betaG16BP) = 0.7 microM, Ki = 122 microM). If alphaG16BP is used as a cofactor, more complex kinetic behavior is observed, but the nonlinear progress curves can be fit to reveal further catalytic parameters (kcat = 74 s(-1), K(betaG1P) = 15 microM, K(betaG16BP) = 0.8 microM, Ki = 122 microM, K(alphaG16BP) = 91 microM for productive binding, K(alphaG16BP) = 21 microM for unproductive binding). These data reveal that variations in the substrate structure affect transition-state affinity (approximately 140,000-fold in terms of rate acceleration) substantially more than ground-state binding (110-fold in terms of binding affinity). When fluoride and magnesium ions are present, time-dependent inhibition of the betaPGM is observed. The concentration dependence of the parameters obtained from fitting these progress curves shows that a betaG1P x MgF3(-) x betaPGM inhibitory complex is formed under the reaction conditions. The overall stability constant for this complex is approximately 2 x 10(-16) M(5) and suggests an affinity of the MgF3(-) moiety to this transition-state analogue (TSA) of < or = 70 nM. The detailed kinetic analysis shows how a special type of TSA that does not exist in solution is assembled in the active site of an enzyme. Further experiments show that under the conditions of previous structural studies, phosphorylated glucose only persists when bound to the enzyme as the TSA. The preference for TSA formation when fluoride is present, and the hydrolysis of substrates when it is not, rules out the formation of a stable pentavalent phosphorane intermediate in the active site of betaPGM.

Surface change of Ras enabling effector binding monitored in real time at atomic resolution

Ras, the prototype of the Ras superfamily, acts as a molecular switch for cell growth. External growth signals induce a GDP-to-GTP exchange. This modifies the Ras surface (Ras(on)GTP) and enables effector binding, which then activates signal-transduction pathways. GTP hydrolysis, catalysed by Ras and GAP, returns the signal to "off" (Ras(off)GDP). Oncogenic mutations in Ras prevent this hydrolysis, and thereby cause uncontrolled cell growth. In the Ras(off)-to-Ras(on) transition, the Ras surface is changed by a movement of the switch I loop that controls effector binding. We monitored this surface change at atomic resolution in real time by time-resolved FTIR (trFTIR) spectroscopy. In the transition from Ras(off) to Ras(on) a GTP-bound intermediate is now identified, in which effector binding is still prevented (Ras(off)GTP). The loop movement from Ras(off)GTP to Ras(on)GTP was directly monitored by the C=O vibration of Thr35. The structural change creates a binding site with a rate constant of 5 s(-1) at 260 K. A small molecule that shifted the equilibrium from the Ras(on)GTP state towards the Ras(off)GTP state would prevent effector binding, even if hydrolysis were blocked by oncogenic mutations. We present a spectroscopic fingerprint of both states that can be used as an assay in drug screening for such small molecules.

Insight into catalysis of a unique GTPase reaction by a combined biochemical and FTIR approach

Rap1 and Rap2 are the only small guanine nucleotide-binding proteins of the Ras superfamily that do not use glutamine for GTP hydrolysis. Moreover, Rap1GAP, which stimulates the GTPase reaction of Rap1 10(5)-fold, does not have the classical "arginine finger" like RasGAP but presumably, introduces an asparagine residue into the active site. Here, we address the requirements of this unique reaction in detail by combining various biochemical methods, such as fluorescence spectroscopy, stopped-flow and time-resolved Fourier transform infrared spectroscopy (FTIR). The fluorescence spectroscopic assay monitors primarily protein-protein interaction steps, while FTIR resolves simultaneously the elementary steps of functional groups labor-free, but it is less sensitive and needs higher concentrations. Combining both methods allows us to distinguish weather mechanistic defects caused by mutation are due to affinity or due to functionality. We show that several mutations of Asn290 block catalysis. Some of the mutants, however, still form a complex with Rap1*GDP in the presence of BeF(x) but not AlF(x), supporting the notion that fluoride complexes are indicators of the ground versus transition state. Mutational analysis also shows that Thr61 is not required for catalysis. While replacement of Thr61 of Rap1 by Leu eliminates GTPase activation by Rap1GAP, the T61A and T61Q mutants have only a minor effect on catalysis, but change the relative rates of cleavage and (P(i)(-)) release. While Rap1GAP(N290A) is completely inactive on wild-type Rap1, it can act on Rap1(T61Q), arguing that Asn290 in trans has a role in catalysis similar to that of the intrinsic Gln in Ras and Rho. Finally, since FTIR works at high, and thus mostly saturating, concentrations, it can clearly separate effects on affinity from purely catalytic modifications, showing that Arg388, conserved between RapGAPs and mutated in the homologous RheBGAP Tuberin, affects binding affinity severely but has no effect on the cleavage reaction itself.

Antagonistic effects of cofilin, beryllium fluoride complex, and phalloidin on subdomain 2 and nucleotide-binding cleft in F-actin

Cofilin/ADF, beryllium fluoride complex (BeFx), and phalloidin have opposing effects on actin filament structure and dynamics. Cofilin/ADF decreases the stability of F-actin by enhancing disorder in subdomain 2, and by severing and accelerating the depolymerization of the filament. BeFx and phalloidin stabilize the subdomain 2 structure and decrease the critical concentration of actin, slowing the dissociation of monomers. Yeast cofilin, unlike some other members of the cofilin/ADF family, binds to F-actin in the presence of BeFx; however, the rate of its binding is strongly inhibited by BeFx and decreases with increasing pH. The inhibition of the cofilin binding rate increases with the time of BeFx incubation with F-actin, indicating the existence of two BeFx-F-actin complexes. Cofilin dissociates BeFx from the filament, while BeFx does not bind to F-actin saturated with cofilin, presumably because of the cofilin-induced changes in the nucleotide-binding cleft of F-actin. These changes are apparent from the increase in the fluorescence intensity of F-actin bound epsilon-ADP upon cofilin binding and a decrease in its accessibility to collisional quenchers. BeFx also affects the nucleotide-binding cleft of F-actin, as indicated by an increase in the fluorescence intensity of epsilon-ADP-F-actin. Phalloidin and cofilin inhibit, but do not exclude each other binding to their complexes with F-actin. Phalloidin promotes the dissociation of cofilin from F-actin and slowly reverses the cofilin-induced disorder in the DNase I binding loop of subdomain 2.

Heterotrimeric G-protein α-Subunit Adopts a “Preactivated” Conformation When Associated with βγ-Subunits

Activation of a heterotrimeric G-protein by an agonist-stimulated G-protein-coupled receptor requires the propagation of structural signals from the receptor binding interface to the guanine nucleotide binding pocket of the G-protein. To probe the molecular basis of this signaling process, we are applying high resolution NMR to track structural changes in an isotope-labeled, full-length G-protein alpha-subunit (G(alpha)) chimera (ChiT) associated with G-protein betagamma-subunit (G(betagamma)) and activated receptor (R(*)) interactions. Here, we show that ChiT can be functionally reconstituted with G(betagamma) as assessed by aluminum fluoride-dependent changes in intrinsic tryptophan fluorescence and light-activated rhodopsin-catalyzed guanine nucleotide exchange. We further show that (15)N-ChiT can be titrated with G(betagamma) to form stable heterotrimers at NMR concentrations. To assess structural changes in ChiT upon heterotrimer formation, HSQC spectra of the (15)N-ChiT-reconstituted heterotrimer have been acquired and compared with spectra obtained for GDP/Mg(2+)-bound (15)N-ChiT in the presence and absence of aluminum fluoride and guanosine 5'-3-O-(thio)triphosphate (GTPgammaS)/Mg(2+)-bound (15)N-ChiT. As anticipated, G(betagamma) association with (15)N-ChiT results in (1)HN, (15)N chemical shift changes relative to the GDP/Mg(2+)-bound state. Strikingly, however, most (1)HN, (15)N chemical shift changes associated with heterotrimer formation are the same as those observed upon formation of the GDP.AlF(4)(-)/Mg(2+)- and GTPgammaS/Mg(2+)-bound states. Based on these comparative analyses, assembly of the heterotrimer appears to induce structural changes in the switch II and carboxyl-terminal regions of G(alpha) ("preactivation") that may facilitate the interaction with R(*) and subsequent GDP/GTP exchange.

Bacterial expression and one-step purification of an isotope-labeled heterotrimeric G-protein alpha-subunit

Heterologous expression systems are often employed to generate sufficient quantities of isotope-labeled proteins for high-resolution NMR studies. Recently, the interaction between the prodomain region of subtilisin and an active, mutant form of the mature enzyme has been exploited to develop a cleavable affinity tag fusion system for one-step generation and purification of full-length soluble proteins obtained by inducible prokaryotic expression. As a first step towards applying high-resolution NMR methods to study heterotrimeric G-protein alpha-subunit (G(alpha)) conformation and dynamics, the utility of this subtilisin prodomain fusion system for expressing and purifying an isotope-labeled G(alpha) chimera (approximately 40 kDa polypeptide) has been tested. The results show that a prodomain fused G(alpha) chimera can be expressed to levels approaching 6-8 mg/l in minimal media and that the processed, mature protein exhibits properties similar to those of G(alpha) isolated from natural sources. To assay for the functional integrity of the purified G(alpha) chimera at NMR concentrations and probe for changes in the structure and dynamics of G(alpha) that result from activation, 15N-HSQC spectra of the GDP/Mg2+ bound form of G(alpha) obtained in the absence and presence of aluminum fluoride, a well known activator of the GDP bound state, have been acquired. Comparisons of the 15N-HSQC spectra reveals a number of changes in chemical shifts of the 1HN, 15N crosspeaks that are discussed with respect to expected changes in the protein conformation associated with G(alpha) activation.

Structure/function studies of phosphoryl transfer by phosphoenolpyruvate carboxykinase

Phosphoenolpyruvate carboxykinase (PCK) catalyzes the conversion of oxaloacetate (OAA) to PEP and carbon dioxide with the subsequent conversion of nucleoside triphosphate to nucleoside diphosphate (NDP). The 1.9 A resolution structure of Escherichia coli PCK consisted of a 275-residue N-terminal domain and a 265-residue C-terminal domain with the active site located in a cleft between these domains. Each domain has an alpha/beta topology and the overall structure represents a new protein fold. Furthermore, PCK has a unique mononucleotide-binding fold. The 1.8 A resolution structure of the complex of ATP/Mg(2+)/oxalate with PCK revealed a 20 degrees hinge-like rotation of the N- and C-terminal domains, which closed the active site cleft. The ATP was found in the unusual syn conformation as a result of binding to the enzyme. Along with the side chain of Lys254, Mg(2+) neutralizes charges on the P beta and P gamma oxygen atoms of ATP and stabilizes an extended, eclipsed conformation of the P beta and P gamma phosphoryl groups. The sterically strained high-energy conformation likely lowers the free energy of activation for phosphoryl transfer. Additionally, the gamma-phosphoryl group becomes oriented in-line with the appropriate enolate oxygen atom, which strongly supports a direct S(N)2-type displacement of this gamma-phosphoryl group by the enolate anion. In the 2.0 A resolution structure of the complex of PCK/ADP/Mg(2+)/AlF(3), the AlF(3) moiety represents the phosphoryl group being transferred during catalysis. There are three positively charged groups that interact with the fluorine atoms, which are complementary to the three negative charges that would occur for an associative transition state.

Novel inhibition mechanism of Bacillus cereus sphingomyelinase by beryllium fluoride

Phosphate analogs have been known to inhibit competitively various phosphatases and phospholipase C and D. We found for the first time that only beryllium fluoride (BeF(x)) among the phosphate analogs studied inhibits Bacillus cereus sphingomyelinase (SMase) activity. The active inhibitory species proved to be not BeF(3)(-) but BeF(2) by the measurement of SMase activity and of (19)F NMR spectroscopy in the presence of a fixed concentration of BeCl(2) and different concentrations of NaF, although both the species have been reported for other kinds of enzymes. The result of kinetic experiment also indicated that the BeF(x) binds in the vicinity of the essential binding site for the substrate and that the Mg(2+) binding to SMase is essential for the binding of BeF(x) to the enzyme.

Concerning the chemical nature of tubulin subunits that cap and stabilize microtubules

There is no definitive evidence on the nature of the cap at microtubule ends that is responsible for dynamic instability behavior. It was, therefore, of interest that steady-state microtubules assembled in 20 mM P(i) buffer and pulsed for 15-60 min with [gamma-(32)P]GTP contained approximately 26 [(32)P]P(i)/microtubule [Panda et al. (2002) Biochemistry 41, 1609-1617]. It was concluded that microtubules are capped with a tubulin-GDP-P(i) subunit at the end of each its 13 protofilaments and that this is responsible for stabilizing microtubules in the growth phase. Also, because microtubules with [(32)P]P(i) were isolated despite the presence of 20 mM P(i), it was concluded that P(i) in terminal tubulin-GDP-P(i) subunits does not exchange with solvent. These observations are inconsistent with our finding that tubulin-GDP-P(i) subunits do not stabilize microtubules and with evidence that the nucleotide, and presumably also P(i), in subunits at microtubule ends exchanges with solvent. We have resolved this discrepancy by finding that during the pulse period the added [(32)P]GTP was almost quantitatively hydrolyzed. The so-formed [(32)P]P(i) labeled the 20 mM P(i) buffer, and this exchanged into tubulin-GDP subunits in the core of the microtubule. Evidence for this was our finding of virtually identical [(32)P]P(i) in microtubules pulsed with [(32)P]GTP with a specific activity that varied 11-fold by using either 100 or 1,100 microM GTP in the reaction. Label uptake was insensitive to the [(32)P]GTP specific activity because in both cases hydrolysis generated 20 mM [(32)P]P(i) with a virtually identical specific activity. Also, approximately 0.4 mol of [(32)P]P(i) /tubulin dimer was found in microtubules when steady-state microtubules in 20 mM P(i) were pulsed with a trace amount of [(32)P]P(i). This stoichiometry is consistent with a 25 mM K(d) previously reported for P(i) binding to tubulin-GDP subunits in microtubules. It is concluded that, under the conditions used for the [(32)P]GTP pulse labeling, (32)P was incorporated into the entire microtubule from [(32)P]P(i) released into the solution, rather than into a tubulin-GDP-P(i) cap, from [(32)P]GTP. Thus, there is no evidence that tubulin-GDP-P(i) subunits accumulate in and stabilize microtubule ends.

Evidence for a transition state analog, MgADP-aluminum fluoride-acetate, in acetate kinase from Methanosarcina thermophila

Aluminum fluoride has become an important tool for investigating the mechanism of phosphoryl transfer, an essential reaction that controls a host of vital cell functions. Planar AlF(3) or AlF(4)(-) molecules are proposed to mimic the phosphoryl group in the catalytic transition state. Acetate kinase catalyzes phosphoryl transfer of the ATP gamma-phosphate to acetate. Here we describe the inhibition of acetate kinase from Methanosarcina thermophila by preincubation with MgCl(2), ADP, AlCl(3), NaF, and acetate. Preincubation with butyrate in place of acetate did not significantly inhibit the enzyme. Several NTPs can substitute for ATP in the reaction, and the corresponding NDPs, in conjunction with MgCl(2), AlCl(3), NaF, and acetate, inhibit acetate kinase activity. Fluorescence quenching experiments indicated an increase in binding affinity of acetate kinase for MgADP in the presence of AlCl(3), NaF, and acetate. These and other characteristics of the inhibition indicate that the transition state analog, MgADP-aluminum fluoride-acetate, forms an abortive complex in the active site. The protection from inhibition by a non-hydrolyzable ATP analog or acetylphosphate, in conjunction with the strict dependence of inhibition on the presence of both ADP and acetate, supports a direct in-line mechanism for acetate kinase.

The phosphoryl-transfer mechanism of Escherichia coli phosphoenolpyruvate carboxykinase from the use of AlF(3)

The mechanism of reversible transfer of the gamma-phosphate group of ATP by Escherichia coli phosphoenolpyruvate carboxykinase (PCK) on to its substrate is of great interest. It is known that metallofluorides are accurate analogs of the transition state in the context of kinase mechanisms. Therefore, two complexes of PCK, one with AlF(3), Mg(2+) and ADP (complex I), the other with AlF(3), Mg(2+), ADP and pyruvate (complex II) were crystallized. The X-ray crystal structures of these two complexes were determined at 2.0 A resolution. The Al atom has trigonal bipyramidal geometry that mimics the transition state of phosphoryl transfer. The Al atom is at a distance of 2.8 A and 2.9 A from an oxygen atom of the beta-phosphoryl group of ADP in complex I and II, respectively. A water molecule in complex I and an oxygen atom of the pyruvate in complex II are located along the axis of the trigonal bipyramid on the side opposite to the beta-phosphoryl oxygen with respect to the equatorial plane, suggesting that the complexes are close mimics of the transition state. Along with the presence of positively charged species around the AlF(3) moiety, these results indicate that phosphoryl transfer occurs via a direct displacement mechanism with associative qualities.

Effect of Li+ upon the Mg2+-dependent activation of recombinant Gialpha1

Although lithium salts have been used in the treatment and prophylaxis of manic-depressive or bipolar patients for 50 years, the mechanism of the pharmacologic action of Li+ is unknown. Based on activity studies of inhibitory and stimulatory guanine-binding (G) proteins in rat cortical membranes, it was proposed that Li+ inhibition of G-proteins may account for its pharmacologic action. We used the purified alpha subunit of the recombinant inhibitory G-protein, rGialpha1, and found that Li+ at therapeutic levels significantly inhibited the formation of the GDP.AlF4-.rGialpha1 complex. Because our studies were conducted with a purified, metal-reconstituted G-protein rather than with cell membrane suspensions, our Li+ inhibition results lend additional support to the G-protein hypothesis for Li+ action.

Characterization of ectophosphatase activities in trypanosomatid parasites of plants

ABSTRACT In the present work ectophosphatase activities of three trypanosomatid parasites of plants were characterized using intact cells. Phytomonas françai, Phytomonas mcgheei, and Herpetomonas sp. hydrolyzed p-nitro-phenylphosphate at a rate of 5.40, 7.28, and 25.58 nmol Pi/mg of protein per min, respectively. Experiments using classical inhibitors of acid phosphatases such as sodium orthovanadate (NaVO(3)) and sodium fluoride (NaF) showed a decrease in phosphatase activities. Lithium fluoride (LiF) and aluminum chloride (AlCl(3)) were also used. Although AlCl3 had no effect, LiF was able to promote a decrease in the phosphatase activities. Interestingly, the inhibition caused by LiF was enhanced by the addition of AlCl3 during the reaction, probably due to the formation of fluoroaluminate complexes. This effect was confirmed by cytochemical analysis. In this assay, electron-dense cerium phosphate deposits were visualized on the external surface of the three parasites.

Beryllium, an adjuvant that promotes gamma interferon production

Beryllium is associated with a human pulmonary granulomatosis characterized by an accumulation of CD4(+) T cells in the lungs and a heightened specific lymphocyte proliferative response to beryllium (Be) with gamma interferon (IFN-gamma) release (i.e., a T helper 1 [Th1] response). While an animal model of Be sensitization is not currently available, Be has exhibited adjuvant effects in animals. The effects of Be on BALB/c mice immunized with soluble leishmanial antigens (SLA) were investigated to determine if Be had adjuvant activity for IFN-gamma production, an indicator of the Th1 response. In this strain of Leishmania-susceptible BALB/c mice, a Th2 response is normally observed after in vivo SLA sensitization and in vitro restimulation with SLA. If interleukin-12 (IL-12) is given during in vivo sensitization with SLA, markedly increased IFN-gamma production and decreased IL-4 production are detected. We show here that when beryllium sulfate (BeSO(4)) was added during in vivo sensitization of BALB/c mice with SLA and IL-12, significantly increased IFN-gamma production and decreased IL-4 production from lymph node and spleen cells were detected upon in vitro SLA restimulation. No specific responses were observed to Be alone. Lymph node and spleen cells from all mice proliferated strongly and comparably upon in vitro restimulation with SLA and with SLA plus Be; no differences were noted among groups of mice that received different immunization regimens. In vivo, when Be was added to SLA and IL-12 for sensitization of BALB/c mice, more effective control of Leishmania infection was achieved. This finding has implications for understanding not only the development of granulomatous reactions but also the potential for developing Be as a vaccine adjuvant.

Formation of helical protein assemblies of IgG and transducin on varied lipid tubules

Helical protein arrays on lipid tubules are valuable assemblies for studying protein structure and protein-lipid interactions through electron microscopy and crystallography. We describe conditions for forming such arrays from two proteins, IgG and transducin, the photoreceptor G protein, using a variety of lipid surfaces. Anti-dinitrophenyl (DNP) IgG arrays formed on DNP-phosphatidylethanolamine (DNP-PE) mixed with either galactosyl-ceramide lipids or phosphatidylcholine (PC) display different pH sensitivities and dimensions, yet have similar helical symmetries. DNP-PE/PC mixtures formed small crystals and large well-ordered flattened tubules. The peripheral membrane protein transducin (G(t)) formed helical arrays either on a mixture of cationic and neutral lipids or on residual photoreceptor lipids. Despite differences in lipid composition, the G(t) arrays have similar structures and show similar sensitivity to activation and variations in ionic environment. G(t) activation causes the helical assemblies to collapse to small vesicles, a process resembling the vesiculation of activated dynamin-lipid tubules. In a preliminary three-dimensional reconstruction, the hapten-bound IgG appears to make two contacts to the central lipid tubule, presumably via the F(ab) domains. The ability to generate a three-dimensional reconstruction without tilts illustrates one advantage of helical structures for two-dimensional crystallography, especially for visualizing protein-lipid interactions.

Microtubule dynamic instability does not result from stabilization of microtubules by tubulin-GDP-Pi subunits

The proposal that microtubule dynamic instability results from stabilization of microtubule ends by tubulin-GDP-Pi subunits (where Pi is inorganic phosphate) [Melki et al. (1996) Biochemistry 35, 12038] was based on studies of GTP hydrolysis and microtubule assembly that showed that tubulin-GDP-Pi subunits can transiently accumulate at microtubule ends. There is no direct evidence that GDP-Pi-subunits can stabilize microtubules under conditions where dynamic instability is observed and this has been inferred from the observation that tubulin-GDP-BeFn subunits stabilize microtubules. To test if tubulin-GDP-Pi stabilizes microtubules we sought evidence for a synergism between the effect of Pi and BeFn. We found, however, that Pi antagonizes the effect of BeFn by displacing it from tubulin subunits. The alternate mechanism in which Pi inhibits BeFn stabilization of microtubules by displacing fluoride from beryllium was ruled out from the 9Be and 19F NMR spectra in the presence and absence of Pi. Further evidence that tubulin-GDP-BeFn is not an analogue of tubulin-GDP-Pi and that tubulin-GDP-Pi is not responsible for maintaining the growth phase in microtubules manifesting dynamic instability was provided by our observation that Pi did not decrease the disassembly rate under conditions where tubulin-GDP-Pi subunits are expected to have formed. Results showing that BeFn binds randomly to subunits in microtubules provided evidence that Pi dissociation from the tubulin-GDP-Pi intermediate formed during GTP hydrolysis occurs randomly rather than processively starting at the growing microtubule tip.

How do kinases transfer phosphoryl groups?

Understanding how phosphoryl transfer is accomplished by kinases, a ubiquitous group of enzymes, is central to many biochemical processes. Qualitative analysis of the crystal structures of enzyme-substrate complexes of kinases reveals structural features of these enzymes important to phosphoryl transfer. Recently determined crystal structures which mimic the transition state complex have added new insight into the debate as to whether kinases use associative or dissociative mechanisms of catalysis.

Inhibition of the hepatitis C virus helicase-associated ATPase activity by the combination of ADP, NaF, MgCl2, and poly(rU). Two ADP binding sites on the enzyme-nucleic acid complex

Hepatitis C virus (HCV) helicase has an intrinsic ATPase activity and a nucleic acid (poly(rU))-stimulated ATPase activity. The poly(rU)-stimulated ATPase activity was inhibited by F- in a time-dependent manner during ATP hydrolysis. Inhibition was the result of trapping an enzyme-bound ADP-poly(rU) ternary complex generated during the catalytic cycle and was not the result of generating enzyme-free ADP that subsequently inhibited the enzyme. However, catalysis was not required for efficient inhibition by F-. The stimulated and the intrinsic ATPase activities were also inhibited by treatment of the enzyme with F-, ADP, and poly(rU). The inhibited enzyme slowly recovered (t1/2 = 23 min) ATPase activity after a 2000-fold dilution into assay buffer. The onset of inhibition by 500 microM ADP and 15 mM F- in the absence of nucleic acid was very slow (t1/2 > 40 min). However, the sequence of addition of poly(rU) to a diluted solution of ADP/NaF-treated enzyme had a profound effect on the extent of inhibition. If the ADP/NaF-treated enzyme was diluted into an assay that lacked poly(rU) and the assay was subsequently initiated with poly(rU), the treated enzyme was not inhibited. Alternatively, if the treated enzyme was diluted into an assay containing poly(rU), the enzyme was inhibited. ATP protected the enzyme from inhibition by ADP/NaF. The stoichiometry between ADP and enzyme monomer in the inhibited enzyme complex was 2, as determined from titration of the ATPase activity ([ADP]/[E] = 2.2) and from the number of radiolabeled ADP bound to the inhibited enzyme ([ADP]/[E] = 1.7) in the presence of excess NaF, MgCl2, and poly(rU). The Hill coefficient for titration of ATPase activity with F- (n = 2.8) or MgCl2 (n = 2.1) in the presence of excess ADP and poly(rU) suggested that multiple F- and Mg2+ were involved in forming the inhibited enzyme complex. The stoichiometry between (dU)18, a defined oligomeric nucleic acid substituting for poly(rU), and enzyme monomer in the inhibited enzyme complex was estimated to be 1 ([(dU)18/[E] = 1.2) from titration of the ATPase activity in the presence of excess ADP, MgCl2, and NaF.

Fluoride activation of the Rho family GTP-binding protein Cdc42Hs

Aluminum tetrafluoride (AlF4-) activation of heterotrimeric G-protein alpha-subunits is a well established aspect of the biochemistry of these proteins; however, until recently it has been thought that AlF4- does not mediate effects on the Ras superfamily of low molecular weight GTP-binding proteins. Recent work demonstrating aluminum fluoride-induced complex formation between Ras and its GTPase-activating proteins (RasGAP and NF1) has provided important insights into the mechanism of GAP-stimulated GTP hydrolysis. We have characterized the AlF4--induced complex formation between the GDP-bound form of the Rho subfamily G-protein Cdc42Hs and a limit functional domain of the Cdc42-GAP using a variety of biochemical techniques. Our results indicate that the apparent affinity of GAP for the AlF4--mediated complex is similar to the affinity observed for the activated (GTP-bound) form of Cdc42 and that beryllium (Be) can replace aluminum in mediating fluoride-induced complex formation. Additionally, the AlF4--induced interaction is weakened significantly by the catalytically compromised GAP(R305A) mutant, indicating that this arginine is critical in transition state stabilization. Unlike Ras, we find that AlF4- and BeF3- mediate complex formation between Cdc42Hs.GDP and downstream target/effector molecules, indicating that there are important differences in the mechanism of effector binding between the Ras and Rho subfamily G-proteins.

Involvement of calcium and G proteins in the acute release of tissue-type plasminogen activator and von Willebrand factor from cultured human endothelial cells

In this study, we investigated the role of Ca2+ and G proteins in thrombin-induced acute release (regulated secretion) of tissue-type plasminogen activator (TPA) and von Willebrand factor (vWF), using a previously described system of primary human umbilical vein endothelial cells (HUVECs). The acute release of TPA and vWF, as induced by alpha-thrombin, was almost zero after chelation of Ca2+i, showing that an increase in [Ca2+]i was required. It did not matter whether the increase in [Ca2+]i came from an intracellular or extracellular Ca2+ source. Thrombin-induced release of TPA and vWF already started at low [Ca2+]i, around 100 nmol/L. Half-maximal release was found at a [Ca2+]i, of 261 nmol/L for TPA and at 222 nmol/L for vWF. The Ca2+ signal was transduced to calmodulin, as calmodulin inhibitors inhibited TPA and vWF release. The Ca2+ ionophore ionomycin dose dependently released vWF; half-maximal vWF release occurred at a [Ca2+]i of 311 nmol/L. In contrast, no TPA release was found at all below a [Ca2+]i of 500 nmol/L. Thus, below 500 nmol/L [Ca2+]i, an increase in [Ca2+]i alone was sufficient to induce vWF release but not sufficient to induce TPA release. Protein kinase C did not appear to be involved in TPA or vWF release, as neither an activator nor an inhibitor of protein kinase C significantly influenced release. Inhibition of phospholipase A2 also did not reduce thrombin-induced TPA and vWF release. The involvement of G proteins was studied by using both saponin-permeabilized and intact cells. GDP-beta-S, which inhibits heterotrimeric and small G proteins, significantly inhibited thrombin-induced vWF and TPA release from permeabilized cells. AlF-4, which activates heterotrimeric G proteins, induced TPA and vWF release in both intact and permeabilized HUVECs. Preincubation of HUVECs with pertussis toxin significantly inhibited thrombin-induced vWF release, due to inhibition of thrombin-induced Ca2+ influx. Pertussis toxin did not affect ionomycin-induced release. The inhibitory effect of pertussis toxin was less obvious in thrombin-induced TPA release, because it was counterbalanced by a positive effect of the toxin on TPA release. Thus, both inhibitory and stimulatory (pertussis toxin-sensitive) G proteins were involved in TPA release. Therefore, thrombin-induced acute release of TPA and vWF differed in two respects. First, below a [Ca2+]i of 500 nmol/L, an increase in Ca2+ was sufficient for vWF release but not for TPA release. Second, pertussis toxin-sensitive G proteins were differentially involved in acute TPA and vWF release.

Structures of active conformations of UMP kinase from Dictyostelium discoideum suggest phosphoryl transfer is associative

UMP/CMP kinase from Dictyostelium discoideum (UmpKdicty) catalyzes the specific transfer of the terminal phosphate of ATP to UMP or CMP. Crystal structures of UmpKdicty with substrates and the transition state analogs AlF3 or BeF2 that lock UmpKdicty in active conformations were solved. The positions of the catalytic Mg2+ and the highly conserved lysine of the P loop are virtually invariant in the different structures. In contrast, catalytic arginines move to stabilize charges that develop during this reaction. The location of the arginines indicates formation of negative charges during the reaction at the transferred phosphoryl group, but not at the phosphate bridging oxygen atoms. This is consistent with an associative phosphoryl transfer mechanism but not with a dissociative one.

Does subunit dissociation necessarily accompany the activation of all heterotrimeric G proteins?

Heterotrimeric (alpha beta gamma) G proteins mediate a variety of signal transduction events in virtually every cell of every eukaryotic organism. The predominant hypothesis is that dissociation of the alpha-subunit from the G beta gamma-subunit complex necessarily accompanies the activation of these proteins, and that the alpha-subunit is primarily responsible for regulating the response of effector molecules. However, there is increasing evidence that both the alpha-subunit and the beta gamma-subunit complex function in regulating effector activity. Furthermore, data for some G proteins suggest that they function as activated heterotrimers rather than as dissociated subunits.

G protein mechanisms: insights from structural analysis

This review is concerned with the structures and mechanisms of a superfamily of regulatory GTP hydrolases (G proteins). G proteins include Ras and its close homologs, translation elongation factors, and heterotrimeric G proteins. These proteins share a common structural core, exemplified by that of p21ras (Ras), and significant sequence identity, suggesting a common evolutionary origin. Three-dimensional structures of members of the G protein superfamily are considered in light of other biochemical findings about the function of these proteins. Relationships among G protein structures are discussed, and factors contributing to their low intrinsic rate of GTP hydrolysis are considered. Comparison of GTP- and GDP-bound conformations of G proteins reveals how specific contacts between the gamma-phosphate of GTP and the switch II region stabilize potential effector-binding sites and how GTP hydrolysis results in collapse (or reordering) of these surfaces. A GTPase-activating protein probably binds to and stabilizes the conformation of its cognate G protein that recognizes the transition state for hydrolysis, and may insert a catalytic residue into the G protein active site. Inhibitors of nucleotide release, such as the beta gamma subunit of a heterotrimeric G protein, bind selectively to and stabilize the GDP-bound state. Release factors, such as the translation elongation factor, Ts, also recognize the switch regions and destabilize the Mg(2+)-binding site, thereby promoting GDP release. G protein-coupled receptors are expected to operate by a somewhat different mechanism, given that the GDP-bound form of many G protein alpha subunits does not contain bound Mg2+.

Role of protein kinase C alpha, Arf, and cytoplasmic calcium transients in phospholipase D activation by sodium fluoride in osteoblast-like cells

The effect of fluoride on phospholipase D (PLD) activation was studied using in vitro culture of Saos-2, MG-63 osteosarcoma cells, and normal osteoblast-like cells derived from human bone explants. Millimolar concentrations of NaF induced a significant accumulation of phosphatidylethanol (PEt) in Saos-2 cells but not in MG-63 and normal osteoblast-like cells. PLD activation was evident at 15 mM and concentration-dependent up to 50 mM. This stimulation was inhibited by deferoxamine, a chelator of Al3+, suggesting that PLD activation involves fluoride-sensitive G proteins. A good correlation was found between the levels of intracellular free Ca2+ and the activation of PLD. The time courses of the two responses were nearly identical. The ability of NaF to induce both responses was largely dependent on the presence of extracellular calcium. The calcium ionophore A23187 reproduced the effect of NaF, and this effect was antagonized by EGTA, suggesting that PLD activation was, at least in part, a calcium-regulated event. Phorbol 12-myristate 13-acetate (PMA) also stimulated PLD activity in human bone cells. Protein kinase C alpha (PKC alpha) and epsilon were expressed in Saos-2 cells. Acute pretreatment of cells with PMA reduced concomitantly the amounts of PKC alpha, but not of PKC epsilon, and the subsequent activation of PLD elicited by PKC activators. The PLD response to NaF was not attenuated but rather enhanced by down-regulation of PKC alpha. Therefore, PKC-alpha-induced PLD activation is unlikely to mediate the effect of NaF. Moreover, PMA and NaF showed a supraadditive effect on PLD activation in Saos-2 cells. This stimulation, in contrast to NaF alone, was not reduced by EGTA. Hence, mobilization of calcium by NaF cannot account for the enhanced PLD activation in response to PMA stimulation. Membrane Arf and RhoA contents were assessed by Western immunoblot analyses. Membranes derived from NaF-stimulated Saos-2 cells contained more Arf and RhoA when compared with membranes derived from control or PMA-stimulated cells. Translocation of the small GTPases was calcium-independent. We conclude that PLD activation by NaF in Saos-2 cells includes a fluoride-sensitive G protein, increases in the levels of intracellular calcium, and Arf/RhoA redistribution to membranes. The results also indicate that the NaF-induced Arf/RhoA translocation exerts in concert with PMA-activated PKC alpha a synergistic effect on the activation of PLD in Saos-2 cells.

In vitro activation of the NADPH oxidase by fluoride. Possible involvement of a factor activating GTP hydrolysis on Rac (Rac-GAP)

The possible mechanism of activation of the NADPH oxidase by fluoride was investigated in the cell-free system. It is shown that the stimulatory effect of fluoride is inhibited by guanosine 5'-O-(2-thiodiphosphate) (GDP[S]) and potentiated by GTP. The effect of fluoride is not additive with GTP[S]. Fluoride activation requires the presence of Mg2+ in millimolar concentration but is independent of Al3+. The activating effect of fluoride is preserved in solubilized membrane extract after removal of the majority of heterotrimeric GTP-binding proteins by immunoadsorption. Fluoride has no direct action either on the nucleotide exchange of GTP hydrolysis of the isolated Rac protein. In contrast, fluoride effectively inhibits Rac-GTPase activity enhanced by a membrane component. In this way, fluoride could prolong the prevalence of Rac in the GTP-bound state and, as a consequence, activate NADPH oxidase. The possibility of the involvement of a membrane-bound Rac GTPase-activating protein activity in the physiological regulation of the enzyme is raised.