Assembly of tubulin with nucleotide analogs

Spatial positioning of EB family proteins at microtubule tips involves distinct nucleotide-dependent binding properties

EB proteins track the ends of growing microtubules and regulate microtubule dynamics both directly and by acting as the hub of the tip-tracking network. Mammalian cells express cell type-specific combinations of three EB proteins with different cellular roles. Here, we reconstitute EB1, EB2 and EB3 tip tracking in vitro. We find that all three EBs show rapid exchange at the microtubule tip and that their signal correlates to the microtubule assembly rate. However, the three signals differ in their maxima and position from the microtubule tip. Using microtubules built with nucleotide analogues and site-directed mutagenesis, we show that EB2 prefers binding to microtubule lattices containing a 1:1 mixture of different nucleotides and its distinct binding specificity is conferred by amino acid substitutions at the right-hand-side interface of the EB microtubule-binding domain with tubulin. Our data are consistent with the model that all three EB paralogues sense the nucleotide state of both β-tubulins flanking their binding site. Their different profile of preferred binding sites contributes to occupying spatially distinct domains at the temporally evolving microtubule tip structure.

Summary:In vitro reconstitution of tip tracking with EB1, EB2 and EB3 shows that these three proteins sense the nucleotide state of both β-tubulins flanking their binding site.

Assays for Nucleotide Competitive Reversible and Irreversible Inhibitors of Ras GTPases

Although the Ras protein has been seen as a potential target for cancer therapy for the past 30 years, there was a tendency to consider it undruggable until recently. This has changed with the demonstration that small molecules with a specificity for (disease related mutants of) Ras can indeed be found, and some of these molecules form covalent adducts. A subgroup of these molecules can be characterized as competing with binding of the natural ligands GTP and GDP. Because of the distinct properties of Ras and related GTPases, in particular the very high nucleotide affinities and associated very low dissociation rates, assays for characterizing such molecules are not trivial. This is compounded by the fact that Ras family GTPases tend to be thermally unstable in the absence of a bound nucleotide. Here, we show that instead of using the unstable nucleotide-free Ras, the protein can be isolated as a 1:1 complex with a modified nucleotide (GDP-β-methyl ester) with low affinity to Ras. With this nucleotide analogue bound to the protein, testing of inhibitors is made experimentally more convenient and we present assays that allow the rapid assessment of the kinetic constants describing the inhibition process.

Nucleoside triphosphate specificity of tubulin

We have determined the binding affinity for binding of the four purine nucleoside triphosphates GTP, ITP, XTP, and ATP to E-site nucleotide- and nucleoside diphosphate kinase-depleted tubulin. The relative binding affinities are 3000 for GTP, 10 for ITP, 2 for XTP, and 1 for ATP. Thus, the 2-exocyclic amino group in GTP is important in determining the nucleotide specificity of tubulin and may interact with a hydrogen bond acceptor group in the protein. The 6-oxo group also makes a contribution to the high affinity for GTP. NMR ROESY experiments indicate that the four nucleotides have different average conformations in solution. ATP and XTP are characterized by a high anti conformation, ITP by a medium anti conformation, and GTP by a low anti conformation. Possibly, the preferred solution conformation contributes to the differences in affinities. When the tubulin E-site is saturated with nucleotide, there appears to be little difference in the ability of the four nucleotides to stimulate assembly. The critical protein concentration is essentially identical in reactions using the four nucleotides. All four of the nucleotides were hydrolyzed during the assembly reaction, and the NDPs were incorporated into the microtubule. We also examined the binding of two gamma-phosphoryl-modified GTP photoaffinity analogues, p(3)-1, 4-azidoanilido-GTP and p(3)-1,3-acetylanilido-GTP. These analogues are inhibitors of the assembly reaction and bind to tubulin with affinities that are 15- and 50-fold lower, respectively, than the affinty for GTP. The affinity of GTP is less sensitive to substitutions at the gamma-phosphoryl position that to changes in the purine ring.

Probing the ATP binding site of tubulin with thiotriphosphate analogues of ATP

Tubulin assembly studies with GTP alpha S diastereoisomers have shown that there is stereoselectivity at the alpha-phosphate binding region of tubulin. GTP alpha S(Sp) bound tighter than GTP alpha S(Rp) and promoted nucleation and assembly better than GTP and GTP alpha S(Rp). ATP and dATP have been reported to bind weakly to tubulin and to be less effective than GTP and dGTP in promoting tubulin assembly. This study was done to learn if ATP alpha S(Sp) and dATP alpha S(Sp) are good promoters of tubulin assembly and to compare these ATP thiotriphosphate analogues to the corresponding GTP analogues in tubulin assembly. Studies were also done with ATP alpha S(Rp), GTP, ATP beta S(Sp) and ATP gamma S. At least three cycles of tubulin (25 microM) assembly-disassembly were found with 1 mM ATP alpha S(Sp) and dATP alpha S(Sp) and both nucleotides were incorporated and hydrolyzed in the polymers. Less dATP alpha S(Sp) (25 microM) than ATP alpha S(Sp) (100 microM) promoted assembly to 50% of the maximum value. The critical concentrations (Cc) for assembly with 1 mM nucleotide were low for ATP alpha S(Sp) (3 microM) and dATP alpha S(Sp) (2 microM) and compared favorably with GTP (5 microM), GTP alpha S(Sp) (2 microM) and dGTP alpha S(Sp) (1 microM). Both 1 mM ATP and dATP were poor promoters of tubulin assembly and were not detected in the polymers. The predominant structures induced by 1 mM (ATP alpha S(Sp) and dATP alpha S(Sp) were bundles of sheets and microtubules, which were more stable to the cold and to Ca(II) than microtubules assembled with GTP, ATP or dATP. ATP alpha S(Rp) (1 mM) did not promote assembly suggesting that there is stereoselectivity at the ATP alpha S alpha-phosphate binding region of tubulin as there is with GTP alpha S diastereoisomers. ATP alpha S(Sp) and dATP alpha S(Sp) mimic GTP alpha S(Sp) and dGTP alpha S(Sp) in tubulin assembly since all four nucleotides promote bundles of tubulin in buffer with glycerol, and the deoxy nucleotides have lower Cc, shorter lags and faster rates for tubulin assembly.

Mutations in the Caenorhabditis elegans beta-tubulin gene mec-7: effects on microtubule assembly and stability and on tubulin autoregulation

We have sequenced 45 mutations in mec-7, a beta-tubulin gene required for the production of 15-protofilament microtubules in the nematode Caenorhabditis elegans, and have correlated sequence alterations with mutant phenotypes. The expression patterns of most alleles have also been determined by in situ hybridization and immunocytochemistry. Most (12/16) complete loss-of-function alleles, which are recessive, result from nonsense mutations, insertions, or deletions; three others disrupt a putative GTP-binding domain. Three of the four loss-of-function, missense mutations result in elevated mec-7 message levels, suggesting a defect in tubulin autoregulation that may be attributable to a loss in the ability to form heterodimers. Most (8/9) mild alleles are caused by missense mutations. Two mild alleles appear to increase microtubule stability and lead to the elaboration of ectopic neuronal processes in mec-7-expressing cells. Most (15/23) mutations that cause severe dominant or semidominant phenotypes are clustered into three discrete domains; four others occur in putative GTP-binding regions. Many of these dominant mutations appear to completely disrupt microtubule assembly.

Incorporation of tubulin subunits into dimers requires GTP hydrolysis

A toroid multisubunit complex of 800–900 kDa has been implicated in assisting protein folding of at least two cytoplasmic proteins, actin and tubulin. This process is dependent on the presence of magnesium ions and ATP hydrolysis. In vitro translation of cDNAs encoding different alpha- and beta-tubulin isotypes also gives rise to the formation of complexes of about 300 kDa. These complexes have been functionally implicated in the incorporation of tubulin monomers within the tubulin heterodimer. This work shows that, in addition to ATP hydrolysis, the incorporation of newly synthesized tubulin subunits into functional heterodimers requires GTP hydrolysis in the presence of magnesium ions. A two-step process is suggested, a first ATP-dependent step in which the 900 kDa complexes are implicated in a similar way to the step taking place in actin folding, and a second GTP-dependent step in which the 300 kDa complexes are involved in the assembly of the heterodimer.

Modulation of tubulin-nucleotide interactions by metal ions: comparison of beryllium with magnesium and initial studies with other cations

With microtubule-associated proteins (MAPs) BeSO4 and MgSO4 stimulated tubulin polymerization as compared to a reaction mixture without exogenously added metal ion, while beryllium fluoride had no effect (E. Hamel et al., 1991, Arch. Biochem. Biophys. 286, 57-69). Effects of both cations were most dramatic at GTP concentrations in the same molar range as the tubulin concentration. We have now compared effects of beryllium and magnesium on tubulin-nucleotide interactions in both unpolymerized tubulin and in polymer. Polymer formed with magnesium had properties similar to those of polymer formed without exogenous cation, except for a 20% lower stoichiometry of exogenous GTP incorporated into the latter. In both polymers the incorporated GTP was hydrolyzed to GDP. Stoichiometry of GTP incorporation into polymers formed with beryllium or magnesium was identical, but much of the GTP in the beryllium polymer was not hydrolyzed. The beryllium polymer was more stable than the magnesium polymer. Beryllium also differed from magnesium in only weakly enhancing the binding of GTP in the exchangeable site of unpolymerized tubulin, while neither cation affected GDP exchange at the site. If both cations were present in a reaction mixture, polymer stability was little changed from that of the beryllium polymer, but most of the GTP incorporated into polymer was hydrolyzed. Six additional metal salts (AlCl3, CdCl2, CoCl2, MnCl2, SnCl2, and ZnCl2) also stimulated MAP-dependent tubulin polymerization, but enhanced polymer stability did not correlate with polymer GTP content. We postulate that enhanced polymer stability is a consequence of cation binding directly to tubulin and/or polymer while deficient GTP hydrolysis in the presence of beryllium, as well as aluminum and tin, is a consequence of tight binding of cation to GTP in the exchangeable site.

Intercellular communication between cultured granulosa cells of the marmoset (Callithrix jacchus)

The influence of follicle-stimulating hormone, forskolin, insulin-like growth factor type I, epidermal growth factor, and 12-tetradecanoyl-phorbol-13-acetate on marmoset granulosa cell communication via gap junctions was investigated by morphological means and microinjection of carboxyfluorescein. Gap junctions between neighbouring granulosa cells were present in all groups. The number, but not length, of gap junctions between marmoset granulosa cells increased when the cells had been treated with follicle-stimulating hormone, insulin-like growth factor type I, and follicle-stimulating hormone plus insulin-like growth factor type I. No effect on gap junctions was seen, after exposure of the cells to the other three substances. Carboxyfluorescein and counting of the surrounding labelled cells showed that supplementation with follicle-stimulating hormone, forskolin, insulin-like growth factor type I and epidermal growth factor from the beginning of cultivation led to an increase in stained cells after 48 h. When treatment was started in 48 h cultures the substances reached their maximal activity within 30 min (forskolin and epidermal growth factor) or 3 h (follicle-stimulating hormone and insulin-like growth factor type I). Spreading of the fluorescent dye was inhibited when the medium was supplemented with 12-tetradecanoyl-phorbol-13-acetate. This effect was maximal after 30 min. Additive effects regarding the coupling of the cells were seen by combining of epidermal growth factor with follicle-stimulating hormone, but not with insulin-like growth factor type I or forskolin plus follicle-stimulating hormone.

Binding of guanine nucleotides and Mg2+ to tubulin with a nucleotide-depleted exchangeable site

Binding of GTP and GDP to tubulin in the presence or absence of Mg2+ was measured following depletion of the exchangeable site--(E-site) nucleotide. The E-site nucleotide was displaced with a large molar excess of the nonhydrolyzable GTP analogue, GMPPCP, followed by the removal of the analogue. Using a micropartition assay, the equilibrium constant measured in 0.1 M 1.4-piperazinediethanesulfonic acid (Pipes), pH 6.9, 1 mM ethylene glycol bis(beta-aminoethyl ether) N,N,N',N'-tetraacetic acid, 1 mM dithiothreitol, and 1 mM MgSO4 at 4 degrees C was 9.1 x 10(6) M-1 for GTP and 4.4 x 10(6) M-1 for GDP. Removal of Mg2+ reduced the binding affinity of GTP by 160-fold while the affinity of GDP remained essentially unchanged. Similar values were obtained if 0.1 M Tris, pH 7.0, was used instead of Pipes. Binding of Mg2+ to tubulin containing GTP, GDP, or no nucleotide at the E-site was also examined by the micropartition method. Tubulin-GTP contained one high affinity Mg2+ site (K alpha = 1.2 x 10(6) M-1) in addition to the one occupied by Mg2+ as tubulin is isolated, while only weak Mg2+ binding to tubulin-GDP and to tubulin with a vacant E-site (K alpha = 10(3) M-1) was observed. It is suggested that Mg2+ binds to the beta and gamma phosphates of GTP, and only to the beta phosphate of GDP, as shown for the H. ras p21 protein.

Role of GTP hydrolysis in microtubule polymerization: evidence for a coupled hydrolysis mechanism

The relationship between GTP hydrolysis and microtubule assembly has been investigated by using a rapid filtration method. Microtubules assembled from phosphocellulose-purified tubulin, double-labeled with [gamma-32P]- and [3H]GTP, were trapped and washed free of unbound nucleotide on glass fiber filters. The transient accumulation of microtubule-bound GTP predicted by uncoupled GTP hydrolysis models [Carlier & Pantaloni (1981) Biochemistry 20, 1918-1924; Carlier et al. (1987) Biochemistry 26, 4428-4437] during the rapid assembly of microtubules was not detectable under our experimental conditions. By calculating hypothetical time courses for the transient accumulation of microtubule-bound GTP, we demonstrate that microtubule-bound GTP would have been detectable even if the first-order rate constant for GTP hydrolysis were 4-5 times greater than the pseudo-first-order rate constant for tubulin subunit addition to microtubules. In a similar manner, we demonstrate that if GTP hydrolysis were uncoupled from microtubule assembly but were limited to the interface between GTP subunits and GDP subunits (uncoupled vectorial hydrolysis), then microtubule-bound GTP would have been detectable if GTP hydrolysis became uncoupled from microtubule assembly at less than 50 microM free tubulin, 5 times the steady-state tubulin concentration of our experimental conditions. In addition, during rapid microtubule assembly, we have not detected any microtubule-bound Pi, which has been proposed to form a stabilizing cap at the ends of microtubules [Carlier et al. (1988) Biochemistry 27, 3555-3559]. Also, several conditions that could be expected to increase the degree of potential uncoupling between GTP hydrolysis and microtubule assembly were examined, and no evidence of uncoupling was found. Our results are consistent with models that propose cooperative mechanisms that limit GTP hydrolysis to the terminal ring of tubulin subunits [e.g., O'Brien et al. (1987) Biochemistry 26, 4148-4156]. The results are also consistent with the hypothesis that a slow conformational change in tubulin subunits after GTP hydrolysis and Pi release occurs that results in destabilized microtubule ends when such subunits become exposed at the ends.

Reexamination of the role of nonhydrolyzable guanosine 5'-triphosphate analogues in tubulin polymerization: reaction conditions are a critical factor for effective interactions at the exchangeable nucleotide site

Recently it was proposed [O'Brien, E. T., & Erickson, H. P. (1989) Biochemistry 28, 1413-1422] that tubulin polymerization supported by guanosine 5'-(beta,gamma-imidotriphosphate) [p(NH)ppG], guanosine 5'-(beta,gamma-methylenetriphosphate) [p(CH2)ppG], and ATP might be due to residual GTP in reaction mixtures and that these nucleotides would probably support only one cycle of assembly. Since we had observed polymerization with these three compounds, we decided to study these reactions in greater detail in two systems. The first contained purified tubulin and a high concentration of glycerol, the second tubulin and microtubule-associated proteins (MAPs). In both systems, reactions supported by nucleotides other than GTP were most vigorous at lower pH values. In the glycerol system, repeated cycles of polymerization were observed with ATP and p(CH2)ppG, but not with p(NH)ppG. With p(NH)ppG, a single cycle of polymerization was observed, and this was caused by contaminating GTP. In the MAPs system, repeated cycles of polymerization were observed with both nonhydrolyzable GTP analogues, even without contaminating GTP, but ATP was not active at all in this system. Binding to tubulin of p(NH)ppG, p(CH2)ppG, and, to a lesser extent, ATP was demonstrated indirectly, since high concentrations of the three nucleotides displaced radiolabeled GDP originally bound in the exchangeable site, with p(NH)ppG the most active of the three compounds in this displacement assay. The failure of GTP-free p(NH)ppG to support tubulin polymerization in our glycerol system even though it displaced GDP from the exchangeable site was further investigated by examining the effects of p(NH)ppG on polymerization and polymer-bound nucleotide with low concentrations of GTP. The two nucleotides appeared to act synergistically in supporting polymerization, so that a reaction occurred with a subthreshold GTP concentration if p(NH)ppG was also in the reaction mixture. Analysis of radiolabeled exchangeable-site nucleotide in polymers formed in reaction mixtures containing both GTP and p(NH)ppG demonstrated that p(NH)ppG which entered polymer did so primarily at the expense of GDP originally bound in the exchangeable site rather than at the expense of GTP. It appears that in the glycerol reaction condition, tubulin-p(NH)ppG cannot initiate tubulin polymerization but that it can participate in polymer elongation. ATP and p(CH2)ppG also entered the exchangeable site during polymerization without GTP in glycerol, as demonstrated by displacement of radiolabeled GDP from polymer when these alternate nucleotides were used.(ABSTRACT TRUNCATED AT 400 WORDS)

Formation and stability of vincristine-tubulin complex in kidney cytosols. Role of GTP and GTP hydrolysis

Vincristine-tubulin complex formed in the 100,000 g fraction of mouse kidney dissociated rapidly at 37 degrees in the absence of guanosine-5'-triphosphate (GTP). In the presence of 2 mM GTP, there was a substantial (2.8-fold) increase in complex stability; NaF (100 mM) but not beta-glycerophosphate (1 mM) also reduced the rate of dissociation. Further, complex was stabilized by other ribonucleoside-5'-triphosphates (but not their respective 5'-monophosphates), and a nonhydrolyzable analogue of GTP. Stability of the VCR-tubulin complex formed in cytosol from kidney and separated from unbound VCR and GTP by gel filtration was influenced by the concentration of GTP. These results appear not to be a consequence of denaturation of tubulin during incubation, as VCR binding activity remained constant under experimental conditions both in the presence and after the removal of GTP. Further, the rate of formation of the VCR-tubulin complex in kidney was also influenced by the concentration of GTP and was increased by the addition of NaF. In the absence of added GTP, virtually no complex was isolated. ATP, CTP, or ITP has little effect on complex formation, suggesting that the effect may be GTP specific. These data suggest that the destabilizing activity in cytosols prepared from mouse kidney, and the failure to form a stable VCR-tubulin complex in kidney, are in part the consequence of rapid hydrolysis of GTP by a pyrophosphohydrolase. Direct measurement of the hydrolysis of GTP showed that the activity in kidney (9.26 nmol/min/mg protein) was 9.3-fold greater than in tumor extracts.

Involvement of GTP-binding "G" proteins in transport through the Golgi stack

GTP gamma S irreversibly inhibits protein transport between successive compartments of the Golgi stack in a cell-free system. Fluoride, potentiated by the addition of aluminum ion, also causes a strong inhibition. These are hallmarks of the involvement of a guanine nucleotide-binding or regulatory "G" protein. Inhibition by GTP gamma S requires a cytosolic inhibitory factor that binds to Golgi membranes during inhibition. Preincubation experiments reveal that GTP gamma S blocks the function of acceptor Golgi but not donor Golgi membranes. More specifically, a processing step in between vesicle attachment and the actual fusion event seems to be affected. Electron microscopy demonstrates a corresponding 5-fold accumulation of non-clathrin-coated buds and vesicles associated with the Golgi cisternae during inhibition by GTP gamma S.

Micromolar Ca2+ requiring protease from human platelets: purification, partial characterization and effect on the cytoskeletal proteins

A calcium-activated neutral protease (CANP) has been purified 2,800 fold, to near homogeneity, from human platelets. The purification procedure involved ammonium sulfate fractionation of the platelet cytosol followed by chromatography on Sephacryl S-200, DEAE-Sephacel, Agarose-Hexylamine, Agarose-Octylamine and alpha-casein-Sepharose 4B affinity gel. The protease consisted of two polypeptides of Mr = 74,000 and 28,000 as judged on sodium dodecyl sulfate-polyacrylamide gel electrophoresis. It hydrolyzed [methyl-14C] alpha-casein at a significant rate of 37 degrees C which was, therefore, used as an exogenous substrate. Microtubules and intermediate filament proteins were also susceptible to hydrolysis by the purified protease. It attained maximum activity at 0.06 uM CaCl2 and displayed two pH maxima: one at 5.5 and the other at 6.5. The protease was fully active in the presence of MnCl2 and was about 75% active with BaCl2 and SrCl2. Among the actinomycete protease inhibitors, leupeptin, antipain and pepstatin, the order of inhibition was: leupeptin greater than antipain greater than pepstatin. The protease was also inhibited by sulfhydryl modifying agents.

The effects of elevated pH and high salt concentrations on tubulin

The effects of incubating phosphocellulose-purified bovine tubulin at 4 degrees C in nucleotide-free buffers at alkaline pH or at high concentrations of NaCl, KCl, (NH4)2SO4, or NH4Cl have been studied. At pH greater than or equal to 7.5 or at NaCl concentrations greater than or equal to 0.7 M, tubulin releases bound nucleotides irreversibly and loses, with apparent first-order kinetics, the ability to assemble into microtubules. In 0.1 M 1,4-piperazinediethanesulfonic acid buffer, pH 6.9, in the presence of 1.3 M NH4Cl, tubulin undergoes more rapid loss of capacity to assemble than it does in NaCl and KCl, but 1.3 M (NH4)2SO4 causes no detectable change in tubulin after 1-h incubation. Incubation at high pH or at high neutral salt concentrations also causes an apparently irreversible change in the ultraviolet difference spectrum and in the sedimentation velocity profile of tubulin. At elevated salt concentrations a decrease of approximately 10% in the molar ellipticity within the wavelength range 220-260 nm is observed. The changes that occur during 1-h exposure to pH 8.0 can be completely prevented by including 1 mM guanosine 5'-triphosphate (GTP) or 4 M glycerol in the buffer, but those which occur at pH 9.0 cannot be prevented by these additions. In 1 M NaCl when the ratio of bound guanine nucleotide to tubulin reaches approximately 1.0, tubulin loses the abilities to assemble into microtubules and to bind colchicine. The rate of loss of nucleotide in 2 M NaCl is decreased in the presence of 1 mM GTP, and tubulin is protected almost completely from 1 M NaCl-induced loss of GTP (and retains the ability to exchange [3H]GTP as well) in the presence of bound colchicine. Investigators who anticipate exposing tubulin to buffers of elevated pH or high concentrations of chaotropic salts should be extremely cautious in interpreting the resulting data unless they can demonstrate that irreversible alteration of the protein has not occurred.

Location of the guanosine triphosphate (GTP) hydrolysis site in microtubules

The rate for GTP hydrolysis remains approximately constant during microtubule assembly from microtubular protein. This indicates that GTP hydrolysis does not accompany tubulin-GTP subunit addition to microtubule ends. We suggest that GTP, within tubulin-GTP subunits that are incorporated into microtubules, is hydrolyzed predominantly at one or both microtubule ends at an interface of a cap of tubulin-GTP subunits and a core of tubulin-GDP subunits.

Synthesis and properties of adenosine-5'-triphosphoro-gamma-1-(5-sulfonic acid)naphthyl ethylamidate: a fluorescent nucleotide substrate for DNA-dependent RNA polymerase from Escherichia coli

A new fluorescent ATP analog, adenosine-5'-triphosphoro-gamma-1-(5-sulfonic acid)naphthyl ethylamidate (gamma-1,5-EDANS)ATP, containing the fluorophore N-(aminoethyl)-5-naphthylamine-1-sulfonic acid attached via a gamma-phosphoamidate bond was synthesized in good yield. It has absorption maxima at 255 and 344 nm and a fluorescence emission maximum at 490 nm. These spectral characteristics permit its uses as an energy acceptor for energy transfer from the intrinsic protein fluorophores and as an energy donor for the energy transfer to the intrinsic Co of Co-substituted RNA polymerases. This analog is a good substrate for Escherichia coli RNA polymerase and can be used to initiate the RNA chain. Incorporation of this analog into the total RNA synthesized was about 60% of that observed for ATP, independent of the templates used. Its Km values (22 and 118 microM) are twofold higher and its Vmax values (45 and 59 nmol/min/mg of enzyme) are 40% lower than those for ATP using calf thymus DNA and poly[d(A-T)], respectively, as template. For abortive initiation reaction using pAR1435 plasmid DNA as template, the Km and Vmax values of this analog are 2.7 times higher and 7 times lower, respectively, than those of ATP. With its desirable spectroscopic properties, (gamma-1,5-EDANS)ATP is a good probe for the studies of nucleotide-protein interactions, active site mapping of RNA polymerase, and other ATP-utilizing biological systems.

Possible treadmilling in zinc(II)-induced sheet polymers of bovine brain microtubule protein

We have examined the ability of zinc(II)-induced sheet polymers, formed from thrice-cycled bovine brain microtubule protein prepared in the absence of glycerol, to exchange with tubulin subunits at steady state. By a rapid filtration assay in which labeled GTP was used as a marker for tubulin addition and loss, we found that steady-state sheet polymers, formed in 0.5 mM-ZnCl2, 1 mM-dithiothreitol, and 100 mM-2-(N-morpholino)ethanesulfonic acid (pH 6.75) in the presence of a GTP-regenerating system at 37 degrees C, incorporated the label in a time-dependent manner to a maximum level. The steady-state uptake of label was inhibited by colchicine, podophyllotoxin and vinblastine. In pulse-chase experiments, we observed that label added onto sheet polymers in a short pulse was retained for a period equal to that required by the polymers to become fully labeled in a continuous pulse; thereafter, the label was lost gradually to a baseline level. An average of 82% of the label was retained in the sheet polymers after a "cold" chase of equal duration to the time of the pulse. Sheet polymers assembled from microtubule protein prepared in the presence of glycerol gave similar results. Using a double-labeling procedure to analyze tubulin addition and loss simultaneously, we found that the rates of steady-state addition and loss were similar. Sheet polymers retained their structural integrity throughout these experiments, as determined by electron microscopy. We believe that the data are consistent with a "treadmilling" mechanism of polymerization and depolymerization, analogous to that documented to occur in steady-state microtubules in vitro. Such a mechanism is discussed in the context of recent findings from structural studies, and a model consistent with established structural data is offered.

Di-and triphosphate derivatives of acyclo- and arabinosylguanine. Effects on the polymerization of purified tubulin

Glutamate- and nucleotide-dependent polymerization of purified calf brain tubulin was used as a model system to study interactions of ribose-modified GDP and GTP analogs with tubulin. Earlier studies (Hamel, E., and Lin, C.M.(1981) Proc. Natl. Acad. Sci. U.S.A. 78,3368-3372) were extended to three additional sets of analogs: the di- and triphosphate derivatives of 9-beta-D-arabinofuranosylguanine (araGDP and araGTP) and acycloguanosine (9-(2-hydroxyethoxymethyl)guanine) (acycloGDP and acycloGTP), as well as the periodate-oxidized and borohydride-reduced derivatives of GDP and GTP (ox-redGDP and ox-redGTP). Disruption of the ribose ring in ox-redGTP resulted in major loss of activity relative to GTP in supporting tubulin polymerization, although the analog's deficiency may result from an inability to displace GDP from the exchangeable site rather than a direct effect on the polymerization reaction itself. The poor activity of ox-redGTP could be largely reversed if nucleoside diphosphate kinase was added to the reaction mixture. Removal of the 2' and 3' carbons entirely, in the form of acycloGTP, resulted in only minimal loss of activity relative to GTP. AraGTP, on the other hand, was more active than GTP in supporting tubulin polymerization. All three GDP analogs were much less effective than GDP in inhibiting tubulin polymerization, although araGDP was significantly more inhibitory than acycloGDP or ox-redGDP. Relative inhibitory activity of these and additional GDP analogs was the same whether GTP or a GTP analog was used to support tubulin polymerization.