Tubulin polymerization with ATP is mediated through the exchangeable GTP site

Glycerol-induced tubulin polymerization supported by non-guanine nucleotides was examined. The electrophoretically homogeneous tubulin was devoid of nucleoside diphosphate kinase activity and 95% saturated with exchangeable GDP and nonexchangeable GTP. All purine ribonucleoside 5'-triphosphates were active but no polymerization occurred with CTP or UTP. All polymerization reactions, as a function of nucleotide concentration, were similar: above a minimum (threshold) concentration, as the amount of nucleotide increased the reaction became progressively more rapid and extensive with a progressively shorter nucleation period. Threshold concentrations of ATP, XTP, ITP and GTP were 0.6 mM, 0.3 mM, 30 microM and 7 microM, respectively. Most ribose- and polyphosphate-modified ATP analogs also supported polymerization at high concentrations, but the activity of these analogs relative to ATP was very similar to the activity of cognate GTP analogs relative to GTP. Polymerization with ATP was associated with an ATPase reaction. ATP hydrolysis was potently inhibited by GDP and GTP and altered by antimitotic drugs in parallel with the effects of these agents on GTP hydrolysis. Substantial amounts of [8-14C]GDP bound in the exchangeable site of tubulin were displaced during polymerization with GTP or ATP, but much higher concentrations of ATP were required for equivalent displacement of the tubulin-bound GDP. Polymerization with GTP or ATP was inhibited in a qualitatively similar manner by GDP, with increasing concentrations of GDP causing a progressive prolongation of the nucleation period and reduction in reaction rate and extent. However, complete inhibition of polymerization required that GDP:GTP much greater than 1, but that GDP:ATP much less than 1. Inhibition appeared to be primarily competitive, since with higher triphosphate concentrations higher GDP concentrations were required for comparable inhibition. We conclude that ATP effects on tubulin polymerization are mediated through a feeble interaction at the exchangeable GTP site.     

Evidence for a distinct ligand binding site on tubulin discovered through inhibition by GDP of paclitaxel-induced tubulin assembly in the absence of exogenous GTP

GDP inhibits paclitaxel-induced tubulin assembly without GTP when the tubulin bears GDP in the exchangeable site (E-site). Initially, we thought inhibition was mediated through the E-site, since small amounts of GTP or Mg²⁺, which favors GTP binding to the E-site, reduced inhibition by GDP. We thought trace GTP released from the nonexchangeable site (N-site) by tubulin denaturation was required for polymer nucleation, but microtubule length was unaffected by GDP. Further, enhancing polymer nucleation reduced inhibition by GDP. Other mechanisms involving the E-site were eliminated experimentally. Upon finding that ATP weakly inhibited paclitaxel-induced assembly, we concluded that another ligand binding site was responsible for these inhibitory effects, and we found that GDP was not binding at the taxoid, colchicine, or vinca sites. There may therefore be a lower affinity site on tubulin to which GDP can bind distinct from the E- and N-sites, possibly on α-tubulin, based on molecular modeling studies.     

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)     

Stoichiometry for guanine nucleotide binding to tubulin under polymerizing and nonpolymerizing conditions

The maximal stoichiometry for [³H]GTP binding to depolymerized tubulin with saturating amounts of added [³H]GTP is 0.4 mol/110,000 g protein. In contrast, 1 mol of radioactive nucleotide is incorporated into microtubules as a result of polymerization with [³H]GTP. The different stoichiometries result from a difference in the nucleotide binding properties of ring protein under polymerizing and nonpolymerizing conditions: ring protein at 0 °C is devoid of binding activity but binds added radioactive guanine nucleotide during microtubule assembly. The radioactive nucleotide which is incorporated into rings during microtubule assembly is not displaced by excess GDP, although it is at a site which is distinct from the N site.     

GTP analogues interact with the tubulin exchangeable site during assembly and upon binding

The question of whether nonhydrolyzable nucleotide analogues and other nucleoside triphosphates support tubulin assembly was addressed. Tubulin which contained residual GTP at the exchangeable site polymerized in the absence of added GTP in the presence of DMSO or glycerol. After maximum absorbance was reached, disassembly occurred at a slow rate. When 0.5 mM GMPPCP, GMPPNP, or ATP was included in the assembly reaction, disassembly did not occur, and about 0.1 mol of these nucleotides per mole of tubulin was incorporated into the protein. When 5 mM nucleotide was used or alkaline phosphatase was included in the case of the nonhydrolyzable analogues, a greater amount of assembly occurred and about 0.7-0.8 mol of analogue was incorporated. The products of the assembly reaction were cold-labile microtubules and protofilament ribbons. After cold-depolymerization of the microtubules and ribbons, a second cycle of assembly produced some microtubules, but cold-stable amorphous polymers were the major product. In addition, when GTP at the exchangeable site was first removed by a cycle of assembly, followed by depolymerization, assembly in the presence of GMPPCP, GMPPNP, or ATP produced a mixture of microtubules and cold-stable polymers, both of which contained bound analogue. Incorporation of GMPPCP, GMPPNP, or ATP into polymerized tubulin always occurred at the expense of GDP at the exchangeable site, the content of which decreased correspondingly. Incubation of tubulin with 5 mM GMPPCP, GMPPNP, or ATP under nonassembly conditions also displaced GDP.(ABSTRACT TRUNCATED AT 250 WORDS)     

Nucleotide specificity in microtubule assembly in vitro

A procedure is described for removing most of the GDP bound at the exchangeable GTP binding site (E site) of tubulin. Microtubule protein containing substoichiometric amounts of GDP at the E site is found to polymerize in response to: (a) two nonhydrolyzable ATP analogues, adenylyl imidodiphosphate (AMP-PNP) and adenylyl beta, gamma-methylenediphosphonate (AMP-PCP); and (b) substoichiometric levels of GTP or dGTP. The results are interpreted as suggesting that: (1) when GDP is removed from tubulin, the E site shows broad specificity for nucleoside triphosphates: (2) microtubule assembly can be induced by the binding of substoichiometric amounts of nucleoside triphosphate to the E site.     

Membrane-dependent guanine nucleotide binding and GTPase activities of soluble protein from bovine rod cell outer segments.

Soluble proteins can be extracted by osmotic shock of purified rod (photoreceptor cell) outer segments that have intact plasma membranes. The soluble proteins include a component that contains tightly bound GDP-Exchange of this GDP with exogenous nucleotide is catalyzed by (and requires) the membranes from the outer segments. ATP does not participate in these reactions. Approximately one-half of the binding sites in the soluble component require GTP as the source of exogenous nucleotide; the remainder accept GTP or GDP with equal facility. When exogenous GTP is the source of bound nucleotide, it is found in the complex in the form of GDP. Exchange of bound nucleotide with GTP is stoichiometrically related to GTPase activity; this activity is highly dependent upon the presence of both membranes and soluble protein. The soluble nucleotide binding protein was purified by making use of the fact that it binds tightly to the membranes (under conditions of moderate ionic strength) in the absence of GTP and can be eluted by solutions containing low concentrations of GTP (but not GDP or ATP, nor can it be eluted by GTP-free solutions of low ionic strength). The purified protein contains two polypeptide chains of molecular weights 41,000 and 37,000; these are the major species that can be extracted from the outer segments by osmotic shock, and they constitute approximately 7% of the total protein of the isolated organelle.     

Dephosphorylation of tubulin-bound guanosine triphosphate during microtubule assembly.

1. Tubulin purified from porcine brain in the presence of GTP contained 0.16 mole of GDP and 0.73 mole of GTP per 60,000 g of protein. 2. Microtubules reconstituted from the purified tubulin contained 0.43 mole of GDP and 0.41 mole of GTP per 60,000 g of protein. Guanine nucleotide bound to the exchangeable site of tubulin was converted to GDP during microtubule assembly, while GTP at the non-exchangeable site remained intact. 3. Guanine nucleotide which had been bound to the exchangeable site of tubulin before microtubule assembly was also exchangeable during disassembly.     

Nucleotide binding and phosphorylation in microtubule assembly in vitro.

Two non-hydrolyzable analogs of GTP, guanylyl-β,γ-methylene diphosphonate and guanylyl imidodiphosphate, have been found to induce rapid and efficient microtubule assembly in vitro by binding at the exchangeable site (E-site) on tubulin. Characterization of microtubule polymerization by several criteria, including polymerization kinetics, nucleotide binding to depolymerized and polymerized microtubules, and microtubule stability, reveals strong similarities between microtubule assembly induced by GTP and non-hydrolyzable GTP analogs. Nucleoside triphosphates which bind weakly or not at all to tubulin, such as ATP, UTP and CTP, are shown to induce microtubule assembly by means of a nucleoside diphosphate kinase (NDP-kinase, EC 2.7.4.6.) activity which is not intrinsic to tubulin. The NDP-kinase mediates microtubule polymerization by phosphorylating tubulin-bound GDP in situ at the E-site. Although hydrolysis of exchangeably bound GTP occurs, it is found to be uncoupled from the polymerization reaction. The non-exchangeable nucleotide binding site on tubulin (N-site) is not directly involved in microtubule assembly in vitro. The N-site is shown to contain almost exclusively GTP which is not hydrolyzed during microtubule assembly. A scheme is presented in which GTP acts as an allosteric effector at the E-site during microtubule assembly in vitro.     

Tubulin polymerisation in the presence of GMP-PCP

Microtubules induced by the binding of GTP or of a non-hydrolysable analog of GTP onto the exchangeable site of tubulin appear very similar according to electron microscopy and polymerisation kinetics criteria. However, we show here that the exchangeable sites or “E” sites of the tubulin subunits remain available for nucleotide exchange inside the GMP-PCP-microtubules contrary to the “E” sites inside the GDP-microtubules. Moreover, under specific conditions, GMP-PCP induces the polymerisation of tubulin into a bidimensional, pseudocrystallin structure. Such a “crystallisation” is inhibited by GTP and GDP.     

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.     

Microtubular protein reaction with nucleotides.

The dissociation constants for GTP and GDP with tubulin were determined to be equal to 1.1 ± 0.4 × 10^?7 M and 1.5 ± .6 × 10^?7 (4°), respectively. A lower limit for the dissociation constant for ATP was established as equal to 6 × 10^?4 M. The equivalent binding of GTP and GDP is not readily consistent with a mechanism in which the role of GTP in microtubule assembly is to bind to the protein to induce a conformation which is able to polymerize. An ATP-induced polymerization of tubulin apparently involves a transphosphorylation reaction in which GTP is formed and mediates the assembly. For this reaction to occur with desalted tubulin trace amounts of GDP are required; in the reaction of 0.1 mM ATP with 22.0 μM tubulin, 0.1 μM GDP induces about 80% as much tubule formation as is seen with 0.1 mM GTP alone.     

Effect of tubulin self-association on GTP hydrolysis and nucleotide exchange reactions

We investigated how the self-association of isolated tubulin dimers affects the rate of GTP hydrolysis and the equilibrium of nucleotide exchange. Both reactions are relevant for microtubule (MT) dynamics. We used HPLC to determine the concentrations of GDP and GTP and thereby the GTPase activity of SEC-eluted tubulin dimers in assembly buffer solution, free of glycerol and tubulin aggregates. When GTP hydrolysis was negligible, the nucleotide exchange mechanism was studied by determining the concentrations of tubulin-free and tubulin-bound GTP and GDP. We observed no GTP hydrolysis below the critical conditions for MT assembly (either below the critical tubulin concentration and/or at low temperature), despite the assembly of tubulin 1D curved oligomers and single-rings, showing that their assembly did not involve GTP hydrolysis. Under conditions enabling spontaneous slow MT assembly, a slow pseudo-first-order GTP hydrolysis kinetics was detected, limited by the rate of MT assembly. Cryo-TEM images showed that GTP-tubulin 1D oligomers were curved also at 36 °C. Nucleotide exchange depended on the total tubulin concentration and the molar ratio between tubulin-free GDP and GTP. We used a thermodynamic model of isodesmic tubulin self-association, terminated by the formation of tubulin single-rings to determine the molar fractions of dimers with exposed and buried nucleotide exchangeable sites (E-sites). Our analysis shows that the GDP to GTP exchange reaction equilibrium constant was an order-of-magnitude larger for tubulin dimers with exposed E-sites than for assembled dimers with buried E-sites. This conclusion may have implications on the dynamics at the tip of the MT plus end.     

Effects of adenyl nucleotides and carbachol on cooperative interactions among G proteins.

Muscarinic agonists and adenyl nucleotides are noncompetitive modulators of sites labeled by [35S]GTP gamma S in washed cardiac membranes from Syrian golden hamsters. Specific binding of the radioligand and its inhibition by either GTP gamma S or GDP reveals three states of affinity for guanyl nucleotides. In the absence of adenyl nucleotide, carbachol promotes an apparent interconversion of sites from higher to lower affinity for GDP; the effect recalls that of guanyl nucleotides on the binding of agonists to muscarinic receptors. In the presence of 0.1 mM ATP gamma S, the binding of [35S]GTP gamma S is increased at concentrations up to about 50 nM and decreased at higher concentrations. At a radioligand concentration of 160 pM, binding exhibits a bell-shaped dependence on the concentration of both ATP gamma S and AMP-PNP; with ADP and ATP, there is a second increase in bound [35S]GTP gamma S at the highest concentrations of adenyl nucleotide. ATP gamma S and AMP-PNP also modulate the effect of GDP, which itself emerges as a cooperative process: that is, binding of the radioligand in the presence of AMP-PNP exhibits a bell-shaped dependence on the concentration of GDP; moreover, the GDP-dependent increase in bound [35S]GTP gamma S is enhanced by carbachol. The interactions among GDP, GTP gamma S, and carbachol can be rationalized quantitatively in terms of a cooperative model involving two sites tentatively identified as G proteins. Both GTP gamma S and GDP exhibit negative homotropic cooperativity; carbachol enhances the homotropic cooperativity of GDP and induces or enhances positive heterotropic cooperativity between GDP and [35S]GTP gamma S. An analogous mechanism may underlie the guanyl nucleotide-dependent binding of agonists to muscarinic receptors. The data suggest that the binding properties of G proteins and their associated receptors reflect cooperative effects within heterooligomeric arrays; agonist-induced changes in cooperativity may facilitate the exchange of GTP for bound GDP and thereby constitute the mechanism of G protein activation in vivo.     

Guanosine 5'-O-(3-thiotriphosphate), a potent nucleotide inhibitor of microtubule assembly

Guanosine 5'-O-(3-thiotriphosphate) (GTP gamma S) and the two diastereoisomers of guanosine 5'-O-(2-thiotriphosphate) (GTP beta S) were prepared enzymatically, and their interactions with tubulin and microtubule-associated proteins (MAPs) in 0.1 M 2-(N-morpholino)ethanesulfonate, 0.5 mM MgCl2 were examined. GTP gamma S did not support microtubule assembly but instead inhibited the reaction. This analog was 1.5-2 times more potent than GDP in inhibiting both tubulin polymerization and GTP hydrolysis under conditions in which these reactions were dependent on MAPs. In contrast to the analog's inhibitory effects on polymerization and hydrolysis, however, radiolabeled GTP gamma S was only feebly bound by purified tubulin at 0 degrees C relative to the binding of GDP and GTP. There was a marked increase in the amount of GTP gamma S bound when the reaction temperature was raised to 37 degrees C or when MAPs were included in the reaction mixture. Only when both MAPs were present and the higher reaction temperature was used did the binding of GTP gamma S exceed that of GDP. Since substitution of sulfur for oxygen in a molecule should decrease its hydrophilic properties, these findings suggest that the exchangeable nucleotide binding site of tubulin becomes more hydrophobic at higher temperatures and in the presence of MAPs. The two isomers of GTP beta S were able to support MAP-dependent polymerization, although a 50-100-fold higher concentration of the analogs as compared to GTP was required. Neither isomer of GTP beta S had a significant inhibitory effect on GTP hydrolysis dependent on tubulin + MAPs.     

Effects of pH on tubulin-nucleotide interactions

Significant GTP-independent, temperature-dependent turbidity development occurs with purified tubulin stored in the absence of unbound nucleotide, and this can be minimized with a higher reaction pH. Since microtubule assembly is optimal at lower pH values, we examined pH effects on tubulin-nucleotide interactions. While the lowest concentration of GTP required for assembly changed little, GDP was more inhibitory at higher pH values. The amounts of exogenous GTP bound to tubulin at all pH values were similar, but the amounts of exogenous GDP bound and endogenous GDP (i.e., GDP originally bound in the exchangeable site) retained by tubulin rose as reaction pH increased. Endogenous GDP was more efficiently displaced by exogenous GTP than GDP at all pH values, but displacement by GTP was 10-15% greater at pH 6 than at pH 7. Dissociation constants for GDP and GTP were about 1.0 microM at pH 6 and 0.02 microM at pH 7. A small increase in the affinity of GDP relative to that of GTP occurs at pH 7 as compared to pH 6, together with a 50-fold absolute increase in the affinity of both nucleotides for tubulin at pH 7. The time courses of microtubule assembly and GTP hydrolysis were compared at pH 6 and pH 7. At pH 6, the two reactions were simultaneous in onset and initially stoichiometric. At pH 7, although the reactions began simultaneously, hydrolysis seemed to lag substantially behind assembly. Unhydrolyzed radiolabeled GTP was not incorporated into microtubules, however, indicating that GTP hydrolysis is actually closely coupled to assembly. The apparent lag in hydrolysis probably results from a methodological artifact rather than incorporation of GTP into the microtubule with delayed hydrolysis.     

The characteristics and effect on catalysis of nucleotide binding to noncatalytic sites of chloroplast F1-ATPase.

The recent finding that the presence of ATP at non-catalytic sites of chloroplast F1-ATPase (CF1) is necessary for ATPase activity (Milgrom, Y. M., Ehler, L. L., and Boyer, P. D. (1990) J. Biol. Chem. 265,18725-18728) prompted more detailed studies of the effect of noncatalytic site nucleotides on catalysis. CF1 containing at noncatalytic sites less than one ADP or about two ATP was prepared by heat activation in the absence of Mg2+ and in the presence of ADP or ATP, respectively. After removal of medium nucleotides, the CF1 preparations were used for measurement of the time course of nucleotide binding from 10 to 100 microM concentrations of 3H-labeled ADP, ATP, or GTP. The presence of Mg2+ strongly promotes the tight binding of ADP and ATP at noncatalytic sites. For example, the ADP-heat-activated enzyme in presence of 1 mM Mg2+ binds ADP with a rate constant of 0.5 x 10(6) M-1 min-1 to give an enzyme with two ADP at noncatalytic sites with a Kd of about 0.1 microM. Upon exposure to Mg2+ and ATP the vacant noncatalytic site binds an ATP rapidly and, as an ADP slowly dissociates, a second ATP binds. The binding correlates with an increase in the ATPase activity. In contrast the tight binding of [3H]GTP to noncatalytic sites gives an enzyme with no ATPase activity. The three noncatalytic sites differ in their binding properties. The noncatalytic site that remains vacant after the ADP-heat-activated CF1 is exposed to Mg2+ and ADP and that can bind ATP rapidly is designated as site A; the site that fills with ATP as ADP dissociates when this enzyme is exposed to Mg2+ and ATP is called site B, and the site to which ADP remains bound is called site C. Procedures are given for attaining CF1 with ADP at sites B and C, with GTP at sites A and/or B, and with ATP at sites A, B, and/or C, and catalytic activities of such preparations are measured. For example, little or no ATPase activity is found unless ATP is at site A, but ADP can remain at site C with no effect on ATPase. Maximal GTPase activity requires ATP at site A but about one-fifth of maximal GTPase is attained when GTP is at sites A and B and ATP at site C. Noncatalytic site occupancy can thus have profound effects on the ATPase and GTPase activities of CF1.     

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-β-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.     

Inhibition of tubulin polymerization with ribose-modified analogs of GDP and GTP reduced inhibition with microtubule-associated proteins and magnesium

Inhibitory effects of ribose-modified GDP and GTP analogs on tubulin polymerization were examined to explore nucleotide structural requirements at the exchangeable GTP binding site. With microtubule-associated proteins and Mg²⁺, GTP-supported polymerization was only modestly inhibited by GDP, and still weaker inhibitory activity was found with two analogs, dGDP and 9-β-D-arabinofuranosylguanine-5′-diphosphate (araGDP). Omission of Mg²⁺ significantly enhanced the inhibitory effects of GDP, dGDP and araGDP and resulted in weak inhibition of the reaction by several other GDP analogs. The relative inhibitory activity of the GDP analogs had no discernable relationship to the relative activity of cognate GTP analogs in supporting microtubule-associated protein-dependent polymerization. One GTP analog, 2′,3′-dideoxyguanosine 5′-triphosphate (ddGTP), supports polymerization both with and without microtubule-associated proteins. The inhibitory activity of GDP and GDP analogs in ddGTP-supported polymerization was much greater in the absence of microtubule-associated proteins than in their presence; and both reactions were more readily inhibited than was microtubule-associated protein-dependent, GTP-supported polymerization. Microtubule-associated protein-independent, ddGTP-supported polymerization was also potently inhibited by GTP and a number of GTP analogs. GTP was in fact twice as inhibitory as GDP. The relative inhibitory activity of the GTP analogs was comparable to the relative inhibitory activity of the cognate GDP analogs and very different from their relative activity in supporting polymerization.     

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.