Microtubular protein reaction with nucleotides

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.

Interaction of tubulin with guanosine 5'-O-(1-thiotriphosphate) diastereoisomers: specificity of the alpha-phosphate binding region

The exchangeable nucleotide-binding site of tubulin has been studied using diastereoisomers A (Sp) and B (Rp) of guanosine 5'-O-(1-thiotriphosphate) (GTP alpha S) in which the phosphorus atom to which sulfur is attached is chiral. GTP alpha S(A) (10 microM) nucleated assembly of purified tubulin (20 microM) into microtubules in buffer containing 0.1 M 2-(N-morpholino)ethanesulfonic acid with 3 mM Mg2+ and 1 mM EGTA, pH 6.6 at 37 degrees C. With 0.2 mM GTP alpha S(A), the critical concentration (Cc; minimum protein concentration required for assembly) was 8 microM tubulin. Neither 0.2 mM GTP nor GTP alpha S(B) promoted microtubule assembly in buffer with 0.5-6.75 mM Mg2+ and 20-70 microM tubulin. The Cc values for GTP alpha S-(A)-induced assembly of tubulin in buffer with 30% glycerol and of microtubule protein (tubulin and microtubule-associated proteins) in buffer were lower than for GTP. GTP alpha S(A)-induced microtubules were more stable to the cold and to Ca2+. GTP alpha S(A) and GTP but not GTP alpha S(B) bound tightly to tubulin at 4 degrees C. Although GTP alpha S(B) did not nucleate assembly, it did bind to tubulin since it was incorporated into the growing microtubule. Both isomers were hydrolyzed in the microtubules. These studies show that GTP alpha S(A) promotes tubulin assembly better than GTP and GTP alpha S(B) and that there is stereoselectivity at the alpha-phosphate binding region of tubulin. The stereoselectivity may be due to different MgGTP alpha S(A) and -(B) interactions with tubulin.

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.

Microtubules and nucleoside diphosphate kinase. Nucleoside diphosphate kinase binds to co-purifying contaminants rather than to microtubule proteins

Nucleoside diphosphate (NDP) kinase has been postulated to generate GTP from the GDP bound to tubulin. The purified chick brain enzyme was studied with respect to its kinetic parameters, and the protein-protein interactions between the NDP kinase and tubulin were examined. No specific interaction is observed between the enzyme and assembled microtubules, tubulin dimers, or tubulin-microtubule-associated protein (MAP) oligomers under a variety of nucleotide conditions. The apparent association is demonstrated to result from NDP kinase binding to a co-purifying contaminant. The absence of detectable NDP kinase-tubulin interactions indicates that NDP kinase does not directly charge up tubulin-GDP.

Mammalian brain microtubules are sensitive to cyclic AMP in vitro

Microtubules assembled in vitro with ATP were depolymerized by the addition of cyclic AMP, which correlates with a stimulation of the endogeneous phosphorylation reaction. When assembled with GTP, however, microtubules were only sensitive to cyclic AMP when ATP was present. This nucleoside triphosphate induced the disassembly of microtubules in a concentration-dependent, cyclic nucleotide-stimulated manner. Since UTP, CTP and the nonhydrolyzable ATP analog adenosine-5'-(beta, gamma-methylene)triphosphate were without comparable effect, it was assumed that phosphorylation of the microtubule-associated proteins may represent a physiological mechanism by which microtubules in the living cell respond to external stimuli.

Direct incorporation of microtubule oligomers at high GTP concentrations

Chick brain microtubule protein consists primarily of a mixture of MAP2:tubulin oligomers and dimeric tubulin. The assembly of this protein is described by a single pseudofirst-order reaction at 20 microM GTP, but by the summation of two pseudofirst-order reactions at 1 mM GTP. The protein contains two GTP-binding species, corresponding to the tubulin dimers and the oligomers, and conditions which alter the dimer: oligomer equilibrium, affect the kinetics of microtubule assembly. The results indicate that the oligomers are only direct assembly intermediates at high GTP concentrations.

Similar affinities of ADP and ATP for G-actin at physiological salt concentrations

The equilibrium constant for the exchange of ATP and ADP at G-actin was determined by fluorimetric titration of G-actin-bound epsilon-ATP by ATP or ADP. The affinity of ATP for G-actin was found to be only about 3-fold higher than the affinity of ADP for G-actin at 37 degrees C, pH 7.5 and physiologically relevant salt concentrations (100 mmol K+/l, 0.8 mmol Mg2+/l, less than 0.01 mmol Ca2+/l).

Modulation of passive permeability by external ATP and cytoskeleton-attacking agents in cultured mammalian cells

External ATP causes a passive permeability change in several transformed cells, but not in untransformed cells. We previously demonstrated that in CHO-K1 cells, a transformed clone of Chinese hamster ovary cells, the external ATP-dependent permeability change was induced when the intracellular ATP concentration was reduced by a mitochondrial inhibitor (Kitagawa, T. and Akamatsu, Y. (1981) Biochim. Biophys. Acta 649, 76-82). A permeability change with similar characteristics was also observed when the CHO cells were treated with external ATP and a cytoskeleton-attacking agent such as vinblastine or cytochalasin B. Just like mitochondrial inhibitors, vinblastine could increase the sensitivity of transformed 3T3 cells to external ATP but showed no effect on passive permeability of normal 3T3 cells. However, in contrast with the effect of the mitochondrial inhibitors, the cytoskeleton drugs caused the permeability change with little reduction of intracellular ATP concentration, suggesting different actions of these two kinds of drug on the permeability change. The present results suggest an important role of cytoskeletal structures in controlling the external ATP-dependent permeability change in transformed cells. Possible effects of intracellular ATP on cytoskeletal structures are also discussed.

Guanosine-5'-triphosphate hydrolysis and tubulin polymerization. Review article

GTP hydrolysis associated with polymerization is a distinctive feature of microtubule assembly. This reaction may be fundamentally linked to the dynamic properties of microtubules in vivo. Kinetic analysis of the connection between microtubule assembly and associated GTP hydrolysis indicates that these two events are kinetically uncoupled, GTP hydrolysis occurring after tubulin incorporation in the microtubule. As a consequence, the combination of the diffusional incorporation of GTP in microtubules at steady-state and of subsequent GTP hydrolysis results in the formation of a steady-state GTP cap at microtubule ends. The interplay between GTP and GDP at microtubule ends is examined. Inhibition by GDP of steady-state GTP hydrolysis at microtubule ends and of microtubule elongation is understood within a tight reversible binding of GDP at microtubule ends generating 'inactive' elongation sites. Nucleotides are freely exchangeable at microtubule ends. This result indicates that the nature of the nucleotide present at microtubule ends must be considered in a model for microtubule assembly. These data are pooled in order to define the general features of a model describing microtubule assembly and treadmilling in terms somewhat different from previously proposed models.

Participation of guanine nucleotides in nucleation and elongation steps of microtubule assembly

Critical concentrations for formation of microtubules from subunits with GTP and its beta, gamma-imido and beta, gamma-methylene analogs are similar when adequate time is given for equilibration. Dilution of microtubules into GTP and GDP yielded values of 0.1 and 0.19 mg/ml for the critical concentration, results similar to those reported by Carlier and Pantaloni [Carlier, M. & Pantaloni, D. (1978) Biochemistry 17, 1908-1915]. GDP is capable of supporting elongation of preformed microtubules, but it efficiently poisons the nucleation events. Reported experiments also demonstrate that the critical tubulin concentration of the tubulin-GDP complex can be accurately measured in both the assembly and disassembly directions. Evidence is presented that GTP is involved in early nucleation events but that microtubules are stabilized in the presence of either GTP or GDP. These results are discussed in terms of a condensation-equilibrium model in which tubulin subunits equilibrate rapidly with microtubule ends, and their affinity for the ends is governed by the nucleotide ligand at the exchangeable site.

Tubulin-associated nucleoside diphosphokinase

Microtubule protein, prepared by cycles of polymerisation and dissociation, contained a nucleoside diphosphokinase (NDP kinase) activity (EC 2.7.4.6). This activity was not intrinsic to the tubulin dimer or the so-called microtubule-associated proteins. The NDP kinase had the following properties. (1) The enzyme existed in a low-molecular-weight form and in association with the complex of microtubule-associated proteins and tubulin (i.e. multimeric tubulin). (2) The low-molecular-weight species was also formed by dissociation of multimeric tubulin by salt or by removal of microtubule-associated proteins on phosphocellulose. (3) GDP bound to the exchangeable site of multimeric tubulin and also GDP derived from the E site of the tubulin dimer was a substrate for the NDP kinase. (4) The NDP kinase showed a 7-fold increase in activity during ATP-dependent microtubule assembly. On the basis of these properties, it is proposed that microtubule protein contains an NDP kinase specifically associated with tubulin and its functions.

Modification of tubulin by tyrosylation in cells and extracts and its effect on assembly in vitro

A post-translational modification of tubulin with potential regulatory significance has been revealed by the discovery of an enzyme (tubulin-tyrosine ligase) in brain extracts which can add a tyrosine residue to the alpha chain, apparently through peptide bond linkage to a C-terminal glutamate. We have investigated whether this modification also occurs in vivo, and whether it alters the extent to which tubulin can assemble in vitro. Cytoplasmic tubulin purified from bovine brain by cycles of assembly was shown to be partially tyrosylated. Carboxypeptidase A digestion of isolated alpha chains liberated about 0.3 equivalent of tyrosine. Brief digestion of native tubulin increased the proportion of alpha chains which could be tyrosylated by ligase, from 25 to 45%. The tubulin assembled to the same extent before and after carboxypeptidase treatment. When tubulin was purified after introducing labeled tyrosine with ligase, the labeled species assembled in the same proportion as unlabeled. Thus tubulin can be incorporated into microbubules in vitro with or without C-terminal tyrosine. An apparent resolution of alpha chain into two components by hydroxylapatite chromatography was shown not to be due to the presence or absence of C-terminal tyrosine. Tubulin-tyrosine ligase was found in extracts of every rat tissue examined, but was not detected in sea urchin eggs before or after fertilization, in Tetrahymena cells or cilia, or in yeast. Cultured neuroblastoma cells fixed tyrosine into tubulin alpha chains under conditions where protein synthesis was inhibited; this in vivo fixation appeared to be into an insoluble moiety of tubulin. Incidental to these studies, a new assay utilizing an enamine substrate for carboxypeptidase was investigated.