Effect of guanine nucleotides on the hydrophobic interaction of tubulin

Inhibition of assembly of bacterial cell division protein FtsZ by the hydrophobic dye 5,5'-bis-(8-anilino-1-naphthalenesulfonate)

To gain further insight into the structural relatedness of tubulin and FtsZ, the tubulin-like prokaryotic cell division protein, we tested the effect of tubulin assembly inhibitors on FtsZ assembly. Common tubulin inhibitors, such as colchicine, colcemid, benomyl, and vinblastine, had no effect on Ca2+-promoted GTP-dependent assembly of FtsZ into polymers. However, the hydrophobic probe 5, 5'-bis-(8-anilino-1-naphthalenesulfonate) (bis-ANS) inhibited FtsZ assembly. The potential mechanisms for inhibition are discussed. Titrations of FtsZ with bis-ANS indicated that FtsZ has one high affinity binding site and multiple low affinity binding sites. ANS (8-anilino-1-naphthalenesulfonate), a hydrophobic probe similar to bis-ANS, had no inhibitory effect on FtsZ assembly. Because tubulin assembly has also been shown to be inhibited by bis-ANS but not by ANS, it supports the idea that FtsZ and tubulin share similar conformational properties. Ca2+, which promotes GTP-dependent FtsZ assembly, stimulated binding of bis-ANS or ANS to FtsZ, suggesting that Ca2+ binding induces changes in the hydrophobic conformation of the protein. Interestingly, depletion of bound Ca2+ with EGTA further enhanced bis-ANS fluorescence. These findings suggest that both binding and dissociation of Ca2+ are capable of inducing FtsZ conformational changes, and these changes could promote the GTP-dependent assembly of FtsZ.

Energy transfer studies of the distances between the colchicine, ruthenium red, and bisANS binding sites on calf brain tubulin

Fluorescence energy transfer experiments were performed in order to measure the spatial separation between the colchine and Ruthenium Red binding sites, the high-affinity bisANS and Ruthenium Red sites, and the allocolchicine and high-affinity bisANS sites on calf brain tubulin. Energy transfer was observed between both colchicine and allocolchicine and Ruthenium Red, resulting in a distance of 40-45 A between these sites on the tubulin molecule. No detectable energy transfer could be observed when allocolchicine was used as fluorescence donor and bisANS as acceptor or when bisANS was used as donor and Ruthenium Red as acceptor. This indicates that the distance of separation between the allocolchicine and bisANS sites is greater than 50 A, while that between the bisANS and Ruthenium Red sites is greater than 72 A. On the basis of these and previous distance measurements (Ward & Timasheff, 1988), two triangles of binding sites have been defined (colchicine-bisANS-E-site and colchicine-bisANS-Ruthenium Red). Since the dihedral angle between them is not known, a schematic model has been drawn with all the sites located in a single plane.(ABSTRACT TRUNCATED AT 250 WORDS)

The disaggregation theory of signal transduction revisited: further evidence that G proteins are multimeric and disaggregate to monomers when activated

We have compared the sedimentation rates on sucrose gradients of the heterotrimeric GTP-binding regulatory (G) proteins Gs, G(o), Gi, and Gq extracted from rat brain synaptoneurosomes with Lubrol and digitonin. The individual alpha and beta subunits were monitored with specific antisera. In all cases, both subunits cosedimented, indicating that the subunits are likely complexed as heterotrimers. When extracted with Lubrol all of the G proteins sedimented with rates of about 4.5 S (consistent with heterotrimers) whereas digitonin extracted 60% of the G proteins with peaks at 11 S; 40% pelleted as larger structures. Digitonin-extracted Gi was cross-linked by p-phenylenedimaleimide, yielding structures too large to enter polyacrylamide gels. No cross-linking of Lubrol-extracted Gi occurred. Treatment of the membranes with guanosine 5'-[gamma-thio]triphosphate and Mg2+ yielded digitonin-extracted structures with peak sedimentation values of 8.5 S--i.e., comparable to that of purified G(o) in digitonin and considerably larger than the Lubrol-extracted 2S structures representing the separated alpha and beta gamma subunits formed by the actions of guanosine 5'-[gamma-thio]triphosphate. It is concluded that the multimeric structures of G proteins in brain membranes are at least partially preserved in digitonin and that activation of these structures in membranes yields monomers of G proteins rather than the disaggregated products (alpha and beta gamma complexes) observed in Lubrol. It is proposed that hormones and GTP affect the dynamic interplay between multimeric G proteins and receptors in a fashion analogous to the actions of ATP on the dynamic interactions between myosin and actin filaments. Signal transduction is mediated by activated monomers released from the multimers during the activation process.

Site-directed mutagenesis of the GTP-binding domain of beta-tubulin

Tubulin binds guanine nucleotides with high affinity and specificity. GTP, an allosteric effector of microtubule assembly, requires Mg2+ for its interaction with beta-tubulin and binds as the MgGTP complex. In contrast, GDP binding does not require Mg2+. The structural basis for this difference is not understood but may be of fundamental importance for microtubule assembly. We investigated the interaction of beta-tubulin with guanine nucleotides using site-directed mutagenesis. Acidic amino acid residues have been shown to interact with nucleotide in numerous nucleotide-binding proteins. In this study, we mutated seven highly conserved aspartic acid residues and one highly conserved glutamic acid residue in the putative GTP-binding domain of beta-tubulin (N-terminal 300 amino acids) to asparagine and glutamine, respectively. The mutants were synthesized in vitro using rabbit reticulocyte lysates, and their affinities for nucleotide determined by an h.p.l.c.-based assay. Our results indicate that the mutations can be placed in six separate categories on the basis of their effects on nucleotide binding. These categories range from having no effect on nucleotide binding to a mutation that apparently abolishes nucleotide binding. One mutation at Asp224 reduced the affinity of beta-tubulin for GTP in the presence but not in the absence of Mg2+. The specific effect of this mutation on nucleotide binding is consistent with an interaction of this amino acid with the Mg2+ moiety of MgGTP. This residue is in a region sharing sequence homology with the putative Mg2+ site in myosin and other ATP-binding proteins. As a result, tubulin belongs to a distinct class of GTP-binding proteins which may be evolutionarily related to the ATP-binding proteins.

Tubulin-G protein interactions involve microtubule polymerization domains

It has been suggested that elements of the cytoskeleton contribute to the signal transduction process and that they do so in association with one or more members of the signal-transducing G protein family. Relatively high-affinity binding between dimeric tubulin and the alpha subunits of Gs and Gi1 has also been reported. Tubulin molecules, which exist in solution as alpha beta dimers, have binding domains for microtubule-associated proteins as well as for other tubulin dimers. This study represents an attempt to ascertain whether the association between G proteins and tubulin occurs at one of these sites. Removal of the binding site for MAP2 and tau from tubulin by subtilisin proteolysis did not influence the association of tubulin with G protein, as demonstrated in overlay studies with [125I]tubulin. A functional consequence of that association, the stable inhibition of synaptic membrane adenylyl cyclase, was also unaffected by subtilisin treatment of tubulin. However, ring structures formed from subtilisin-treated tubulin were incapable of effecting such inhibition. Stable G protein-tubulin complexes were formed, and these were separated from free tubulin by Octyl-Sepharose chromatography. Using this methodology, it was demonstrated that assembled microtubules bound G protein quite weakly compared with tubulin dimers. The alpha subunit of Gi1 and, to a lesser extent, that of Go were demonstrated to inhibit microtubule polymerization. In aggregate, these data suggest that dimeric tubulin binds to the alpha subunits of G protein at the sites where it binds to other tubulin dimers during microtubule polymerization. Interaction with signal-transducing G proteins, thus, might represent a role for tubulin dimers which is independent of microtubule formation.

Effects of antimitotic agents on tubulin-nucleotide interactions

The interaction of antimitotic drugs with guanine nucleotides in the tubulin-microtubule system is reviewed. Antimitotic agent-tubulin interactions can be covalent, entropic, allosteric or coupled to other equilibria (such as divalent cation binding, alternate polymer formation, or the stabilization of native tubulin structure). Antimitotics bind to tubulin at a few common sites and alter the ability of tubulin to form microtubules. Colchicine and podophyllotoxin compete for a common overlapping binding site but only colchicine induces GTPase activity and large conformational changes in the tubulin heterodimer. The vinca alkaloids, vinblastine and vincristine, the macrocyclic ansa macrolides, maytansine and ansamitocin P-3, and the fungal antimitotic, rhizoxin, share and compete for a different binding site near the exchangeable nucleotide binding site. The macrocyclic heptapeptide, phomopsin A, and the depsipeptide, dolastatin 10, bind to a site adjacent to the vinca alkaloid and nucleotide sites. Colchicine, vinca alkaloids, dolastatin 10 and phomopsin A induce alternate polymer formation (sheets for colchicine, spirals for vinblastine and vincristine and rings for dolastatin 10 and phomopsin A). Maytansine, ansamitocin P-3 and rhizoxin inhibit vinblastine-induced spiral formation. Taxol stoichiometrically induces microtubule formation and, in the presence of GTP, assembly-associated GTP hydrolysis. Analogs of guanine nucleotides also alter polymer morphology. Thus, sites on tubulin for drugs and nucleotides communicate allosterically with the interfaces that form longitudinal and lateral contacts within a microtubule. Microtubule associated proteins (MAPs), divalent cations, and buffer components can alter the surface interactions of tubulin and thus modulate the interactions between antimitotic drugs and guanine nucleotides.