Studies of crystallization conditions for native and subtilisin-cleaved pig brain tubulin

Regulation of microtubule dynamic instability by the carboxy-terminal tail of β-tubulin

This work examines how the carboxy-terminal tail domain of β-tubulin governs microtubule dynamic instability and the structure of plus ends using complementary in vivo and in vitro experiments.

Dynamic instability is an intrinsic property of microtubules; however, we do not understand what domains of αβ-tubulins regulate this activity or how these regulate microtubule networks in cells. Here, we define a role for the negatively charged carboxy-terminal tail (CTT) domain of β-tubulin in regulating dynamic instability. By combining in vitro studies with purified mammalian tubulin and in vivo studies with tubulin mutants in budding yeast, we demonstrate that β-tubulin CTT inhibits microtubule stability and regulates the structure and stability of microtubule plus ends. Tubulin that lacks β-tubulin CTT polymerizes faster and depolymerizes slower in vitro and forms microtubules that are more prone to catastrophe. The ends of these microtubules exhibit a more blunted morphology and rapidly switch to disassembly after tubulin depletion. In addition, we show that β-tubulin CTT is required for magnesium cations to promote depolymerization. We propose that β-tubulin CTT regulates the assembly of stable microtubule ends and provides a tunable mechanism to coordinate dynamic instability with ionic strength in the cell.

Prion protein inhibits microtubule assembly by inducing tubulin oligomerization

A growing body of evidence points to an association of prion protein (PrP) with microtubular cytoskeleton. Recently, direct binding of PrP to tubulin has also been found. In this work, using standard light scattering measurements, sedimentation experiments, and electron microscopy, we show for the first time the effect of a direct interaction between these proteins on tubulin polymerization. We demonstrate that full-length recombinant PrP induces a rapid increase in the turbidity of tubulin diluted below the critical concentration for microtubule assembly. This effect requires magnesium ions and is weakened by NaCl. Moreover, the PrP-induced light scattering structures of tubulin are cold-stable. In preparations of diluted tubulin incubated with PrP, electron microscopy revealed the presence of approximately 50 nm disc-shaped structures not reported so far. These unique tubulin oligomers may form large aggregates. The effect of PrP is more pronounced under the conditions promoting microtubule formation. In these tubulin samples, PrP induces formation of the above oligomers associated with short protofilaments and sheets of protofilaments into aggregates. Noticeably, this is accompanied by a significant reduction of the number and length of microtubules. Hence, we postulate that prion protein may act as an inhibitor of microtubule assembly by inducing formation of stable tubulin oligomers.

XKCM1 acts on a single protofilament and requires the C terminus of tubulin

The stability of microtubules during the cell-cycle is regulated by a number of cellular factors, some of which stabilize microtubules and others that promote breakdown. XKCM1 is a kinesin-like protein that induces microtubule depolymerization and is required for mitotic spindle assembly. We have examined the binding and depolymerization effects of XKCM1 on different tubulin polymers in order to learn about its mechanism of action. Zinc-induced tubulin polymers, characterized by an anti-parallel protofilament arrangement, are depolymerized by XKCM1, indicating that this enzyme acts on a single protofilament. GDP-tubulin rings, which correspond to the low-energy state of tubulin, are stable only under conditions that inhibit XKCM1 depolymerizing activity, but can be stabilized by XKCM1 bound to AMPPNP. Tubulin polymers made of subtilisin-treated tubulin (lacking the tubulin C-terminal tail) are resistant to XKCM1-induced depolymerization, suggesting that the interaction of the acidic tail of tubulin with basic residues in XKCM1 unique to Kin I proteins is required for depolymerization.

Crystallization of a macromolecular ring assembly of tubulin liganded with the anti-mitotic drug podophyllotoxin

The interaction of the anti-cancer drug podophyllotoxin with a high-molecular-weight assembly of tubulin has been employed to produce three-dimensional crystals from avian erythrocyte tubulin as well as from pig brain tubulin. Avian erythrocyte tubulin crystals belong to the space group C2 with unit cell dimensions a = 740 A, b = 330 A, c = 460 A, beta = 128 degrees. The basis of these crystals is ring oligomers with a molecular mass of approximately 6 x 10(6) Da. So far, the crystals diffract to 8-A resolution and a first complete data set to 12-A resolution has been collected under cryogenic conditions. The crystals grew from conventionally purified tubulin consisting of multiple isoforms and different posttranslational modifications. Thus, the use of highly homogeneous tubulin preparations should improve the diffraction quality of these crystals.

Cryo-electron microscopy of GDP-tubulin rings

Rings of guanosine diphosphate (GDP)-tubulin formed in the presence of divalent cations have been studied using conventional negative stain and cryo-electron microscopy. The structure of such rings resembles that of depolymerizing microtubule ends and corresponds to an "unconstrained" conformation of tubulin in its GDP state. The use of cryo-techniques has allowed us to image the ring polymers free from dehydration and flattening artifacts. Preparations of frozen-hydrated GDP-tubulin rings are generally heterogeneous and contain a mixture of double, triple, and incomplete rings, as well as spirals and some rare single rings. Images of different polymer types can be identified and classified into groups that are then amenable for averaging and single particle reconstruction methods. Identifying the differences in tubulin structure, between straight and curve protofilaments, will be important to understand the molecular bases of dynamic instability in microtubules.

Structure of tubulin at 6.5 A and location of the taxol-binding site

Tubulin, the major component of microtubules, is a heterodimer of two chains, alpha and beta, both of relative molecular mass 50,000 (Mr50K) and with 40-50% identity. The isotypic variety and conformational flexibility of tubulin have so far made it impossible to obtain crystals for X-ray work. Structural knowledge of tubulin has been limited to about 20 A from X-ray diffraction of oriented microtubules, and from electron microscopy of microtubules and zinc-induced crystalline sheets in negative stain. The sheets consist of protofilaments similar to those in microtubules but associated in an antiparallel arrangement, and their two-dimensional character is ideal for high-resolution electron microscopy. Here we present a three-dimensional reconstruction of tubulin to 6.5 A resolution, obtained by electron crystallography of zinc-induced two-dimensional crystals of the protein. The alpha- and beta-subunits appear topologically similar, in agreement with their sequence homology. Several features can be defined in terms of secondary structure. An apparent alpha-helical portion, adjacent to both interdimer and inter-protofilament contacts, is tentatively attributed to a segment near the carboxy terminus of the protein. We can assign the alpha- and beta-subunits on the basis of projection studies of the binding of taxol, which show one taxol site per tubulin heterodimer, in agreement with the known stoichiometry of taxol in microtubules. These studies indicate that taxol affects the interaction between protofilaments; to our knowledge, this is the first time that a ligand-binding site has been visualized in the tubulin molecule.

Assembly of purified GDP-tubulin into microtubules induced by taxol and taxotere: reversibility, ligand stoichiometry, and competition

Purified tubulin fully liganded to GDP at the exchangeable nucleotide binding site has been prepared by a new direct nucleotide exchange procedure. This normally inactive GDP-tubulin is driven to assemble into microtubules by the binding of the antitumor drug taxol or its more soluble side-chain analogue Taxotere in Mg(2+)-containing buffer, and it disassembles by cooling the solution. Therefore this ligand-induced equilibrium microtubule assembly system dispenses with the requirement of a gamma-phosphate-metal cation ligand bound at the nucleotide site for tubulin to be active. GDP-tubulin can also form characteristic pseudo-ordered aggregates of double rings. These aggregates dissociate upon warming or by addition of GTP. Back-substitution of the nucleotide gamma-phosphate permits glycerol-induced assembly without taxol and reduces the critical protein concentration required for drug-induced microtubule assembly by a factor of 2.6 +/- 0.1. The ligand-induced assembly is maximal at taxol or Taxotere concentrations equimolar with tubulin, and both drugs bind to assembled tubulin with a stoichiometry of 0.99 +/- 0.04 ligand per alpha beta dimer. Taxotere apparently competes with taxol for the same binding site, with 1.9 +/- 0.1 times larger effective affinity. Similarly, the Taxotere-induced assembly of GDP-tubulin or GTP-tubulin proceeds with a critical protein concentration 2.1 +/- 0.1 times smaller than with taxol.

Subtilisin cleavage of tubulin heterodimers and polymers

Native pig brain tubulin in heterodimer or polymer form was subjected to limited proteolysis by subtilisin, which is known to cleave at accessible sites within the last 50 amino acids of the highly variable carboxyl-termini of the alpha and beta subunits. Heterodimeric tubulin or tubulin polymerized in the presence of 4 M glycerol or taxol was used in these experiments. Digested tubulin was purified by cycles of polymerization and depolymerization, ammonium sulfate precipitation, or ion-exchange chromatography in the absence or presence of nonionic detergent; however, smaller cleaved products of about 34,000 to 40,000 MW remained associated with the major cleaved subunits, alpha' and beta', under all purification conditions. In order to determine the effect of subtilisin cleavage on tubulin heterogeneity, purified native or subtilisin-cleaved tubulin was subjected to isoelectric focusing, followed by SDS-PAGE. The total number of isotypes was reduced from 17-22 for native alpha,beta tubulin to 7-9 for subtilisin-cleaved alpha',beta' tubulin. When tubulin heterodimers were cleaved, a single major beta' isotype was evident; however, when tubulin polymerized in 4 M glycerol was cleaved, two major beta' isotypes were found. Monoclonal antibodies that recognize a beta carboxyl-terminal peptide, residues 410-430, reacted with both major beta' isotypes, indicating that subtilisin cleavage occurred within the last 20 of the 450 amino acids. In order to establish whether this difference was in fact associated with polymer or heterodimer forms of tubulin, digestion was carried out in the presence of taxol, which stabilizes tubulin polymers. A single major beta' isotype different from the cleaved heterodimer, but coincident with one of the bands of the cleaved glycerol-induced polymers, was found when taxol-treated tubulin was digested. This result suggests the presence of more than one subtilisin site in the beta subunit, near residues 430-435, with different accessibility to the enzyme in the heterodimer and polymer form.