Preparation and characterization of des-C-terminal tubulin

Quantitative analysis of tau-microtubule interaction using FRET

The interaction between the microtubule associated protein, tau and the microtubules is investigated. A fluorescence resonance energy transfer (FRET) assay was used to determine the distance separating tau to the microtubule wall, as well as the binding parameters of the interaction. By using microtubules stabilized with Flutax-2 as donor and tau labeled with rhodamine as acceptor, a donor-to-acceptor distance of 54 ± 1 Å was found. A molecular model is proposed in which Flutax-2 is directly accessible to tau-rhodamine molecules for energy transfer. By titration, we calculated the stoichiometric dissociation constant to be equal to 1.0 ± 0.5 µM. The influence of the C-terminal tails of αβ-tubulin on the tau-microtubule interaction is presented once a procedure to form homogeneous solution of cleaved tubulin has been determined. The results indicate that the C-terminal tails of α- and β-tubulin by electrostatic effects and of recruitment seem to be involved in the binding mechanism of tau.

Preparation and properties of pure tubulin S

Limited proteolysis of the tubulin dimer (alphabeta) by subtilisin occurs more rapidly with beta than with alpha tubulin. This leads to the formation of an intermediate hybrid dimer, alphabeta(s), before both C termini are cleaved to form tubulin S(alpha(s)beta(s)). The three forms of tubulin usually coexist in subtilisin-treated preparations and such cross-contamination can be reliably detected only by running SDS-polyacrylamide gels well beyond expulsion of the dye front. Previously published preparations have not ruled out such contamination or have formed poorly reversible polymers. Because ion exchange separation incurred substantial protein losses, we have developed a new protocol for rapid preparation of tubulin S (alpha(s)beta(s), free of alphabeta or alphabeta(s)) that is based on proteolysis at low ionic strength. This increases the relative rate of C terminal cleavage of beta tubulin. The product forms sheets, bundles, or rings that are depolymerized by cold, salt, and podophyllotoxin, partially depolymerized by Ca2+, and has a decreased critical concentration for polymerization that can be further decreased by taxol. We have also found a method for forming nearly pure alphabeta(s) dimers by using methods that retard proteolysis of the C terminus of alpha tubulin.

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)

Multiple sites for subtilisin cleavage of tubulin: effects of divalent cations

Limited digestion of pig brain GDP-tubulin by subtilisin was carried out in the presence of Mg2+, Mn2+, Ca2+, Zn2+, or Be2+. Isoelectric focusing, followed by SDS-PAGE, revealed characteristic divalent cation-dependent changes in the alpha- and beta-tubulin cleavage patterns. Previous studies revealed that the beta-cleavage pattern is different for heterodimers and microtubules [Lobert and Correia, 1992: Arch. Biochem. Biophys. 296: 152-160]. Divalent cation effects on subtilisin digestion of tubulin indicate different classes of divalent cation binding sites. Western blot analysis locates the proteolytic zone at residue 430 or higher in both subunits for all conditions. Turbidity and electron microscopy reveal that GDP-tubulin cleaved by subtilisin in the presence of Mg2+, Ca2+, or Mn2+ forms sheets of rings. Mn2+ induces ring formation in uncleaved GDP-tubulin. Isotype-depleted tubulin was generated by the removal of class III beta-tubulin using immunoaffinity chromatography. Subtilisin digestion of the depleted fraction and the purified class III beta-tubulin demonstrates that cleavage occurs at three to four distinct sites. Thus, subtilisin-digested tubulin is more heterogeneous than was previously reported and the cleavage sites depend on solution conditions, divalent cations, and the state of assembly. This has important implications for experiments that utilize subtilisin-digested tubulin for studying microtubule-associated protein binding.

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.

Bundling of microtubules by synapsin 1. Characterization of bundling and interaction of distinct sites in synapsin 1 head and tail domains with different sites in tubulin

Synapsin 1 is a nerve terminal phosphoprotein whose role seems to encompass the linking of small synaptic vesicles to the cytoskeleton. Synapsin 1 can join small synaptic vesicles to neuronal spectrin, microfilaments and microtubules; it can also bundle microtubules and microfilaments. In this paper, the mode of interaction between synapsin 1 and microtubules has been investigated. Bundling is shown to be highly cooperative: the apparent Hill coefficient is 3.06 +/- 0.3, and bundling is half-maximal at 0.63 +/- 0.02 microM. Bundling occurs either when whole synapsin 1 preparations (containing monomers and oligomers) or when monomeric synapsin 1 is added to microtubules. However, it is not clear that synapsin 1 remains monomeric in the presence of microtubules. Synapsin 1-microtubule mixtures contain two types of filament. One type is characterised by microtubules often with synapsin 1 bound to their surface. The other type is composed of filaments of diameter 15 +/- 5 nm. This filament type is granular and made up in part of 14-nm-diameter particles. These dimensions are consistent with their being made up of polymerised synapsin 1. It is possible that microtubules induce the polymerisation of synapsin 1. Synapsin 1 had independent tubulin binding sites in the N-terminal head domain and in the C-terminal tail domain. Whole synapsin 1 can interact with tubulin after it has been digested to remove the tubulin C terminus (des-C-terminal tubulin). The interaction of des-C-terminal tubulin with synapsin 1 appears to be via the head domain, since 125I-des-C-terminal tubulin only shows specific binding to the head domain on gel blots. By contrast intact tubulin binds to both head and tail domains. Binding to the tail domain can be inhibited by a synthetic peptide representing the microtubule-associated protein 2 (MAP2) binding site of class II beta tubulin. These results suggest a model for microtubule bundling by synapsin 1 in which independent sites in the head and tail domains of synapsin 1 cross-link microtubules by interactions with two distinct sites in tubulin.

A novel microtubule-binding motif identified in a high molecular weight microtubule-associated protein from Trypanosoma brucei

The major component of the cytoskeleton of the parasitic hemoflagellate Trypanosoma brucei is a membrane skeleton which consists of a single layer of tightly spaced microtubules. This array encloses the entire cell body, and it is apposed to, and connected with, the overlying cell membrane. The microtubules of this array contain numerous microtubule-associated proteins. Prominent among those is a family of high molecular weight, repetitive proteins which consist to a large extent of tandemly arranged 38-amino acid repeat units. The binding of one of these proteins, MARP-1, to microtubules has now been characterized in vitro and in vivo. MARP-1 binds to microtubules via tubulin domains other than the COOH-termini used by microtubule-associated proteins from mammalian brain, e.g., MAP2 or Tau. In vitro binding assays using recombinant protein, as well as transfection of mammalian cell lines, have established that the repetitive 38-amino acid repeat units represent a novel microtubule-binding motif. This motif is very similar in length to those of the mammalian microtubule-associated proteins Tau, MAP2, and MAP-U, but both its sequence and charge are different. The observation that the microtubule-binding motifs both of the neural and the trypanosomal proteins are of similar length may reflect the fact that both mediate binding to the same repetitive surface, the microtubule, while their sequence and charge differences are in agreement with the observation that they interact with different domains of the tubulins.

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

A survey of crystallization conditions for pig brain tubulin, using standard vapor diffusion techniques in sitting drops or capillaries, has resulted in irregular, fragile needles or plates with a largest dimension of 0.5 mm. These occurred in 2.5% PEG (MW 3350), 0.1 M Pipes, pH 6.2 and 6.4, 2-16 mM MgSO4, 10-15 mM DTE, and 0.1 mM GDP at 8 degrees C. When GTP replaced GDP these aggregates did not form under any of the conditions surveyed (temperature: 8-10 degrees C; MgSO4: 2-16 mM; pH 6-7; PEG, MW 3350: 1.25-12.5%). EM observations demonstrated that sheets of rings appear in crystal solutions in the presence of GDP or GTP. These results are consistent with the results of Howard and Timasheff (1986, Biochemistry 25, 8292-8300) that tubulin rings form in the presence of GDP or GTP but more readily in GDP. Tubulin crystallization experiments are hampered by tubulin's high degree of heterogeneity. Much of the variability lies in the carboxyl terminal region. Conditions for limited digestion of the heterodimer by subtilisin, removing only the carboxyl terminus, were determined. Reduction of heterogeneity was demonstrated by isoelectric focusing. The solubilities of native and subtilisin-cleaved tubulin in MgSO4, (NH4)2SO4, PEG (MW 1450, 3350, 10,000), DMSO, and MPD were compared. Subtilisin-cleaved tubulin precipitated more readily than native tubulin under all conditions surveyed, consistent with the removal of the highly acidic carboxyl terminus. Vapor diffusion experiments using subtilisin-cleaved tubulin under conditions where native tubulin forms needles or plates resulted in similar aggregates.