CELL STRUCTURE AND FUNCTION 24-: 329—335 (1999)
`(C) 1999 by Japan Society for Cell Biology
`
`Modulation of Microtubule Dynamics by Drugs: A Paradigm for the Actions
`of Cellular Regulators
`
`Leslie Wilson*, Dulal Panda, and Mary Ann Jordan
`
`Department of Molecular, Cellular, and Developmental Biology, University of California, Santa Barbara, CA
`93106, USA
`
`ABSTRACT. Microtubules are intrinsically dynamic polymers. Two kinds of dynamic behaviors, dynamic in-
`stability and treadmilling, are important for microtubule function in cells. Both dynamic behaviors appear to
`be tightly regulated, but the cellular molecules and the mechanisms responsible for the regulation remain
`largely unexplored. While microtubule dynamics can be modulated transiently by the interaction of regula-
`tory molecules with soluble tubulin, the microtubule itself is likely to be the primary target of cellular mole-
`cules that regulate microtubule dynamics. The antimitotic drugs that modulate microtubule dynamics serve as
`excellent models for such cellular molecules. Our laboratory has been investigating the interactions of small
`drug molecules and stabilizing microtubule-associated proteins (MAPS) with microtubule surfaces and ends. We
`find that drugs such as colchicine, vinblastine, and taxol, and stabilizing MAPs such as tau, strongly modu-
`late microtubule dynamics at extremely low concentrations under conditions in which the microtubule poly-
`mer mass is minimally alfected. The powerful modulation of the dynamics is brought about by the binding of
`only a few drug or MAP molecules to distinct binding sites at the microtubule surface or end. Based upon our
`understanding of the well-studied drugs and stabilizing MAPs, it is clear that molecules that regulate dynam-
`ics such as Kin 1 and stathmin could bind to a large number of distinct tubulin sites on microtubules and
`employ an array of mechanisms to selectively and powerfully regulate microtubule dynamics and dynamics-
`dependent cellular functions.
`
`Key words:
`
`microtubules/dynamics/antimitotic drugs/microtubule-associated proteins/mitosis
`
`Microtubule dynamics
`
`Treadmilling and Dynamic Instability
`Microtubules are not simple equilibrium polymers
`(e.g., 6, 14, for recent reviews). Soluble tubulin binds
`GTP reversibly at a site in the ,8 subunit and the GTP
`becomes hydrolyzed to GDP and P, as or shortly after
`the tubulin polymerizes onto a growing microtubule
`end. The irreversible hydrolysis of GTP during tubu-
`lin addition to the microtubule creates two unique dy—
`namic behaviors. One such behavior, treadmilling (11,
`13, 14), characterized by net growth at the plus end of a
`microtubule and net shortening at the minus end, is due
`to the difference in critical tubulin subunit concentra-
`
`tions at plus and minus ends. The second behavior,
`dynamic instability (10, 15, 28),
`is characterized by
`
`
`* To whom correspondence should be addressed.
`Tel: +1—805—893—2819, Fax: +1—805—893—8094
`E—mail: wilson@lifesci.lscf.ucsb.edu
`
`switching at microtubule ends between episodes of rela-
`tively slow sustained growth and rapid shortening. At
`or near steady state, both behaviors can coexist
`in
`microtubule populations. Even when an individual
`microtubule continually changes its overall length due
`to extensive dynamic instability excursions, net tubulin
`addition occurs at plus ends and loss at minus ends
`(Fig.
`l). The extent to which individual microtubule
`populations display treadmilling and dynamic instabil-
`ity behaviors appears highly dependent upon the con-
`ditions (7).
`
`Spindle microtubule dynamics
`Treadmilling and dynamic instability behaviors ap-
`pear to be critical to many microtubule—dependent cell
`functions. Rapid microtubule dynamics are especially
`prominent in mitosis and are essential for proper spin-
`dle assembly and function. In interphase cells, microtu—
`bules exchange their tubulin with soluble tubulin in the
`cytoplasmic pool with half times of several min to sev—
`
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`minus end
`
`plus end
`
`L. Wilson et a].
`
`
`
`Fig. 1. Simultaneous treadmilling and dynamic instability, producing a net flow of tubulin subunits from plus to minus ends. Shown are
`consecutive “snapshots” of a microtubule exhibiting growth and shortening at plus and minus ends, with net growth at the plus end and net
`shortening at the minus end. The shaded subunits represent a marked segment. To:zero time; t1 to t6 represent arbitrary equal units of time.
`
`eral hr. However, with the onset of mitosis, a popula—
`tion of highly dynamic mitotic spindle microtubules re-
`places the interphase microtubules. These microtubules
`are 10 to 100 times more dynamic than microtubules in
`interphase cells (1, 23). The extremely rapid dynamics
`of mitotic spindle microtubules, which are highly sensi-
`tive to modulation by antimitotic drugs, play a crucial
`role in the intricate movements of the chromosomes.
`
`For example, extensive growth and shortening excur-
`sions appear to be responsible for the initial attach-
`ment of the chromosomes at their kinetochores to the
`
`forming spindle. Rapid treadmilling of microtubules
`also occurs during metaphase and anaphase (16) where
`it may be involved in the flow of signals from kineto—
`chores to the poles (14).
`
`Kinetic modulation of microtubule dynamics by anti-
`mitotic drugs as paradigms for the actions of cellular
`regulatory molecules
`
`A large and chemically diverse number of natural
`product drugs inhibit cell proliferation by acting on
`spindle microtubules. Most of the known antimitotic
`
`agents including colchicine and the anticancer drug vin-
`blastine inhibit microtubule polymerization.
`In con-
`trast, the anti-cancer drug taxol has an opposite action
`on microtubules; stimulation of polymerization and
`stabilization of spindle microtubules. Not surprisingly,
`the actions of these antimitotic drugs on spindle func-
`tion were thought to be caused by reduction or en—
`hancement of the spindle microtubule mass. However,
`a sophisticated understanding is now emerging at the
`molecular level of the mechanisms by which these com-
`pounds act on microtubule polymerization and dynam-
`ics. The early idea that these agents acted on microtu-
`bule-dependent cell functions only by destroying the
`microtubules or preventing their polymerization was
`simplistic. These drugs bind to the surfaces or ends of
`microtubules at discrete sites and modulate polymeriza-
`tion dynamics at drug concentrations well below those
`required to change the polymer mass (Table I). It
`now seems likely that the natural product drugs have
`evolved to mimic the actions of endogenous cellular
`molecules whose functions involve regulation of micro-
`tubule dynamics. The drugs can be used as tools to
`probe the roles of microtubule dynamics in cell func-
`
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`Modulation of Microtubule Dynamics
`
`Table 1.
`EFFECTS or COLCHICINE, VINBLASTINE, TAXOL, AND TAU ON THE DYNAMIC INSTABILITY
`or: INDIVIDUAL MICROTU’BULES AT STEADY STATE
`
`
`Colchicine
`Vinblastine
`Taxol
`Tau
`
`Concentration (yM)
`0.05
`0.1
`0.1
`0.15
`
`
`
` Percent of Control
`
`75
`69
`66
`67
`Growing rate
`56
`39
`52
`53
`Shortening rate
`100
`100
`71
`71
`Catastrophe frequency
`200
`60
`295
`195
`Rescue frequency
`38
`53
`39
`63
`Dynamicity
`
`Polymer mass 117 95 92 136
`
`
`
`
`Colchicine data are taken from Panda et a]. (19); vinblastine data are from Panda et al. (21); taxol
`data are from Derry et al. (4); and the tau data are from Panda er al. (20).
`
`tions. However, their mechanisms can also be consid-
`ered as paradigms for the mechanisms of action of cel—
`lular molecules that regulate microtubule dynamics.
`The mechanisms of some of the best understood drugs
`are described below in the context of their serving as
`such paradigms.
`
`Colchicine
`
`Colchicine has played a fundamental role in elucida-
`tion of the properties and functions of tubulin and
`microtubules since it was first found to bind to “the
`subunits of microtubules” in cell extracts more than 30
`years ago (2, 3, 29). The binding reaction between col-
`chicine and tubulin is a two-step process that begins
`with formation of a reversible, low—affinity pre-equilib—
`rium complex. This is followed by one or more slow
`steps in which conformational changes in tubulin lead
`to formation of a poorly reversible final—state tubulin-
`colchicine (TC) complex (reviewed in 8, 31). While the
`nature of the slow conformational changes that occur
`during formation of the final state TC complex re-
`main unknown, it seems likely that the conformational
`changes are responsible for the powerful effects of TC
`complex on tubulin exchange at microtubule ends. The
`specific location of the colchicine binding site in tubu-
`lin is not known precisely, but appears to be located at
`the interface between the a and 13 tubulin subunits in
`tubulin dimers (18).
`Colchicine inhibits microtubule polymerization in
`vitro at concentrations well below the concentration of
`
`tubulin free in solution (reviewed in 31), indicating that
`it inhibits microtubule polymerization by binding to
`microtubule ends rather than to the soluble tubulin
`
`pool. Free colchicine itself either may not bind directly
`to microtubule ends or it does so very inefficiently. In-
`stead, it must first bind to soluble tubulin and form a
`final-state TC complex, which then incorporates at the
`ends in small numbers along with large numbers of free
`tubulin molecules (25, 26). When TC complexes are in—
`
`corporated at microtubule ends, the ends remain com-
`petent to grow.
`Significantly, low concentrations of TC complex in-
`hibit tubulin exchange at microtubule ends at concen-
`trations far below those required to induce microtu-
`bule disassembly. For example, in an experiment using
`0.1 yM TC complex, incorporation of only 1—2 mole-
`cules of colchicine per plus end inhibited by 50% the
`rate of tubulin incorporation by treadmilling. As time
`progressed TC—complexes were continuously incorpo-
`rated with very little change in inhibition (25). Inhibi-
`tion occurred in the absence of significant microtubule
`depolymerization.
`Early experiments with radiolabeled GTP-tubulin in-
`dicated that incorporation of TC complex at microtu-
`bule ends stabilized the ends; a surprising action for a
`drug that inhibits microtubule polymerization. Direct
`evidence that TC complex kinetically stabilizes plus
`ends of microtubules has been obtained by video mi-
`croscopy (19). Specifically, when added at low con-
`centrations to steady-state MAP—free microtubules,
`TC complex strongly reduces the rate and extent both
`of growing and shortening (Table I). In addition, it
`strongly increases the percentage of time the microtu-
`bules remain in an attenuated state, neither growing nor
`shortening detectably. It also strongly increases the fre—
`quency of switching from shortening to growth or to
`an attenuated state (the rescue frequency) and strongly
`decreases the frequency of switching from growth or
`attenuation to shortening (the catastrophe frequency)
`(Table 1). Thus, the switching mechanism responsible
`for gain and loss of the stabilizing cap at plus ends ap—
`pears to be affected by colchicine.
`We hypothesize that the conformational changes in—
`duced in tubulin by the binding of colchicine play a
`major role in the ability of TC complexes to stabilize
`microtubule ends. TC complexes may assume a confor-
`mation that disrupts the tubulin lattice at or near the
`end in a way that impairs the efficiency of new tubulin
`
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`addition but does not destroy the ability of tubulin to
`be incorporated.
`Importantly,
`the incorporated TC
`complex must bind more tightly to its tubulin neigh-
`bors so that the normal rate and extent of tubulin dis-
`
`sociation is reduced. Finally, because TC complexes
`strongly reduce the catastrophe frequency and increase
`the rescue frequency, we hypothesize that TC complex
`may modulate the mechanism responsible for gain and
`loss of the stabilizing GTP cap.
`
`Vinblastine
`
`Vinblastine binds to tubulin dimers at a site that ap-
`pears to be located on the ,8 subunit. The site appears to
`be exposed at the plus ends of the microtubule (18). It is
`distinct from the colchicine site. Vinblastine binds with
`
`high aflinity to tubulin at the microtubule ends and with
`markedly reduced aflinity to tubulin buried in the tubu-
`lin lattice (30). Vinblastine exerts two distinct actions on
`microtubule polymerization and dynamics that appear
`to be due to binding of the drug to these two classes of
`sites. Vinblastine inhibits tubulin exchange at plus
`ends of MAP-rich microtubules by 50% when only 1—2
`Vinblastine molecules are bound at the end (see 31).
`Like TC complex,
`low concentrations of Vinblastine
`(in the sub—micromolar range) powerfully suppress both
`dynamic instability and treadmilling behaviors without
`appreciably depolymerizing the microtubule.
`Vinblastine suppresses dynamic instability behavior
`selectively at plus ends (21, 27). The drug strongly re-
`duces the rate and extent of growing and shortening and
`increases the percentage of time microtubules spend in
`an attenuated state, neither growing nor shortening de-
`tectably. Also, like TC complex, Vinblastine increases
`the rescue frequency and decreases the catastrophe fre—
`quency, indicating that its action may involve stabiliza—
`tion of the GTP cap. Vinblastine induces a conforma-
`tional change in tubulin that increases the affinity of
`tubulin for itself (12, 17) an action that undoubtedly
`is responsible for the increased stability of the micro-
`tubule ends. It is quite interesting, that at low concen-
`trations, Vinblastine and colchicine, which act by differ-
`ent molecular mechanisms at different tubulin binding
`sites, exert similar stabilizing effects on microtubule dy—
`namics. Consistent with the postulated exposure of the
`Vinblastine binding sites at the extreme plus ends and
`absence at the minus ends (18), Vinblastine does not
`suppress dynamics at minus ends but in contrast, it de-
`stabilizes these ends (21).
`Binding of Vinblastine to microtubules at somewhat
`higher concentrations (>5 micromolar) depolymerizes
`the microtubules by peeling of protofilaments at both
`microtubule ends (Fig. 2). Thus, binding of Vinblastine
`to the low afiinity sites appears to weaken the lateral in-
`teractions between protofilaments. At still higher con-
`centrations (>100 nM), the drug induces formation of
`
`L. Wilson et a1.
`
`paracrystalline arrays composed of tubulin in the form
`of oriented protofilaments and bound Vinblastine, both
`in cells and in vitro (see 31).
`Thus, the Vinblastine binding site represents another
`site in tubulin involved in possible regulation of dy-
`namics. It is of potential significance in the context of
`anticipating how cellular regulators of microtubule
`polymerization and dynamics might act that microtu-
`bule polymerization and dynamics respond diiferently
`to Vinblastine, depending upon the location of the drug
`binding sites in the microtubule lattice. Interestingly,
`recent evidence indicates that Kin 1, a kinesin—like pro-
`tein, induces microtubule depolymerization by induc-
`ing protofilament peeling at microtubule ends in a man—
`ner that appears similar to that of Vinblastine (5). Con-
`ceivably, this protein, postulated to be a regulator of
`microtubule polymerization, may be acting at the Vin-
`blastine binding site and inducing depolymerization by
`a mechanism similar to that of Vinblastine.
`
`Taxol
`
`The taxol binding site is yet another distinct site in
`microtubules that may play an important role in the
`regulation of dynamics. The site appears to be located
`on the inside microtubule surface, and access to it by
`taxol might occur through small pores in the surface
`lattice (18). Taxol profoundly affects the polymeriza-
`tion characteristics and stability of microtubules. It
`stimulates microtubule polymerization in vitro, pro-
`moting nucleation and reducing the critical tubulin sub-
`unit concentration (i.e., soluble tubulin concentration
`at steady state) to near zero (9, 24).
`Analysis of the eifects of low taxol concentrations on
`dynamic inStability indicates that the drug powerfully
`suppresses the rate and extent of shortening and great-
`ly increases the percentage of time the microtubules
`spend in the attenuated state in the absence of apprecia-
`ble change in the polymer mass (4). Like the actions of
`TC complex and Vinblastine, a small ratio of bound
`taxol molecules to total tubulin in a microtubule is
`
`sufficient to produce a high degree of stabilization. For
`example, one taxol molecule bound per 100 tubulin
`molecules inhibits the shortening rate 50% (4). Such
`data suggest that small numbers of bound regulatory
`molecules that act by stabilizing microtubule dynamics
`might act in analogy to taxol by binding to only a very
`few tubulin molecules in the microtubule polymer. In
`support of this idea, very low ratios of the microtu-
`bule—stabilizing MAP tau bound to tubulin in a micro-
`tubule are sufliczent to induce a powerful suppression of
`dynamics (20). [nterestingly, the suppressive action of
`small numbers of tau molecules on microtubule dynam-
`ics is remarkably similar to that of taxol. In addition,
`small peptides representing the tubulin binding do-
`mains of tau suppress dynamics in a manner qualita-
`
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`Modulation of Microtubule Dynamics
`
`
`
`Fig. 2. Vinblastine—induced protofilament peeling. A microtubule end with attached spirals, negatively stained without agitation after incuba—
`tion of a steady-state suspension of microtubules with Vinblastine (200 ,uM) for 2 min (1.1 mg microtubule protein/ml). This high vinblastine
`concentration rapidly induces extensive protofilament peeling. A drop of microtubule suspension was placed directly onto a parlodion and
`carbon—coated grid to which one drop of buffered 40% sucrose had previously been applied. Following aspiration of the liquid, cytochrome
`c (1 mg/ml in water) followed by 3 drops water and 1% w/uranyl acetate were applied sequentially for 15—20 5 and then aspirated. The bar:
`100 nm.
`
`tively similar to that of full-length tau. Because the tau
`binding site and taxol binding site are almost certainly
`distinct, the data indicate that interaction with tubulin
`in two distinct domains can bring about a similar stabi-
`lization of dynamics.
`
`Lessons to be learned from understanding drug
`mechanisms
`
`Several lessons can be learned from analysis of the
`actions of drugs on microtubule dynamics. Microtu—
`bules have many distinct regions along their surfaces
`
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`and at their ends that serve as binding sites for drugs.
`Binding of the drugs to the different sites modulates dy-
`namics in a variety of ways. The location of the bind-
`ing sites with respect to the tubulin surface or end ap—
`pears to be a major determinant of the response by the
`microtubule to drug binding. Drugs acting at the micro-
`tubule plus end such as colchicine and vinblastine stabi—
`lize dynamics, possibly by increasing the stability of the
`stabilizing cap. Drugs interacting along the surface of
`the microtbule can either stabilize (taxol) or destabilize
`(high concentrations of vinblastine) the microtubule
`lattice. The response of a microtubule to 'a drug such as
`colchicine will be different when the drug is initially
`added to a steady—state population of microtubules
`than after the microtubules attain their new steady state
`in the continuous presence of the drug.
`The many drug binding sites on tubulin and micro-
`tubules most likely are the same sites to which endo-
`genous regulators of dynamics bind. A search for
`endogenous regulators of microtubule dynamics (e.g.,
`catastrophe factors, minus-end depolymerizing fac-
`tors) is beginning to reveal such regulators (6). If as we
`suspect, the known drugs are mimicking the actions of
`natural regulators of dynamics, the knowledge gained
`from understanding how drugs modulate dynamics will
`greatly aid in elucidating the mechanisms of natural
`regulators.
`
`Acknowledgments. The work described in this paper was supported
`by United States Public Health Service grants NS 13560 and CA
`57291.
`
`References
`
`2.
`
`1. BELMONT, L., HYMAN, A.A., SAWIN, KB, and MITCHISON,
`T.J. 1990. Real-time visualization of cell cycle dependent
`changes in microtubule dynamics in cytoplasmic extracts. J. Cell
`Biol, 40: 95—107.
`BORISY, G.G. and TAYLOR, E.W. 1967. The mechanism of ac—
`tion of colchicine. Binding Of colchicine 3H to cellular protein.
`J. Cell Biol, 34: 525—533.
`3. BORISY, G.G. and TAYLOR, E.W. 1967. The mechanism of ac-
`tion of colchicine. Colchicine binding to sea urchin eggs and the
`mitotic apparatus. J. Cell Biol, 34: 535—548.
`4. DERRY, W.B., WILSON, L., and JORDAN, M.A. 1995. Sub-
`stoichiometric binding of taxol suppresses microtubule dy—
`namics. Biochemistry, 34: 2203—2211.
`5. DESAI, A., VERMA, S., MITCHISON, T.J., and WALCZAK, C.
`1999. Kin I Kinesins are microtubule-destabilizing enzymes.
`Cell, 96: 69—78.
`6. DESAI, A. and MITCHISON, T.J. 1997. Microtubule polymeriza—
`tion dynamics. Annu. Rev. Cell Dev. Biol, 13: 83—117.
`FARRELL, K.W., JORDAN, M.A., MILLER, HR, and WILSON, L.
`1987. Phase dynamics at microtubule ends: the coexistence of
`microtubule length changes and treadmilling. J. Cell Biol, 104:
`1035—1046.
`Interactions of colchicine with tubulin.
`8. HASTIE, S.B. 1991.
`Pharmacol. Ther., 512: 377—401.
`
`7.
`
`L. Wilson et a1.
`
`HOWARD, W.iD. and TIMASHEFF, SN. 1988. Linkages between
`the effects of taxol, colchicine, and GTP on tubulin polymeriza-
`tion. J. Biol. Chem, 263: 1342—1346.
`HORIO, T. and HOTANI, H. 1986. Visualization of the dy—
`namic instability of individual microtubules by dark-field mi-
`croscopy. Nat xre, 321: 605—607.
`HOTANI, H. and HORIo, T. 1988. Dynamics of microtubules
`visualized by dark field microscopy: treadmilling and dynamic
`instability. Cell Motil. Cytoskeleton, 10: 229—236.
`LOBERT, S., FRANKEURTER, A., and CORREIA, J.J . 1995. Bind-
`ing of vinblastine to phosphocellulose—purified and a/S-class III
`tubulin:
`the role of nucleotides and fi-tubulin isotypes. Bio-
`chemistry, 34: 8050—8060.
`MARGOLIS, R.L. and WILSON, L. 1978. Opposite end assembly
`and disassembly of microtubules at steady state in vitro. Cell,
`13: 1—8.
`MARGOLIS, R.IL. and WILSON, L. 1998. Microtubule treadmill-
`ing: what goes around comes around. BioEssays, 20: 830—836.
`MITCHISON, T J. and KIRSCHNER, M. 1984. Dynamic instabil-
`ity of microtubule growth. Nature, 312: 237—242.
`MITCHISON, T.J. and SALMON, ED. 1992. Poleward kineto-
`chore fiber movement occurs during both metaphase and ana-
`phase A in newt lung cell mitosis. J. Cell Biol, 119: 569—582.
`NA, G.G. and TIMASHEFF, SN. 1980. Thermodynamic linkage
`between tubulin self association and the binding of vinblastine.
`Biochemistry, 19: 1347—1354.
`NOGALES, E., WHITTAKER, M., MILLIGAN, R.A., and DOWNING,
`K.H. 1999. High resolution model of the microtubule. Cell, 96:
`79—88.
`PANDA, D., DAIJo, J.E., JORDAN, M.A., and WILSON, L. 1995.
`Kinetic stabilization Of microtubule dynamics at steady state in
`vitro by substoichiometric concentrations Of tubulin-colchicine
`complex. Biochemistry, 34: 9921-9929.
`PANDA, D., GOODE, B.L., FELNSTEIN, S.C., and WILSON, L.
`1995. Kinetic stabilization of microtubule dynamics at steady
`state by tau and microtubule binding domains of tau. Bio-
`chemistry, 34: 11117—11127.
`PANDA, D., JORDAN, M.A., CHIN, K., and WILSON, L. 1996.
`Differential effects of vinblastine on polymerization and dynam-
`ics at opposite microtubule ends. J. Biol. Chem., 271: 29807—
`29812.
`RODIONOV, V., NADEZIIDINA, E., and BORISY, G. 1999. Cen-
`trosomal control of microtubule dynamics. Proc. Natl. Acad.
`Sci. USA, 96: 115—120.
`SAXTON, W.M., STEMPLE, D.L., LESLIE, R.J., SALMON, E.D.,
`ZAVORTNIK, M'., and MCINTOSH, J .R. 1984. Tubulin dynamics
`in cultured mammalian cells. J. Cell Biol, 99: 4093—4100.
`SCHIFF, P.B., FANT, J., and HORWITZ, S.B. 1979. Promotion
`of microtubule assembly in vitro by taxol. Nature, 277: 665—
`667.
`SKOUFIAS, D. and WILSON, L. 1992. Mechanism of inhibition
`of microtubule polymerization by colchicine: Inhibitory proten-
`cies of unliganded colchicine and tubulin-colchicine complexes.
`Biochemistry, .31: 738—746.
`STERNLICHT, H. and RINGEL, I. 1979. Colchicine inhibition of
`microtubule assembly via copolymer formation. J. Biol. Chem,
`254: 10540—10550.
`TOSO, R.J., JORDAN, M.A., FARRELL, K.W., MATSUMOTO, B.,
`and WILSON, L. 1993. Kinetic stabilization of microtubule
`dynamic instability in Vitro by vinblastine. Biochemistry, 32:
`1285—1293.
`WALKER, R.A., O’BRIEN, E.T., PRYER, N.K., SOBOEIRO, M.F.,
`VOTER, W.A., ERICKSON, H.P., and SALMON, ED. 1988. Dy-
`
`10.
`
`11.
`
`12.
`
`13.
`
`14.
`
`15.
`
`16.
`
`17.
`
`18.
`
`19.
`
`20.
`
`21.
`
`22.
`
`23.
`
`24.
`
`25.
`
`26.
`
`27.
`
`28.
`
`334
`
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`Modulation of Microtubule Dynamics
`
`29.
`
`namic instability of individual microtubules analyzed by video
`light microscopy; rate constants and transition frequencies. J.
`Cell Biol., 107: 1437—1448.
`WILSON, L. and FRIEDKIN, M. 1967. The biochemical events of
`mitosis II. The in vivo and in vitro binding of colchicine in
`grasshopper embryos and its possible relation to inhibition of
`mitosis. Biochemistry, 6: 3126—3135.
`
`30.
`
`31.
`
`WILSON, L., JORDAN, M.A., MORSE, A., and MARGOLIS, KL.
`1982.
`Interaction of vinblastine with steady-state microtubules
`in vitro. J. M01. Biol., 159: 125—149.
`WILSON, L. and JORDAN, M .A. 1994. Pharmacological probes
`of microtubule function. In Microtubules (J. Hyams, and C.
`Lloyd, eds.). John Wiley and Sons, New York, pp.59—84.
`
`335
`
`IMMUNOGEN 2059, Pg. 7
`Phigenix v. Immunogen
`|PR2014—00676
`
`IMMUNOGEN 2059, pg. 7
`Phigenix v. Immunogen
`IPR2014-00676
`
`