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Polyamine sharing between tubulin dimers favours microtubule nucleation and elongation via facilitated diffusion.

Mechulam A, Chernov KG, Mucher E, Hamon L, Curmi PA, Pastré D - PLoS Comput. Biol. (2009)

Bottom Line: We suggest for the first time that the action of multivalent cations on microtubule dynamics can result from facilitated diffusion of GTP-tubulin to the microtubule ends.The mechanism of facilitated diffusion requires an attraction force between two tubulins, which can result from the sharing of multivalent counterions.The results presented here show that polyamines can be of particular importance for the regulation of the microtubule network in vivo and provide the basis for further investigations into the effects of facilitated diffusion on cytoskeleton dynamics.

View Article: PubMed Central - PubMed

Affiliation: Laboratoire Structure-Activité des Biomolécules Normales et Pathologiques, Université Evry-Val d'Essonne, Evry, France.

ABSTRACT
We suggest for the first time that the action of multivalent cations on microtubule dynamics can result from facilitated diffusion of GTP-tubulin to the microtubule ends. Facilitated diffusion can promote microtubule assembly, because, upon encountering a growing nucleus or the microtubule wall, random GTP-tubulin sliding on their surfaces will increase the probability of association to the target sites (nucleation sites or MT ends). This is an original explanation for understanding the apparent discrepancy between the high rate of microtubule elongation and the low rate of tubulin association at the microtubule ends in the viscous cytoplasm. The mechanism of facilitated diffusion requires an attraction force between two tubulins, which can result from the sharing of multivalent counterions. Natural polyamines (putrescine, spermidine, and spermine) are present in all living cells and are potent agents to trigger tubulin self-attraction. By using an analytical model, we analyze the implication of facilitated diffusion mediated by polyamines on nucleation and elongation of microtubules. In vitro experiments using pure tubulin indicate that the promotion of microtubule assembly by polyamines is typical of facilitated diffusion. The results presented here show that polyamines can be of particular importance for the regulation of the microtubule network in vivo and provide the basis for further investigations into the effects of facilitated diffusion on cytoskeleton dynamics.

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Schematic representation of the mechanism of facilitated diffusion.(A) Attraction between two tubulin heterodimers mediated by polyamines.The attraction is mediated by the two C-terminal tails of one tubulindimer, which, for entropic reasons (separation distance betweenthe two C-termini tails on a tubulin dimer), are likely toshare polyamines with two C-terminal tails of another tubulin dimer. (B)In the absence of an attraction force, many collisions between tubulinand nucleus do not result in association, whereas, in the presence of anattraction force, both facilitated diffusion via sliding and an increaseof the interaction lifetime favour association to the growing nucleus.(C) In the presence of an attraction force, sliding of tubulin along thecylindrical surface of MT favours tubulin association to the MTends.
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pcbi-1000255-g001: Schematic representation of the mechanism of facilitated diffusion.(A) Attraction between two tubulin heterodimers mediated by polyamines.The attraction is mediated by the two C-terminal tails of one tubulindimer, which, for entropic reasons (separation distance betweenthe two C-termini tails on a tubulin dimer), are likely toshare polyamines with two C-terminal tails of another tubulin dimer. (B)In the absence of an attraction force, many collisions between tubulinand nucleus do not result in association, whereas, in the presence of anattraction force, both facilitated diffusion via sliding and an increaseof the interaction lifetime favour association to the growing nucleus.(C) In the presence of an attraction force, sliding of tubulin along thecylindrical surface of MT favours tubulin association to the MTends.

Mentions: Multivalent cations like polyamines can induce an electrostatic attractionbetween two highly negatively charged surfaces. The theoretical treatment ofthis force has been the subject of extensive studies [20]–[23]in particular to explain the phenomenon of DNA condensation by polyamines [24]–[26]. Concerningmicrotubules, their self-attraction can also be triggered by multivalent saltsand can lead to the formation of bundles [27]. The energybenefit of association between two like-charged bodies is generally due to thecorrelations between the multivalent counterions condensed on their surfaces.Here we will show that the interaction between two tubulin heterodimers, whichare highly negatively charged proteins with a net charge about 20–30e−, at pH around 7 [19], may also beinfluenced by this mechanism. Let us first estimate the attraction energybetween two tubulin heterodimers in the presence of polyamines. Theαβ-tubulin heterodimer is a nonspherical globule, havingdimensions of 46×80×65 Å with two long C-terminaltails (∼35 Å) [3]. Most of the tubulincharge, at least 40%, is concentrated in the C-terminal tails so thatthe charge distribution on its surface is non homogeneous. We then need toinvestigate the electrostatic properties of the C-terminal tails and of theremaining of the heterodimer molecule separately. As described in details inTextS1, we obtained after theoretical developments the energy gain for thesetwo interactions. The attraction energy for two heterodimers without theC-terminal tails is positive for putrescine and spermidine and negative only forspermine (−0.26 KBT), which means a weakenergy benefit. As this result was obtained by considering that there is onlyspermine on heterodimer surfaces, we might expect that the resulting attractionforce is negligible in physiological conditions where spermine competes withother cations or proteins for tubulin neutralization. On the other hand, theattraction energy becomes significantly larger between two interacting tubulinC-terminal tails. We obtained in this case UC valuesof +4.7, −2.6, −5 and −6.2KBT for cation valenceZ = 1, 2, 3 and 4respectively, showing the presence of an attraction energy when Z>1 andwhich increases with the valence of ions. The energy benefit per counterion incorrelations remains lower than KBT for divalentputrescine (see Text S1) so that it could hardly induce an attraction force betweenC-terminal tails due to thermal agitation. On the other hand, we expect thattrivalent spermine and tetravalent spermidine can induce a significantattraction force between the C-terminal tails of tubulin dimers (see Text S1 andFigure 1A).


Polyamine sharing between tubulin dimers favours microtubule nucleation and elongation via facilitated diffusion.

Mechulam A, Chernov KG, Mucher E, Hamon L, Curmi PA, Pastré D - PLoS Comput. Biol. (2009)

Schematic representation of the mechanism of facilitated diffusion.(A) Attraction between two tubulin heterodimers mediated by polyamines.The attraction is mediated by the two C-terminal tails of one tubulindimer, which, for entropic reasons (separation distance betweenthe two C-termini tails on a tubulin dimer), are likely toshare polyamines with two C-terminal tails of another tubulin dimer. (B)In the absence of an attraction force, many collisions between tubulinand nucleus do not result in association, whereas, in the presence of anattraction force, both facilitated diffusion via sliding and an increaseof the interaction lifetime favour association to the growing nucleus.(C) In the presence of an attraction force, sliding of tubulin along thecylindrical surface of MT favours tubulin association to the MTends.
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Related In: Results  -  Collection

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Show All Figures
getmorefigures.php?uid=PMC2599886&req=5

pcbi-1000255-g001: Schematic representation of the mechanism of facilitated diffusion.(A) Attraction between two tubulin heterodimers mediated by polyamines.The attraction is mediated by the two C-terminal tails of one tubulindimer, which, for entropic reasons (separation distance betweenthe two C-termini tails on a tubulin dimer), are likely toshare polyamines with two C-terminal tails of another tubulin dimer. (B)In the absence of an attraction force, many collisions between tubulinand nucleus do not result in association, whereas, in the presence of anattraction force, both facilitated diffusion via sliding and an increaseof the interaction lifetime favour association to the growing nucleus.(C) In the presence of an attraction force, sliding of tubulin along thecylindrical surface of MT favours tubulin association to the MTends.
Mentions: Multivalent cations like polyamines can induce an electrostatic attractionbetween two highly negatively charged surfaces. The theoretical treatment ofthis force has been the subject of extensive studies [20]–[23]in particular to explain the phenomenon of DNA condensation by polyamines [24]–[26]. Concerningmicrotubules, their self-attraction can also be triggered by multivalent saltsand can lead to the formation of bundles [27]. The energybenefit of association between two like-charged bodies is generally due to thecorrelations between the multivalent counterions condensed on their surfaces.Here we will show that the interaction between two tubulin heterodimers, whichare highly negatively charged proteins with a net charge about 20–30e−, at pH around 7 [19], may also beinfluenced by this mechanism. Let us first estimate the attraction energybetween two tubulin heterodimers in the presence of polyamines. Theαβ-tubulin heterodimer is a nonspherical globule, havingdimensions of 46×80×65 Å with two long C-terminaltails (∼35 Å) [3]. Most of the tubulincharge, at least 40%, is concentrated in the C-terminal tails so thatthe charge distribution on its surface is non homogeneous. We then need toinvestigate the electrostatic properties of the C-terminal tails and of theremaining of the heterodimer molecule separately. As described in details inTextS1, we obtained after theoretical developments the energy gain for thesetwo interactions. The attraction energy for two heterodimers without theC-terminal tails is positive for putrescine and spermidine and negative only forspermine (−0.26 KBT), which means a weakenergy benefit. As this result was obtained by considering that there is onlyspermine on heterodimer surfaces, we might expect that the resulting attractionforce is negligible in physiological conditions where spermine competes withother cations or proteins for tubulin neutralization. On the other hand, theattraction energy becomes significantly larger between two interacting tubulinC-terminal tails. We obtained in this case UC valuesof +4.7, −2.6, −5 and −6.2KBT for cation valenceZ = 1, 2, 3 and 4respectively, showing the presence of an attraction energy when Z>1 andwhich increases with the valence of ions. The energy benefit per counterion incorrelations remains lower than KBT for divalentputrescine (see Text S1) so that it could hardly induce an attraction force betweenC-terminal tails due to thermal agitation. On the other hand, we expect thattrivalent spermine and tetravalent spermidine can induce a significantattraction force between the C-terminal tails of tubulin dimers (see Text S1 andFigure 1A).

Bottom Line: We suggest for the first time that the action of multivalent cations on microtubule dynamics can result from facilitated diffusion of GTP-tubulin to the microtubule ends.The mechanism of facilitated diffusion requires an attraction force between two tubulins, which can result from the sharing of multivalent counterions.The results presented here show that polyamines can be of particular importance for the regulation of the microtubule network in vivo and provide the basis for further investigations into the effects of facilitated diffusion on cytoskeleton dynamics.

View Article: PubMed Central - PubMed

Affiliation: Laboratoire Structure-Activité des Biomolécules Normales et Pathologiques, Université Evry-Val d'Essonne, Evry, France.

ABSTRACT
We suggest for the first time that the action of multivalent cations on microtubule dynamics can result from facilitated diffusion of GTP-tubulin to the microtubule ends. Facilitated diffusion can promote microtubule assembly, because, upon encountering a growing nucleus or the microtubule wall, random GTP-tubulin sliding on their surfaces will increase the probability of association to the target sites (nucleation sites or MT ends). This is an original explanation for understanding the apparent discrepancy between the high rate of microtubule elongation and the low rate of tubulin association at the microtubule ends in the viscous cytoplasm. The mechanism of facilitated diffusion requires an attraction force between two tubulins, which can result from the sharing of multivalent counterions. Natural polyamines (putrescine, spermidine, and spermine) are present in all living cells and are potent agents to trigger tubulin self-attraction. By using an analytical model, we analyze the implication of facilitated diffusion mediated by polyamines on nucleation and elongation of microtubules. In vitro experiments using pure tubulin indicate that the promotion of microtubule assembly by polyamines is typical of facilitated diffusion. The results presented here show that polyamines can be of particular importance for the regulation of the microtubule network in vivo and provide the basis for further investigations into the effects of facilitated diffusion on cytoskeleton dynamics.

Show MeSH
Related in: MedlinePlus