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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 tubulin                            dimer, which, for entropic reasons (separation distance between                                the two C-termini tails on a tubulin dimer), are likely to                            share polyamines with two C-terminal tails of another tubulin dimer. (B)                            In the absence of an attraction force, many collisions between tubulin                            and nucleus do not result in association, whereas, in the presence of an                            attraction force, both facilitated diffusion via sliding and an increase                            of the interaction lifetime favour association to the growing nucleus.                            (C) In the presence of an attraction force, sliding of tubulin along the                            cylindrical surface of MT favours tubulin association to the MT                        ends.
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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 tubulin dimer, which, for entropic reasons (separation distance between the two C-termini tails on a tubulin dimer), are likely to share polyamines with two C-terminal tails of another tubulin dimer. (B) In the absence of an attraction force, many collisions between tubulin and nucleus do not result in association, whereas, in the presence of an attraction force, both facilitated diffusion via sliding and an increase of the interaction lifetime favour association to the growing nucleus. (C) In the presence of an attraction force, sliding of tubulin along the cylindrical surface of MT favours tubulin association to the MT ends.

Mentions: Multivalent cations like polyamines can induce an electrostatic attraction between two highly negatively charged surfaces. The theoretical treatment of this force has been the subject of extensive studies [20]–[23] in particular to explain the phenomenon of DNA condensation by polyamines [24]–[26]. Concerning microtubules, their self-attraction can also be triggered by multivalent salts and can lead to the formation of bundles [27]. The energy benefit of association between two like-charged bodies is generally due to the correlations between the multivalent counterions condensed on their surfaces. Here we will show that the interaction between two tubulin heterodimers, which are highly negatively charged proteins with a net charge about 20–30 e−, at pH around 7 [19], may also be influenced by this mechanism. Let us first estimate the attraction energy between two tubulin heterodimers in the presence of polyamines. The αβ-tubulin heterodimer is a nonspherical globule, having dimensions of 46×80×65 Å with two long C-terminal tails (∼35 Å) [3]. Most of the tubulin charge, at least 40%, is concentrated in the C-terminal tails so that the charge distribution on its surface is non homogeneous. We then need to investigate the electrostatic properties of the C-terminal tails and of the remaining of the heterodimer molecule separately. As described in details in Text S1, we obtained after theoretical developments the energy gain for these two interactions. The attraction energy for two heterodimers without the C-terminal tails is positive for putrescine and spermidine and negative only for spermine (−0.26 KBT), which means a weak energy benefit. As this result was obtained by considering that there is only spermine on heterodimer surfaces, we might expect that the resulting attraction force is negligible in physiological conditions where spermine competes with other cations or proteins for tubulin neutralization. On the other hand, the attraction energy becomes significantly larger between two interacting tubulin C-terminal tails. We obtained in this case UC values of +4.7, −2.6, −5 and −6.2 KBT for cation valence Z = 1, 2, 3 and 4 respectively, showing the presence of an attraction energy when Z>1 and which increases with the valence of ions. The energy benefit per counterion in correlations remains lower than KBT for divalent putrescine (see Text S1) so that it could hardly induce an attraction force between C-terminal tails due to thermal agitation. On the other hand, we expect that trivalent spermine and tetravalent spermidine can induce a significant attraction force between the C-terminal tails of tubulin dimers (see Text S1 and Figure 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 tubulin                            dimer, which, for entropic reasons (separation distance between                                the two C-termini tails on a tubulin dimer), are likely to                            share polyamines with two C-terminal tails of another tubulin dimer. (B)                            In the absence of an attraction force, many collisions between tubulin                            and nucleus do not result in association, whereas, in the presence of an                            attraction force, both facilitated diffusion via sliding and an increase                            of the interaction lifetime favour association to the growing nucleus.                            (C) In the presence of an attraction force, sliding of tubulin along the                            cylindrical surface of MT favours tubulin association to the MT                        ends.
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Related In: Results  -  Collection

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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 tubulin dimer, which, for entropic reasons (separation distance between the two C-termini tails on a tubulin dimer), are likely to share polyamines with two C-terminal tails of another tubulin dimer. (B) In the absence of an attraction force, many collisions between tubulin and nucleus do not result in association, whereas, in the presence of an attraction force, both facilitated diffusion via sliding and an increase of the interaction lifetime favour association to the growing nucleus. (C) In the presence of an attraction force, sliding of tubulin along the cylindrical surface of MT favours tubulin association to the MT ends.
Mentions: Multivalent cations like polyamines can induce an electrostatic attraction between two highly negatively charged surfaces. The theoretical treatment of this force has been the subject of extensive studies [20]–[23] in particular to explain the phenomenon of DNA condensation by polyamines [24]–[26]. Concerning microtubules, their self-attraction can also be triggered by multivalent salts and can lead to the formation of bundles [27]. The energy benefit of association between two like-charged bodies is generally due to the correlations between the multivalent counterions condensed on their surfaces. Here we will show that the interaction between two tubulin heterodimers, which are highly negatively charged proteins with a net charge about 20–30 e−, at pH around 7 [19], may also be influenced by this mechanism. Let us first estimate the attraction energy between two tubulin heterodimers in the presence of polyamines. The αβ-tubulin heterodimer is a nonspherical globule, having dimensions of 46×80×65 Å with two long C-terminal tails (∼35 Å) [3]. Most of the tubulin charge, at least 40%, is concentrated in the C-terminal tails so that the charge distribution on its surface is non homogeneous. We then need to investigate the electrostatic properties of the C-terminal tails and of the remaining of the heterodimer molecule separately. As described in details in Text S1, we obtained after theoretical developments the energy gain for these two interactions. The attraction energy for two heterodimers without the C-terminal tails is positive for putrescine and spermidine and negative only for spermine (−0.26 KBT), which means a weak energy benefit. As this result was obtained by considering that there is only spermine on heterodimer surfaces, we might expect that the resulting attraction force is negligible in physiological conditions where spermine competes with other cations or proteins for tubulin neutralization. On the other hand, the attraction energy becomes significantly larger between two interacting tubulin C-terminal tails. We obtained in this case UC values of +4.7, −2.6, −5 and −6.2 KBT for cation valence Z = 1, 2, 3 and 4 respectively, showing the presence of an attraction energy when Z>1 and which increases with the valence of ions. The energy benefit per counterion in correlations remains lower than KBT for divalent putrescine (see Text S1) so that it could hardly induce an attraction force between C-terminal tails due to thermal agitation. On the other hand, we expect that trivalent spermine and tetravalent spermidine can induce a significant attraction force between the C-terminal tails of tubulin dimers (see Text S1 and Figure 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