The formation of the protective cap is a cooperative event with the addition of each new tubulin-GTP subunit increasing the number of binding sites available for further subunit attachment [, ]. This may act in parallel with facilitated, or directed diffusion in which the probability of a subunit encountering an MT-fibre free β-tubulin-GTP increases as the length and number of MT-fibres increases []. The polarity of fibres, by virtue of the directs a colliding subunit toward the plus, or αβ-heterodimers, growth, end. Both these mechanisms are illustrated in Figures 1.5 and 1.6 [, ]. Figure 1.5. of the MT capping model. The layer of Schematic ‘monte carlo’ illustration subunits forms a protective cap at the end of the MT fibre. The attachment of subunit provides an extra binding site for a subsequent attachment. each β-tubulin-GTP Eventually the number of free subunits in the vicinity of the fibre tip falls below a critical concentration (CC) and the cap is lost, leading to rapid dissociation of the unprotected termed catastrophe. light [dark green, β-tubulin-GTP; dark red, β-tubulin-GDP; Image printed with permission from NCBI PubMed, Bethesda, MD, green/red, α-tubulin]. USA. Martin et al. Biophysical Journal 1993; 65(2):578-96 []. Figure 1.6. Illustration of facilitated diffusion. Collisions between subunits and the MT fibre are random. (A) In the classical view the probability that a free subunit will β-tubulin-GTP encounter a fibre tip is low. (B) The presence of an attractive force, by virtue of the heterodimer polarity, facilitates colliding subunits to along the fibre towards the tip, ‘slide’ ‘+’ thereby increasing the probability of subunit attachment. Image printed with permission of PLos. Mechulam et al. Computational Biology 2009; 5(1) [].
The formation of the protective cap is a cooperative event with the addition of each new tubulin-GTP subunit increasing the number of binding sites available for further subunit attachment [, ]. This may act in parallel with facilitated, or directed diffusion in which the probability of a free β-tubulin-GTP subunit encountering an MT-fibre increases as the length and number of MT-fibres increases []. The polarity of by virtue of the αβ-heterodimers, directs a colliding subunit toward the plus, or growth, end. Both these mechanisms are illustrated in Figures 1.5 and 1.6 [, ]. Figure 1.5. Schematic ‘monte carlo’ illustration of the MT capping model. The layer of subunits forms a protective cap at the end of the MT fibre. The attachment of each β-tubulin-GTP subunit provides an extra binding site for a subsequent attachment. Eventually the number of free subunits in the vicinity of the fibre tip falls below a critical concentration (CC) and the cap is lost, leading to rapid dissociation of the unprotected termed catastrophe. [dark green, β-tubulin-GTP; dark red, β-tubulin-GDP; light green/red, α-tubulin]. Image printed with permission from NCBI PubMed, Bethesda, MD, USA. Martin et al. Biophysical Journal 1993; 65(2):578-96 []. Figure 1.6. Illustration of facilitated diffusion. Collisions between subunits and the MT fibre are random. (A) In the classical view the probability that a free β-tubulin-GTP subunit will encounter a fibre tip is low. (B) The presence of an attractive force, by virtue of the polarity, facilitates colliding subunits to ‘slide’ along the fibre towards the ‘+’ tip, thereby increasing the probability of subunit attachment. Image printed with permission of PLos. Mechulam et al. Computational Biology 2009; 5(1) []. fibres, heterodimer