Microtubule assembly, nucleation initiation

Although the self-assembly of polymeric structures can involve nucleation and elongation steps (See Actin Assembly Kinetics Microtubule Assembly Kinetics), one can simplify the assembly process through what is known as seeded assembly. At an initial monomer concentration [M], seeded assembly is induced by the addition of pre-assembled polymeric structures consequently the polymer number concentration must remain constant. The rate of monomer incorporation into indefinite length polymers can be written as follows ... 

The self-assembly of tubulin to form microtubules was described initially in a classic polymerization model of nucleated helical polymerization by Maruyama and Oosawa (4). Assembly involves two phases a nucleation phase followed by an elongation phase. With purified systems in vitro, nucleation can... 

Figure 3 A typical microtubule assembly reaction is initiated by warming a solution of ice-cold tubulin dimers to 37°C in the presence of GTP. Tubulin dimers (adjacent white and gray circles) slowly form nucleating seeds (heptameric tubulin aggregate), which catalyze a rapid phase of microtubule elongation (growing microtubule) enroute to a steady state condition of microtubule formation and destruction. The assembly reaction is monitored by measuring the change in absorbance at 350 nm. In vitro incubation of microtubules with 2,5-HD or in vivo exposure of animals to 2,5-HD followed by tubulin purification yields pyrrolylated tubulin with altered assembly behavior. 2,5-HD-modified tubulin quickly forms numerous seeds, resulting in more rapid assembly into greater numbers of shorter microtubules compared to the control.

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