Obermann H, Mandelkow E M, Lange G, Mandelkow E
Max-Planck Unit for Structural Molecular Biology, Hamburg, Federal Republic of Germany.
J Biol Chem. 1990 Mar 15;265(8):4382-8.
Microtubules are capable of performing synchronized oscillations of assembly and disassembly which has been explained by reaction mechanisms involving tubulin subunits, oligomers, microtubules, and GTP. Here we address the question of how microtubule nucleation or their number concentration affects the oscillations. Assembly itself requires a critical protein concentration (Cc), but oscillations require in addition a critical microtubule number concentration (CMT). In spontaneous assembly this can be achieved with protein concentrations Cos well above the critical concentration Cc because this enhances the efficiency of nucleation. Seeding with microtubules can either generate oscillations or suppress them, depending on how the seeds alter the effective microtubule number concentration. The relative influence of microtubule number and total protein concentrations can be varied by the rate at which assembly conditions are induced (e.g. by a temperature rise): Fast T-jumps induce oscillations because of efficient nucleation, slow ones do not. Oscillations become damped for several reasons. One is the consumption of GTP, the second is a decrease in microtubule number, and the third is that the ratio of microtubules in the two phases (growth-competent and shrinkage-competent) approach a steady state value. This ratio can be perturbed, and the oscillations restarted, by a cold shock, addition of seeds, addition of GTP, or fragmentation. Each of these is equivalent to a change in the effective microtubule number concentration.
微管能够进行组装和拆卸的同步振荡，这已通过涉及微管蛋白亚基、寡聚体、微管和GTP的反应机制得到解释。在此，我们探讨微管成核或其数量浓度如何影响振荡这一问题。组装本身需要临界蛋白浓度（Cc），但振荡还需要临界微管数量浓度（CMT）。在自发组装中，这可以通过远高于临界浓度Cc的蛋白浓度Cos来实现，因为这会提高成核效率。用微管接种既可以产生振荡，也可以抑制振荡，这取决于种子如何改变有效微管数量浓度。微管数量和总蛋白浓度的相对影响可以通过诱导组装条件的速率（例如通过温度升高）来改变：快速的温度跃变由于有效的成核作用会诱导振荡，缓慢的则不会。振荡会因多种原因而衰减。一是GTP的消耗，二是微管数量的减少，三是两个阶段（有生长能力和有收缩能力）的微管比例接近稳态值。这种比例可以通过冷休克、添加种子、添加GTP或片段化来扰动，并使振荡重新开始。这些中的每一个都等同于有效微管数量浓度的变化。
