微管是一种具有极性的细胞骨架。它是由13 条原纤维（protofilament）构成的中空管状结构，直径22—25纳米。每一条原纤维由微管蛋白二聚体线性排列而成。

微管蛋白二聚体由结构相似的α和β-微管蛋白构成，两种亚基均可结合GTP，α-微管蛋白结合的GTP从不发生水解或交换，是α-微管蛋白的固有组成部分；而作为GTP酶，β-微管蛋白可水解结合的GTP，结合的GDP可交换为GTP，称为可交换位。微管和微丝一样，具有生长速度较快解离速度较慢的(+)端和生长速度较慢解离速度较快的(-)端。微管在细胞内起支撑作用。另外它还是两种运载分子，驱动蛋白和动力蛋白（Dynein）的行走轨道。微管可能连带附在其上的运动蛋白会发放信号促进粘着斑的解聚，后者是粘着斑的周转和尾部与底质分离过程中重要的一步。

推荐大家阅读两篇文献：

Dynamic instability of microtubule growth

TIM MITCHISON & MARC KIRSCHNER

Department of Biochemistry and Biophysics, University of California at San Francisco, San Francisco, California 94143, USA

We report here that microtubules in vitro coexist in growing and shrinking populations which interconvert rather infrequently. This dynamic instability is a general property of microtubules and may be fundamental in explaining cellular microtubule organization.

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What makes microtubules dynamic?

J Cell Sci 1990 95:329-334

Introduction
Microtubules show the remarkable property of being ableto switch, apparently at random, between a state of steadygrowth and one of relatively rapid shrinking. This articleconsiders recent developments in formulating the possiblemolecular mechanism of this phenomenon of dynamicinstability, and surveys the currently unresolvedproblems in our understanding.Microtubule dynamic instability has been observedboth in vitro and in vivo (Mitchison and Kirschner,1984a,b; Horio and Hotani, 1986; Cassin~erise t al. 1987;Farrell et al. 1987; Walker et a1. 1988; Cassimeris et al.1988; Sammak and Borisy, 1988; Schulze & Kirschner,1988). Such observations have become possible throughadvances in optical nlicroscopy using dark-field (Horioand Hotani, 1986) and DIC optics (Salmon et a/. 1989)with video-enhanced processing. An example is given inFig. 1, which shows that the dynamic behaviour is theproperty of an individual nlicrotubule and not simply theeffect of local solution conditions, such as lack of GTP.The dynamic processes are characterised by the growthand shrinking rates RG and Rs, and by the average timespent in a given state, i.e. lifetimes, TG and Ts (or thecorresponding frequencies 1 1 ~ ) .F or a given set ofconditions, the average excursion length at a given end isgiven by the product Lc=Rc.TG (and Ls similarly).These quantities are typically several micrometers; i.e.the system undergoes essentially macroscopic excursionsin short times, a dynamic condition that is not obviouslyexplicable in terms of simple association and dissociationreaction mechanisms.Observations of changes in length distribution in amicrotubule population (Mitchison and Kirschner,1984b), and the kinetics of incorporation of tubulin intosteady state microtubules (Martin et al. 1988), areconsistent with the in~plications of dynamic instability(Bayley et al. 19896). Attention has therefore mostrecently been focussed on the potential molecular originsof the dynamic transitions of growing and shrinkingmicrotubules.

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Video shows growth and shrinkage of microtubules. More info about dynamic instability of microtubules: Microtubules may grow steadily, and then shrink rapidly by loss of tubulin dimers at the plus end. The rapid disassembly is referred to as catastrophe. In vitro, the tendency to grow or shrink may be a function of tubulin concentration. As microtubules grow, tubulin dimers are depleted. Below a critical tubulin concentration, rapid shrinkage at the plus end has been attributed to loss of a "GTP cap." Hydrolysis of GTP by b-tubulin, as polymerization brings it into contact with a-tubulin, takes time. A rapidly growing microtubule may accumulate a few layers of tubulin-GTP at the plus end. A GTP cap stabilizes the plus end of a microtubule. If the concentration of tubulin heterodimers is low, dissociation of tubulin-GTP may expose tubulin-GDP at the plus end, causing that end to become unstable. Rapid shrinkage ensues. Fraying or curving of protofilaments is observed at the ends of rapidly disassembling microtubules. This may be due to a change in conformation when b-subunits at the plus end have bound GDP instead of GTP. Tubulin heterodimers with GDP bound to the b subunit form ring shaped assemblies in vitro. Straight protofilaments form only when both tubulins have bound GTP. Dynamic instability of microtubules in vivo is regulated by interaction with other proteins. For example, during prophase of mitosis, microtubules grow out from the centrosome. If the plus end of a microtubule makes contact with a chromosome, it becomes stabilized. Otherwise rapid disassembly at the plus end ensues, and the tubulin dimers are available for growth of another microtubule. Text Ref: http://www.rpi.edu/dept/bcbp/molbiochem/MBWeb/mb2/part1/microtub.htm