Self-organisation and forces in the microtubule cytoskeleton Franc Ëois Ne  de  lec  , Thomas Surrey and Eric Karsenti Modern microscopy techniques allow us to observe speci®cally tagged proteins in live cells. We can now see directly that many cellular structures, for example mitotic spindles, are in fact dynamic assemblies. Their apparent stability results from out-of-equilibrium stochastic interactions at the molecular level. Recent studies have shown that the spindles can form even after centrosomes are destroyed, and that they can even form around DNA-coated beads devoid of kinetochores. Moreover, conditions have been produced in which microtubule asters interact even in the absence of chromatin. Together, these observations suggest that the spindle can be experimentally deconstructed, and that its de®ning characteristics can be studied in a simpli®ed context, in the absence of the full division machinery. Addresses EMBL, Cell Biology and Biophysics Programme, Meyerhofstrasse 1, 69117 Heidelberg, Germany  e-mail: nedelec@embl-heidelberg.de Current Opinion in Cell Biology 2003, 15:118±124 This review comes from a themed issue on Cell structure and dynamics Edited by Michel Bornens and Laura M Machesky 0955-0674/03/$ ± see front matter ß 2003 Elsevier Science Ltd. All rights reserved. DOI 10.1016/S0955-0674(02)00014-5 Introduction In biology, the term `self-organisation' can have different meanings. Most commonly, it refers to an organisation processinwhichmultipleagents(e.g.molecules,animals, etc.) follow behavioural rules based on local information. Self-organisation requires the absence of a preconceived vision of the ®nal organisationÐa plan that would be followed by the agents, or imposed on them by a leader [1]. In physics and chemistry, this de®nition is not of much use, simply because the systems studied are made of agents that are too simple, of molecules that lack intelli- gence. Thus, the above de®nition of `self-organisation' always applies, and the term is used instead as a synonym of out-of-equilibrium organisation [2,3]. In this more precise sense, a self-organised system continuously con- sumes and dissipates energy to maintain itself. This should be opposed to `self-assembling' systems, which instead release free energy during their organisation, leading to static structures in which no energy ¯ows. Self-assembly and self-organisation thus cover the two basic possibilities withrespecttoenergyrequirements.Aswillbeillustrated later, typical organisation phenomena contain both self- assembling and self-organising parts. The cytoskeleton as a self-organising system The cytoskeleton is the basis of the internal architecture of eukaryotic cells, and its organisation seems to emerge mostly from self-organisation. This is already the case for the key components of the cytoskeleton: the polar ®la- ments generated by the non-covalent assembly of tubulin or actin subunits. This assembly is coupled to GTP and ATP hydrolysis, for microtubules and actin, respectively [4,5]. The dynamic ®laments found in cells are self-orga- nised structures because they persistently consume energy. Microtubules or actin ®laments can also be said to be self-assembled structures, because they can be formed even when GTP hydrolysis is inhibited [6] Ð thatis,withoutsustainedconsumptionofenergy.Itmight be perplexing that both self-assembly and self-organisa- tion can produce microtubules; however, the two types of microtubules are of a different nature. Their tubulin lattices, although similar, look different on electron micrographs [7,8]. This difference would become striking if we could detect the forces and tensions inside the structure. While a static microtubule grown in the pre- sence of a non-hydrolysable GTP analogue is a tube that has little tensions, a dynamic microtubule grown in the presence of GTP is a tube ready to crack. During growth, the energy supplied by GTP hydrolysis is stored in the lattice as mechanical strain. This strain powers the fast shortening of disassembling microtubules [9±11]. This simple example shows how dif®cult it could be to make the distinction between self-organisation and self-assem- bly. Forces are not easy to see, and the best way to detect self-organisation is to look at the dynamical properties of the system. Dynamic instability and treadmilling are phenomena that require energy dissipation, and which could not emerge from a pure self-assembly process [12±15]. Thus, tubulin or actin monomers self-organise into dynamic®laments.Thenextlevelofcomplexityemerges from the self-organisation of those ®laments into various three-dimensional patterns. This involves other compo- nents, such as regulators of ®lament nucleation and dynamics, as well as molecular motors [16]. In contrast to chemical systems, such as the Belouzov±Zhabotinsky reaction [17,18], mechanical forces are central to the self- organisation of the cytoskeleton, simply because of the sizeofthose®laments.Indeed,whilesmallmoleculescan diffuse and react rapidly to organise themselves in space, 118 Current Opinion in Cell Biology 2003, 15:118±124 www.current-opinion.com