Microtubule architecture: inspiration for novel carbon nanotube-based biomimetic materials Francesco Pampaloni 1 and Ernst-Ludwig Florin 2 1 Cell Biology and Biophysics Unit, European Molecular Biology Laboratory, Meyerhofstrasse 1, D-69117 Heidelberg, Germany 2 Center for Nonlinear Dynamics, University of Texas, Austin, TX 78712, USA Microtubules are self-assembling biological nanotubes that are essential for cell motility, cell division and intra- cellular trafficking. Microtubules have outstanding mechanical properties, combining high resilience and stiffness. Such a combination allows microtubules to accomplish multiple cellular functions and makes them interesting for material sciences. We review recent experiments that elucidate the relationship between molecular architecture and mechanics in microtubules and examine analogies and differences between micro- tubules and carbon nanotubes, which are their closest equivalent in nanotechnology. We suggest that a long- term goal in bionanotechnology should be mimicking the properties of microtubules and microtubule bundles to produce new functional nanomaterials. Introduction Microtubules are cytoskeletal biopolymers that, along with actin and intermediate filaments, accomplish essential functions at each stage of the cell’s life cycle. They ensure the mechanical stability of the mitotic spindle, provide oriented tracks for intracellular trafficking of organelles and support the cell’s shape during migration [1]. At the cell length scale, microtubules are very stiff filaments. The average Young’s modulus of a microtubule, considered as a simple isotropic tube, is 2 GPa. Thus, microtubules are as stiff as hard plastic and about one hundred times stiffer than the other cytoskeleton components, actin and inter- mediate filaments [1]. Interestingly, microtubules are not only stiff, but also highly resilient. Their efficient combi- nation of high stiffness (relative to the other cytoskeletal filaments) and resilience is due to the anisotropic molecu- lar architecture of microtubules and allows them to accom- plish multiple tasks in the cell. On the one hand, high stiffness is required to resist the large pushing forces occurring during mitotic spindle elongation at the end of anaphase. On the other hand, high resilience allows micro- tubules to search the cellular space laterally for binding partners and to keep growing in a different direction with- out breaking when encountering obstacles. As microtubules are extraordinarily versatile struc- tures, the question arises of what could be learned from them for the design of novel structural and multifunctional materials for applications in material sciences and biona- notechnology. Carbon nanotubes (CNTs), one of the most promising products of nanotechnology, are the closest technological counterpart of microtubules. CNTs are an extremely stiff material; their Young’s modulus is 1 TPa, about five times higher than that of steel (210 GPa) [2]. Similar to microtubules, CNTs are also highly resilient. It is generally acknowledged that CNTs will play a major role in the development of new materials, with applications ranging from ‘super-tough’ composite fibers [3] to drug-delivery systems [4]. Molecular control over the CNT assembly process would be desirable for the full exploitation of their nanoscale properties and for the reproducible fabrication of CNT-based materials. How- ever, although CNTs aggregate into bundles or sheets spontaneously, their assembly into designed composite structures remains difficult to understand and to direct at molecular level. Microtubules and CNTs are surprisingly similar in their mechanical behavior despite their very different chemical composition (proteins and non-covalent interactions in the case of microtubules, carbon and covalent bonds in the case of CNTs) and elastic moduli. Here, we describe key structural aspects of microtubules and review recent results on their mechanics. We then compare CNTs and microtubules side-by-side with respect to structural and elastic properties. Finally, we discuss examples of how microtubules and microtubule-based structures could provide insights for the design and assem- bly of novel CNT-based biomimetic materials. Structure of microtubules The basic building unit of microtubules is the heterodi- meric protein tubulin, consisting of a and b subunits. Tubulin dimers self-assemble head-to-tail (-ab-ab-) into linear protofilaments (PFs) (Figure 1). The cylindrical wall most commonly comprises 13 PFs in vivo, but this number can vary from 9 to 16 in vitro [5]. Tubulins of adjacent PFs are laterally linked through homologous monomer con- tacts a-a, b-b (except at the ‘seam’, Figure 1). In the microtubule, stacked PFs are slightly longitudinally dis- placed with respect to each other (0.9 nm in a 13-PF microtubule). This displacement results in a left-handed helical surface lattice. Although the biological function of Opinion Corresponding authors: Pampaloni, F. (pampalon@embl.de); Florin, E.-L. (florin@chaos.utexas.edu). 302 0167-7799/$ – see front matter ß 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.tibtech.2008.03.002 Available online 21 April 2008