LETTERS 600 nature materials | VOL 2 | SEPTEMBER 2003 | www.nature.com/naturematerials E merging technologies are creating increasing interest in smart materials that may serve as actuators in micro- and nanodevices 1–3 . Mechanically active polymers currently studied include a variety of materials 4–9 . ATP-driven motor proteins, the actuators of living cells 10 , possess promising characteristics 11–13 , but their dependence on strictly defined chemical environments can be disadvantagous 14 . Natural proteins that deform reversibly by entropic mechanisms might serve as models for artificial contractile polypeptides with useful functionality 15 , but they are rare 16 . Protein bodies from sieve elements of higher plants 17–19 provide a novel example. sieve elements form microfluidics systems for pressure-driven transport of photo- assimilates throughout the plant 20–22 . Unique protein bodies in the sieve elements of legumes act as cellular stopcocks, by undergoing a Ca 2+ -dependent conformational switch in which they plug the sieve element 23 . In living cells, this reaction is probably controlled by Ca 2+ - transporters in the cell membrane 23 . Here we report the rapid, reversible, anisotropic and ATP-independent contractility in these protein bodies in vitro. Considering the unique biological function of the legume ‘crystalloid’ protein bodies and their contractile properties, we suggest to give them the distinctive name forisome (‘gate-body’; from the Latin foris, the wing of a gate). We scrutinized the Ca 2+ response in vitro, using forisomes isolated from broad bean (Vicia faba) sieve elements. Individual sieve elements were ripped open with fine glass pipettes in Ca 2+ -free medium (see Methods).The forisome was then pressed with the pipette tip against the opposite cell wall to enable it to stick to the pipette and be transferred to Ca 2+ -free medium in test vessels that allowed microscopic observation. In our first attempts, forisomes were heavily damaged during the extraction, but still swelled and shrank isodiametrically in response to media with or without Ca 2+ , respectively (Fig. 1a). Forisomes that had retained their shape (the length varied between 18 and 34 μm, and diameter between 2.2 and 4 μm) during isolation and therefore appeared intact, did not only swell radially in response to Ca 2+ , but also contracted longitudinally (Fig. 1b). Thus, the condensed form of forisomes corresponds to a longitudinally expanded state, whereas forisomes in dispersed conformation are longitudinally contracted. In contraction, forisomes shortened by 29.4% ± 6.7% (means ± s.d., n = 15) and increased in diameter (measured across their centre) by 119.5% ± 47.9%. This corresponds to a more than threefold Ca 2+ - induced volume increase. The response to Ca 2+ , which invariably occurred in all of the more than 200 intact forisomes that we isolated individually, was fully reversible by transfer to Ca 2+ -free solutions ATP-independent contractile proteins from plants MICHAEL KNOBLAUCH* 1 , GUNDULA A. NOLL 1 , TORSTEN MÜLLER 2 , DIRK PRÜFER 3 , INGRID SCHNEIDER-HÜTHER 1 , DÖRTE SCHARNER 1 , AART J. E. VAN BEL 1 AND WINFRIED S. PETERS 1,4 1 Institut für Allgemeine Botanik der Justus Liebig-Universität, Senckenbergstr. 17-21, D-35390 Gießen, Germany 2 Institut für Biologie, Humboldt Universität zu Berlin, Invalidenstr. 42, D-10115 Berlin, Germany 3 Fraunhofer Institut für Molekularbiologie und Angewandte Ökologie, Bereich Molekularbiologie, Auf dem Aberg 1, D-57392 Schmallenberg-Grafschaft, Germany 4 Arbeitskreis Kinematische Zellforschung, Biozentrum der Johann Wolfgang Goethe-Universität, Marie-Curie-Str. 9, D-60439 Frankfurt am Main, Germany *e-mail: michael.knoblauch@bio.bot1.uni-giessen.de Published online: 24 August 2003; doi:10.1038/nmat960 Figure 1 Response to Ca 2+ of forisomes isolated from Vicia faba sieve elements. a, Isotropic swelling of a forisome that had been damaged mechanically during isolation. The forisome is attached to the tip of a glass pipette (tip diameter 4 μm). Swelling and shrinkage were induced reversibly by application of Ca 2+ (10 mM CaCl 2 , 50 mM KCl, 10 mM HEPES pH 7.3) or Ca 2+ -free medium (as before, with CaCl 2 replaced by 10 mM disodium EDTA), respectively, as indicated. b, Anisotropic Ca 2+ response with longitudinal contraction by about 30% in an intact forisome (solutions as in a). The length of the expanded forisome (left) is 24 μm. A movie showing this experiment is available in the Supplementary Information, Movie S1. ©2003 Nature Publishing Group