Nature Materials 2, 600–604 (2003) In the above paper, we reported that the threshold concentration of free Ca2+ required to induce longitudinal contraction in forisomes was in the nanomolar range. Due to errors in estimating Ca2+ concentrations in Ca2+-buffered media, these calculated figures were incorrect.

Forisomes are elongate protein bodies of up to 30 gm length from cells of the sieve tube network of higher plants. In this natural microfluidics system, they act as reversible stopcocks by undergoing rapid conformational changes which involve more than three-fold increases in volume. The conformational switch is controlled by Ca2+ with a threshold concentration in the nM range. We here report recent progress in our attempts to define the technological potential of this novel biological actuator.

Emerging technologies are creating increasing interest in smart materials that may serve as actuators in micro- and nanodevices1,2,3. Mechanically active polymers currently studied include a variety of materials4,5,6,7,8,9. ATP-driven motor proteins, the actuators of living cells10, possess promising characteristics11,12,13, but their dependence on strictly defined chemical environments can be disadvantagous14. Natural proteins that deform reversibly by entropic mechanisms might serve as models for artificial contractile polypeptides with useful functionality15, but they are rare16. Protein bodies from sieve elements of higher plants17,18,19 provide a novel example. sieve elements form microfluidics systems for pressure-driven transport of photo-assimilates throughout the plant20,21,22. Unique protein bodies in the sieve elements of legumes act as cellular stopcocks, by undergoing a Ca2+-dependent conformational switch in which they plug the sieve element23. In living cells, this reaction is probably controlled by Ca2+-transporters in the cell membrane23. 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).