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Forisome: a smart plant protein inside a phloem system

Amy Shen
Washington University in St. Louis
Nanotechnology

With the discovery of the plant protein forisome, a novel, smart non-living, ATP-independent biological material became available to the designer of smart materials for advanced actuating and sensing. Forisomes are unique contractile protein bodies that function as cellular stopcocks in sieve tubes of the Fabaceae. They can block individual sieve elements by undergoing reversible conversions between a low-volume, spindle-shaped, crystalloid conformation and a high volume, spheroidal, disordered conformation; this conversion is accompanied by longitudinal shortening and radial expansion. The in vitro studies show that forisomes (1-3 micron wide and 10-30 micron long) can be repeatedly stimulated to contract and expand anisotropically by shifting either the ambient pH or the ambient calcium ion concentration.

Possible applications of forisomes include micro-valves, micro-actuators, and other smart sensing activities where one may currently see materials such as synthetic hydrogels or shape memory alloys. In order to pursue forisome synthesis as a smart material and to understand its biological function, a detailed understanding of its material properties is required. In this talk, I will present some current forisome studies on: (A) that the forisome is positively birefringent in the crystalline condensed conformation and that this birefringence is missing in the disordered dispersed conformation; (B) that the process of forisome dispersion is accompanied by a tripling of forisome volume, a doubling of equatorial perimeter, and a statistically significant 20% decrease in polar perimeter; (C) that forisome action is most readily explained by protein fibrils which self-assemble during forisome condensation to lie parallel to the axis of the forisome spindle, and diffuse apart during forisome dispersion to form a disordered mesh tacked together by occasional lateral chains. Detailed conformational kinetics of forisomes and materials characterizations of forisomes will also be discussed.

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