Motility powered by supramolecular springs and ratchets
Authors: L. Mahadevan and P. Matsudaira
Publication: Mahadevan, L., and Matsudaira, P. "Motility powered by supramolecular springs and ratchets." Science (2000) vol. 288 (5463) pp. 95-99.
Keywords: Brownian motion
Biological movements are brought about in various ways, and not all of these involve the action of motor proteins. Apart from motor proteins, biological movements can be brought about by components that act like springs and ratchets. Biological springs store potential energy in various protein filament conformations, while biological ratchets act by biasing random Brownian motion and turning it into directional polymerization.
Leeuwenhoek observed in 1676 that small, single-celled organisms on leaves could retract and spring forth repeatedly. It turns out that these quick movements are brought about by rapid calcium ion binding. The mechanism is shown in Figure 1. The spring structure is made from a bundle of filaments held together by negatively charged ions (blue). Contraction of the spring is driven by entropic forces. When calcium ions are released (red), the charge is neutralized and the filaments lose their extended structure, condensing due to entropy.
A second type of spring is observed in some marine invertebrate sperm cells. The spring is made up of a bundle of coiled actin filaments. This is shown in Figure 2. In the coiled state, the actin filaments are over-twisted. A sudden calcium-dependent change in conformation then causes the filaments to untwist and slide past each other, releasing the stored chemical energy.
Even some viruses possess similar spring-like components to inject their DNA into bacterial cells.
In ratchet systems, motion is driven by polymerization. Polymerizing filaments are cross-linked, forming a gel that provides the force for movement. Certain bacteria can burst out of a cell (see Figure 3) using asymmetric nucleation of host cell actin polymerization on one side of the bacterial cell wall. Part (C) of Figure 3 shows the idea of a Brownian ratchet, where small fluctuations in the position of the moving object (black) allow for the addition of a new monomer at the end of a polymerizing filament. Cross-linking anchors the filament, and constant polymerization prevents the moving object from fluctuating backward. Thus, the random processes of diffusion and object fluctuation are biased to occur in one direction. Filaments act cooperatively to generate force in this way.
By organizing proteins into special geometries like filaments and bundles, Nature amplifies small conformational changes into meso- and macroscopic movements. Even though these biological springs and ratchets are based on different proteins, they are usually similar in important ways. Viscous forces and Brownian motion dominate on biological length scales, and motion results either from a rapid release of stored chemical energy (often in metastable protein conformations) or from the slower rectification of polymerization in a specified direction.
 Mahadevan, L., and Matsudaira, P. "Motility powered by supramolecular springs and ratchets." Science (2000) vol. 288 (5463) pp. 95-99. [DOI:10.1126/science.288.5463.95]