P. A. Janmey, J. X. Tang, Josef Käs, Jagesh V. Shah
Brigham & Women's Hospital, Harvard Medical School, LMRC301, Boston, MA 02115.
Bundles and networks of F-actin are a common feature of cells and biological tissues, ranging from the isotropic meshworks at the cortex of motile cells to the paracrystalline arrays of actin filaments in microvilli. The formation of such structures is generally orchestrated by the activity of specific binding proteins, but the thermodynamic driving force for their formation is largely unknown. Like DNA, actin filaments are anionic with linear charge densities sufficiently high to stabilize electrostatic interactions with polycations even at physiological ionic strength. Theories of polyelectrolytes developed to account for cation-induced condensation of DNA apply equally well to F-actin, and provide an explanation for the ability of numerous polycationic proteins to be efficient, regulated, bundling factors even in the absence of stereospecific actin binding sites.
The viscoelastic properties of networks formed by F-actin differ strongly from those of microtubules and intermediate filaments. Recent advances in visualization of single filaments within networks and in theoretical modeling of semi-flexible polymers have set the stage for developing a quantitative explanation for the actin's viscoelasticity. The specific features of actin rheology suggest some aspects of its possible biological function, and recent theoretical models suggest that the molecular basis of the elasticity of actin networks differs radically from that of rubber-like materials. The effects of metal ions and polyvalent protein ligands on the structure and rheology of actin networks likewise provide data related both to the biological function of these networks and to the molecular structures underlying their mechanical properties.