Mechanical response of single filamin A (ABP-280) molecules and its role in the actin cytoskeleton

Filamin A produces isotropic cross-linked three-dimensional orthogonal networks with actin filaments in the cortex and at the leading edge of cells. Filamin A also links the actin cytoskeleton to the plasma membrane via its association with various kinds of membrane proteins. Recent new findings strongly support that filamin A plays important roles in the mechanical stability of plasma membrane and cortex, formation of cell shape, mechanical responses of cells, and cell locomotion. To elucidate the mechanical properties of the actin/filamin A network and the complex of membrane protein-filamin A-actin cytoskeleton, the mechanical properties of single human filamin A (hsFLNa) molecules in aqueous solution were investigated using atomic force microscopy. Ig-fold domains of filamin A can be unfolded by the critical external force (50-220 pN), and this unfolding is reversible, i.e., the refolding of the unfolded chain of the filamin A occurs when the external force is removed. Due to this reversible unfolding of Ig-fold domains, filamin A molecule can be stretched to several times the length of its native state. Based on this new feature of filamin A as the 'large-extensible linker', we describe our hypothesis for the mechanical role of filamin A in the actin cytoskeletons in cells and discuss its biological implications. In this review, function of filamin A in actin cytoskeleton, mechanical properties of single filamin A proteins, and the hypothesis for the mechanical role of filamin A in the actin cytoskeletons are discussed.