DYNAMIC CROSS-LINKING BY ALPHA-ACTININ DETERMINES THE MECHANICAL-PROPERTIES OF ACTIN FILAMENT NETWORKS

We used smooth muscle ar-actinin to evaluate the contribution of cross -linker dynamics to the mechanical properties of actin filament networ ks. Recombinant actin-binding domain (residues 2-269) binds actin fila ments with a K-d of 1 mu M at 25 degrees C, 20 times stronger than act in-binding domain produced by thermolysin digestion of native alpha-ac tinin (residues 25-257). Between 8 and 25 degrees C the rate constants for recombinant actin-binding domain to bind to (0.8-2.7 mu M-1 s(-1) ) and dissociate from (0.2-2.4 s(-1)) actin filaments depend on temper ature. At 8 degrees C actin filaments cross-linked with alpha-actinin are stiff and nearly solid, whereas at 25 degrees C the mechanical pro perties approach those of actin filaments alone. In these experiments, high actin concentrations kept most of the alpha-actinin bound to act in and temperature varied a single parameter, cross-linker dynamics, b ecause the mechanical properties of pure actin filaments (a viscoelast ic gel) or biotinylated actin filaments cross-linked irreversibly by a vidin (a stiff viscoelastic solid) depend little on temperature. These results show that the rate of exchange of dynamic cross-links between actin filaments is an important determinant of the mechanical propert ies of the networks.