Dynamic cross-linking by α-actinin determines the mechanical properties of actin filament networks

We used smooth muscle α-actinin to evaluate the contribution of cross- linker dynamics to the mechanical properties of actin filament networks. Recombinant actin-binding domain (residues 2-269) binds actin filaments with a K(d) of 1 μM at 25°C, 20 times stronger than actin-binding domain produced by thermolysin digestion of native α-actinin (residues 25-257). Between 8 and 25°C the rate constants for recombinant actin-binding domain to bind to (0.8-2.7 μM^-1 s^-1) and dissociate from (0.2-2.4 s^-1) actin filaments depend on temperature. At 8°C actin filaments cross-linked with α-actinin are stiff and nearly solid, whereas at 25°C the mechanical properties approach those of actin filaments alone. In these experiments, high actin concentrations kept most of the α-actinin bound to actin and temperature varied a single parameter, cross-linker dynamics, because the mechanical properties of pure actin filaments (a viscoelastic gel) or biotinylated actin filaments cross-linked irreversibly by avidin (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 properties of the networks.