Cofilin binding to muscle and non-muscle actin filaments: isoform-dependent cooperative interactions.

I have monitored equilibrium binding of human cofilin to rabbit skeletal muscle (alpha) and human non-muscle (85% beta, 15% gamma) actin filaments from the quenching of pyrene actin fluorescence. Filament binding is cooperative and stoichiometric (i.e. one cofilin molecule per actin subunit) for both actin isoforms. The Hill coefficient for binding to betagamma-actin filaments (n(H)=3.5) is greater than for muscle actin (n(H)=2.3). Analysis of equilibrium binding using a nearest-neighbor cooperativity model indicates that the intrinsic affinities for binding to an isolated site are comparable (10-14 microM) for both filament isoforms but the cooperative free energy is greater for binding betagamma-actin filaments. The predicted cofilin cluster sizes and filament binding densities are small at concentrations of cofilin where efficient filament severing is observed, indicating that a few bound cofilin molecules are sufficient to destabilize the filament lattice and promote fragmentation. The analysis used in this study provides a framework for evaluating proton and ion linkage and effects of regulatory proteins on cofilin binding and severing of actin filaments.