Affinity of Myosin S-1 for F-Actin, Measured by Time-Resolved Fluorescence Anisotropy

The association constant for myosin subfragment-1 (S-1) and actin was measured, using a new application of fluorescence depolarization which capitalizes on the fact that S-1 has high rotational mobility while F-actin does not. Uncoupling of the time dependences of the anisotropy decay and the association/dissociation phenomena allowed the experimentally determined anisotropy decay curve to be fitted by a sum of two terms weighted by the mole fractions of the free and bound S-1. At 4 degrees C, ionic strength 0.16 M, and pH 7.0, the association constant Ka is (1.73 ± 0.35) × 106 M-1 at infinite dilution. This makes the -Δ Go of binding of F-actin to S-1 similar to the -Δ Go of binding of ATP to S-1, and the possible physiological relevance of the similarity to muscle contraction is discussed.