Quenching of tryptophan fluorescence in skeletal myosin rod

摘要

The fibrous region of myosin, called myosin rod when isolated from myosin after proteolysis, is a two-stranded coil made of identical chains of nearly 1000 residues. Myosin from rabbit skeletal muscle has two tryptophans per chain located at identical hydrophobic d sites in the heptad repeat which forms the basis for hydrophobic dimerization. Steady-state quenching of these tryptophans by KI shows downward curvature in a classic Stern-Volmer plot; analysis using a modified S-V equation indicates that only about 80% of the tryptophans are accessible to this charged quencher. The emission intensity decays are complex and are fit to lifetimes of 5.3, 1.6, and 0.4 ns with relative amplitudes of 0.78, 0.13, and 0.09. Lifetime resolved quenching studies indicate that only the long lifetime component is quenched by iodide; the collisional rate constant for quenching this component is very similar to that calculated from the steady-state quenching data. The long lifetime component thus corresponds to a population of solvent accessible tryptophans that may be on the surface of the coil protein. These studies suggest that the tryptophans in myosin rod may be in equilibrium between accessible and inaccessible sites at the coil interface.