Local Conformation of Rabbit Skeletal Myosin Rod Filaments Probed by Intrinsic Tryptophan Fluorescence

Rabbit skeletal myosin rod contains two tryptophan residues per chain (four per coiled-coil) that are located about 50 and 175 Å from the N-terminus of the light meromyosin (LMM) region of rod. We have characterized the local polarity, excited-state photophysics, solvent accessibility, and rotational dynamics of these tryptophans in myosin rod filaments at 125 mM KCl using steady-state and time-resolved fluorescence techniques. The fluorescence decays were described using a complex bimodal distribution with a discrete long-lifetime component of 5.44 ns (amplitude of 0.51) and a Gaussian distribution of short lifetimes with mean of 0.105 ns and width of 2.15 ns (amplitude 0.49). The discrete long-lifetime species was efficiently quenched by the neutral quencher acrylamide with a bimolecular collision constant (k_q) of 0.85 × 10⁹ M⁻¹ s⁻¹. The emission spectrum, lifetime distribution, and quenching behavior of the tryptophans in myosin rod monomers (at 0.5 M KCl) were quite similar. Time-resolved anisotropy decays of the rod monomers and filaments exhibited nearly identical double-exponential decays to a constant. Each had a fast, subnanosecond component (amplitude 0.07), probably corresponding to fast wobble of the tryptophans on the coiled-coil surface, a slower, ≈6-ns component (amplitude ≈0.04), corresponding to an unidentified internal, segmental mode of motion of the coiled-coil, and a constant (r_∞) of 0.15. This study demonstrated that there were no significant differences in the chemical or physical environment of the rod tryptophans in the two aggregation states of the protein, indicating that the region of LMM containing these tryptophans was not tightly aggregated into the body of the thick filament. This work suggests that the myosin crossbridge may be nearly 300 Å longer than previous models have postulated.