Tryptophan dynamics of the FK506 binding protein: Time-resolved fluorescence and simulations

The FK506-binding protein (FKBP12) is important in the immunosuppressant action of FK506 and rapamycin. We have investigated Trp side chain dynamics in FKBP12, with and without a bound immunosuppressant, by measuring the Trp time-resolved fluorescence anisotropy decay r(t). The r(t) for W59 in aqueous uncomplexed FKBP12 at 20°C is well described by a single exponential with a recovered initial anisotropy, r(o)/(eff), of 0.192 and an overall rotational correlation time for the protein, φ(p), of 4.7 ns; r(o)/(eff) = 0.214 and φ(p) = 4.2 ns for the FKBP12/FK506 complex. Using an expression for the order parameter squared, namely S² = r(o)/(eff)/r(o)/(T), where r(o)/(T) is the vitrified steady-state excitation anisotropy, we recovered an S² of 0.75 for W59 fluorescence in uncomplexed FKBP12 and S² ≃ 1 in the FKBP12/FK506 complex. Results obtained for the FKBP12/rapamycin complex are similar to those found for the FKBP12/FK506 complex. Minimum perturbation mapping simulations were performed on the free and complexed forms of FKBP12 and the results were generally in agreement with the experimental data.