Mechanism of microtubule kinesin ATPase.

A six-step mechanism is derived for the activation of kinesin K379 ATPase by microtubules. The data are fitted by the kinetic scheme [Formula see text] where T, D, and P refer to nucleotide triphosphate, nucleotide diphosphate, and inorganic phosphate, respectively; MtK refers to the complex of a K379 unit with the microtubule binding site. The initial binding and release steps, 1 and 6, are treated as rapid equilibria: k2 = 200 s-1, k3 = 100 s-1, k5 = 35-40 s-1, maximum steady-state rate = 25 s-1 (50 mM NaCl, 20 degrees C). k2 was obtained from the maximum rate of fluorescence enhancement with mant-ATP as substrate, k3 was obtained from the hydrolysis transient phase for ATP or mant-ATP, and k5 was obtained from the rate of decrease in fluorescence of mant-ADP in the reaction [Formula see text]. A large excess of ATP was present with the Mt to block rebinding of mant-ADP. The rate was measured as a function of microtubule concentration and extrapolated to give the maximum rate k5. The same method was used to obtain k5 for ADP by mixing K.ADP with microtubules plus excess mant-ATP. The enhancement of fluorescence for the binding of mant-ATP is followed by a decrease in fluorescence with a rate constant of 35-40 s-1. Since the decrease must occur after hydrolysis, it may be correlated with a step or steps leading to the low fluorescence MtK.D state. In the kinetic scheme, steps 4 and 5 both contribute to determining the maximum turnover rate. At higher ionic strengths or lower protein concentrations, the MtK complex is dissociated by ATP. The maximum rate is 12 +/- 2 s-1 in 50 mM NaCl; consequently, hydrolysis occurs before dissociation. The dissociation constant of MtK in the presence of ADP is twice as large as the dissociation constant in the presence of ATP and four times larger than the KM for microtubule activation. The proposed kinetic scheme, which treats the K379 units of a dimer as independent, provides a satisfactory description of the transient and steady-state properties of the system with the possible exception of results at very low substrate concentrations.