Pathway of the Microtubule-Dynein ATPase and the Structure of Dynein: A Comparison with Actomyosin

Abstract

Dynein and myosin show several important similarities in design as well as some interesting differences in detail. Both ATPases function as crossbridges that undergo microscopic movements to drive the sliding of filaments, which results in macroscopic movements. They share a common design employing globular heads attached to flexible strands. Each head contains one ATP-binding site and one filament-binding site, and the binding of ATP induces an extremely rapid dissociation of the crossbridge-filament "rigor" complex. Following ATP hydrolysis, which is readily reversible, the crossbridge reassociates with the filament and returns to its original state with the release of products. Thus, the nucleotide-induced changes in conformation are effectively used to couple the hydrolysis of ATP to the dissociation and reassociation of the crossbridge in order to produce a force for net movement according to the Lymn-Taylor-Eisenberg model. The utilization of nucleotide-binding energy to induce a change in conformation can be rationalized in terms of our understanding of enzyme catalysis in general, whereby substrate binding energy is used to induce a change in conformation that stabilizes the transition state for catalysis. In these crossbridge ATPases, the substrate-induced change in conformation also serves to weaken the crossbridge-filament interaction. The pathway is symmetrical, with a return to the tight (filament) binding state coupled to product release. The ball on a string design may provide a reasonable basis to explain how a unidirectional force is obtained from a symmetrical cycle; opposite changes in conformation with the binding and release of the nucleotide produce a significant force only when pulling on the flexible strand. Moreover, the very rapid dissociation of the crossbridge following ATP binding limits the time that a negative force is in effect and also prevents a rigor crossbridge from retarding the sliding movements generated by other crossbridges. Myosin and dynein exhibit nearly identical kinetic constants governing ATP binding and the ATP-induced dissociation of the crossbridge. These appear as invariant steps that may reflect the basic principles of enzyme catalysis as applied to the mechanochemical cycle. The rates of ATP hydrolysis and synthesis by myosin and dynein differ slightly, but in each case the reactions are readily reversible with an equilibrium constant less than one. Steps involving the loss and rebinding of products occur at rates two to three orders of magnitude faster for dynein than for myosin.(ABSTRACT TRUNCATED AT 400 WORDS)