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Abstract

This work describes the crystal structures of the GTPase domain of dynamin 1 from Rattus norwegicus and the motor domain of myosin II from Dictyostelium discoideum in atomic detail. The crystal structure of the nucleotide-free GTPase domain of dynamin 1 shows that the fold of the GTPase domain in mammalian dynamin closely resembles that in dynamin A from D. discoideum. The presented structural model contains all loop regions and therefore allows for the first time the complete description of the nucleotide binding region of a dynamin family member. In contrast to other GTPases the switch I threonine, which is important for the coordination of GTP is kept in its catalytically active conformation in dynamin 1. Furthermore, on the basis of the structural model it was possible to propose a role for Arg59 as accelerator of GTP hydrolysis. The crystal structure of the myosin head domain shows a novel conformation of the motor protein wherein both nucleotide-binding switch motives are in an open position. This hitherto unknown state is referred to as O/O conformation in a new nomenclature. From the structural model it becomes apparent that both nucleotide and actin binding region are not only able to communicate via switch II but also via switch I. Positional change of switch I alters the geometry of the actin binding site at the outer end of the 50K cleft via the central sheet. Thereby the switch positions are not entirely uncoupled. Switch I can only open if switch II is already open. In the O/O conformation switch II is more open than observed in previous structures, which in combination with an open switch I could result in release of ADP. The results of this work complement the previous picture of the actomyosin ATPase cycle accounting for both the position of switch II and switch I. After the power stroke and the dissociation of phosphate by opening of switch II, switch I has to open as well to release ADP. After reaching the rigor state switch I closes upon ATP binding and the myosin head dissociates from the actin filament. In the actomyosin ATPase cycle the new O/O conformation can be placed in immediate vicinity to the rigor state.