The « small GTPase structure/function relationship » project emerged one year ago in the Structural Motility group at the Curie Institute. Our goal is to gain 3D structural information on several small GTPases of the H-Ras superfamily that are involved in cancer. We have chosen to study these proteins in complex with their cellular partners in order to visualize the protein-protein interactions that are at the origin of their functions. Our structures will provide critical information leading to better understanding of their mode of action, their functions and their roles in cancer. The cellular biology environment of the Curie Institute is ideal for the development of this project. We are not only informed on unpublished results that involve small GTPases in the cancer field but we can develop very fruitful collaborations to perform a full structural/functional study of these proteins. In this context, we are interested in the study of three families of small GTPases : The RGK proteins, as atypical small GTPases. RGK proteins are critical regulators of cytoskeleton remodelling and voltage-dependent calcium channels (VDCCs). These small GTPases exhibit several atypical features in their primary sequence and their mode of regulation. To elucidate how this atypical family of small GTPases function, we propose to determine the structures of an RGK protein in complex with calmodulin which plays a role in its regulation and with one of its effectors that is involved in the VDCCs signalling pathway. The Arf proteins, structure/function studies. The Arf small GTPases regulate vesicular traffic and organelle structure. A new role for ARF6 in cytokinesis has been reported involving the JIP proteins (scaffold proteins regulating the MAP Kinase pathways), and two reverse microtubule motors, Kinesin1 and the Dynein-Dynactin complex. One intriguing observation is that ARF6 and the two reverse motors interact with the same region of the JIP proteins ; some interactions are exclusive, while others are cooperative. To better understand the role of each of these proteins in cytokinesis, we plan to determine the structures of the JIP proteins in complex with ARF6 and with the kinesin-1 light chain, respectively. The Rho proteins, as a target for bacterial toxins. Rho proteins are main regulators of the actin cytoskeleton organization and cell division. These critical functions make these small GTPases favorite targets of bacterial toxins. One of these toxins, the C3 exoenzyme, has become powerful as a pharmacological tool to study their functions, since it exhibits high specificity for one subfamily of Rho proteins. We are interested in understanding in detail, the structural basis that allows the C3 exoenzyme to specifically target and modify this Rho subfamily. To do so, we propose to solve the structure of several intermediates of the C3 exoenzyme-RhoA complex along the enzymatic reaction.