Role of Rho GTPases of the RhoBTB family in tumorigenesis

We have recently identified a new subfamily of Rho GTPases that we have called RhoBTB. Members of this subfamily are present in mammals, Drosophila and Dictyostelium, therefore widely distributed in the metazoans. RhoBTB Proteins are characterized by a modular organization, consisting of a GTPase domain, a proline rich region, a tandem of two BTB domains and a C-terminal region of unknown function. BTB domains have been shown recently to participate in the formation of cullin-3 dependent ubiquitin ligase complexes. The remarkable modular architecture of these proteins suggests that they could play a pivotal role as scaffolds, functioning as target recognition elements for ubiquitination and degradation of signalling components. We have determined the gene structure of the human and mouse RHOBTB genes and have analyzed their expression profile. RhoBTB proteins are well represented in cancer tissues, and hybridization of a cancer profiling array corroborates this observation. Indeed, a recent reports indicate that RhoBTB2 might function as a tumor suppressor.

We have initiated the characterization of the three mammalian RhoBTB isoforms. For in depth studies we are currently concentrating on RhoBTB3, which is widely expressed and is present in all cell culture lines analyzed so far. We have studied the subcellular localization and are searching for binding partners of this protein in a two hybrid screening. We intend to investigate the function of RhoBTB3 using gen inactivation by means of the recently developed RNA interference method. We also plan to investigate the role RhoBTB GTPases in tumorigenesis. As a first step, we plan to study the expression of each RHOBTB gene in diverse normal and tumoral tissues using in situ hybridization and polyclonal antibodies that have being raised recently. These studies are being performed in parallel with the investigation of the Dictyostelium RhoBTB homologue, RacA.