Mitochondria constitute a real cellular network whose morphology is conditioned by a constant equilibrium between active processes of fission and fusion of biological membranes. Unique among membrane fusion systems, that of mitochondria does not require SNAREs but two sets of GTPases, the mitofusins and OPA1/Mgm1, involved in outer and inner membrane fusion respectively. Consequently, these GTPases do not only participate in oxidative phosphorylation and apoptosis but their mutations are also directly linked to several neurodegenerative syndromes such as the Charcot-Marie Tooth and Parkinson diseases. We have previously shown that Fzo1, the yeast mitofusin, is subject to an exquisite regulation by the Ubiquitin-Proteasome system. We will aim at dissecting the role of this regulation in discrete steps of the mitochondrial fusion process.