In eukaryotic cells, biochemical processes occur in specialized membrane-enclosed compartments - organelles - whose integrity must be preserved to ensure cellular health.
Recently, it was discovered that mitochondria - the cellular power plants – establish physical contacts with a myriad of other organelles, with crucial functional implications. In particular, mitochondria are tethered to the endoplasmic reticulum, peroxisomes, lysosomes , to the plasma membrane and to themselves.

Fig: In eukaryotes, physical contacts between mitochondria and other organelles allows optimized performance. Mitochondria associate with each other (allowing their fusion), with the endoplasmic reticulum (ER - the production site for lipids and membrane proteins), the peroxisomes (necessary for several catabolic and anabolic reactions e.g. lipid synthesis and detoxification of reactive oxygen species), the lysosomes (where waste materials within the cell are digested to reusable building blocks) and the plasma membrane (PM - which separates each cell from its outside environment). The respective tethers are called MECA, ERMES, POMES and vCLAMP.

Mitochondria are very dynamic organelles that constantly fuse and divide. This plasticity is crucial for embryonic development, for neuronal survival, and is associated with mitophagy of damaged mitochondria and apoptosis.

We want to understand how mitochondrial fusion is regulated, using genetics, biochemistry and cell biology.
Deficiencies in mitochondrial fusion cause Charcot–Marie–Tooth disease, the most common form of hereditary peripheral neuropathy and were also linked to Parkinson's disease, the most frequent movement disorder.

How does ubiquitylation regulate mitochondrial fusion?

From yeast to mammals, mitochondrial fusion is regulated by post-translational modifications – like ubiquitylation - of its central component, mitofusin.

Ubiquitin, a 76-small amino acid protein, can be covalently attached to lysine residues of target proteins as a single moiety, or in the form of different ubiquitin chains. This leads to distinct cellular functions, from protein degradation to stabilization of protein complexes. Ubiquitylation begins with the activation of the modifier by E1 enzymes, followed by its transfer to E2 conjugating enzymes, and its ligation to the target substrate by E3 ubiquitin ligases, like the SCF complexes, where the variable F-box subunit mediates substrate recognition.

Fig.: Protein post-translational modification by ubiquitylation. (A) Different possibilities of post-translational modification of target substrates by monoubiquitylation or subsequent polyubiquitylation. (B) Enzymatic cascade allowing ubiquitylation. Ubiquitin is successively transferred from enzymes E1 to E2. Then, ubiquitin is either directly transferred to the E3 before its final conjugation with the substrate or, alternatively, transferred from the E2 to the substrate brought into its proximity by the E3.
We found that the yeast mitofusin Fzo1 is ubiquitylated by two independent pathways of ubiquitylation/deubiquitylation, which perform a regulatory or a quality control role. Each of these pathways either activates or inhibits mitochondrial fusion, thus pointing to a sophisticated mechanism of regulating mitochondrial dynamics.

Fig.: Model for the role of Fzo1 ubiquitylation in OM fusion. Ubp2 and Ubp12 regulate two distinct ubiquitylation path-ways, which inhibit or promote mitochondrial fusion, respectively, leading to fragmentation or to increased connectivity of the mitochondrial network. Ubp2 acts on a quality control pathway and removes destabilizing ubiqui-tylation from Fzo1. Ubp12 acts on a regulatory pathway, where ubiquitylation stabilizes Fzo1 and allows fusion (adapted from Wiedemann et al., 2013)