Ubiquitin-dependent remodeling of the actin cytoskeleton drives cell fusion

Cell-cell fusion is a frequent and essential event during development, and its dysregulation causes diseases ranging from infertility to muscle weakness. Fusing cells repeatedly need to remodel their plasma membrane by orchestrated formation and disassembly of cortical actin filaments, but how the dynamic reorganization of the actin cytoskeleton control is still poorly understood. Here, we identified a ubiquitin- dependent toggle switch that establishes reversible actin bundling during mammalian cell fusion. We found that EPS8-IRSp53 complexes stabilize cortical actin bundles at sites of cell contact, which in part pushes cells towards each other. Conversely, EPS8 monoubiquitylation by CUL3KCTD10 displaces EPS8-IRSp53 from membranes and counteracts actin bundling, a dual activity that allows apposed cells to progress with fusion. We conclude that cytoskeletal rearrangements during development are precisely controlled by ubiquitylation, raising the possibility to modulate the efficiency of cell-cell fusion for therapeutic benefit. Overall design: RNA-seq analysis was performed on undiffernitataed myoblasts cells seeded at subcondluent densities, and treated with ON-TARGETplus Non-targeting Control (Horizon Discovery; D-001810-03 UGGUUUACAUGUUUUCUGA), or ON-TARGET plus siRNA siKctd10 #1 mouse (Horizon Discovery; J-057526-05 CCAGCAAUUCUGACGAUAA)