Structural Basis of Lipid Membrane Binding by Human Ferlins

Ferlins, ancient membrane proteins with a unique architecture, are central to multiple essential, Ca ²⁺ -dependent vesicle fusion processes. Despite numerous functional studies and their link to burdening human diseases, a mechanistic understanding of how these multi-C ₂ domain proteins interact with lipid membranes to promote their remodeling and fusion is currently lacking. Here, we elucidate the near-complete cryo-electron microscopy structures of human myoferlin and dysferlin in their Ca ²⁺ and lipid-bound states. We show that ferlins adopt compact, ring-like tertiary structures achieved upon membrane binding. The top arch of the ferlin ring, comprising the C ₂ C-C ₂ D region, is rigid and varies little across the observed functional states. In contrast, the N-terminal C ₂ B and the C-terminal C ₂ F-C ₂ G domains cycle between alternative conformations and, in response to Ca ²⁺ , close the ferlin ring, promoting tight interaction with the target membrane. Probing key domain interfaces validates the observed architecture and informs a model of how ferlins engage lipid bilayers in a Ca ²⁺ - dependent manner. This work reveals the general principles of human ferlin structures and provides a framework for future analyses of ferlin-dependent cellular functions and disease mechanisms.