Structural insights into lipid membrane binding by human ferlins.

Ferlins are ancient membrane proteins with a unique architecture, and play central roles in crucial processes that involve Ca(2+)-dependent vesicle fusion. Despite their links to multiple human diseases and numerous functional studies, a mechanistic understanding of how these multi-C(2) domain-containing proteins interact with lipid membranes to promote membrane remodelling and fusion is currently lacking. Here we obtain 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 upon membrane binding. The top arch of the ferlin ring, composed of the C(2)C-C(2)D region, is rigid and exhibits only little variability 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.