The crystal structure of the C₂A domain of otoferlin reveals an unconventional top loop region.

Otoferlin (Otof), whose genetic mutations cause profound deafness in humans, is a protein composed of at least six C(2) domains, which are known as Ca(2)(+)-binding and phospholipid-binding regions. Mammalian ferlin proteins are proposed to act in membrane fusion events, with Otof being specifically required for exocytosis in auditory hair cells. Ferlin C(2) domains exhibit a rather low level of sequence similarity to those of synaptotagmins, protein kinase C isoforms, or phospholipases. Here, we report the crystal structure of the N-terminal C(2) domain of Otof (C₂A) at 1.95-Å resolution. In contrast to previous predictions, we found that this C(2) domain is complete with eight β-strands. Comparing the structure of Otof C₂A to those of other C(2) domains revealed one top loop in Otof to be significantly shorter. This results in a depression of the surface, which is positively charged for the Otof C₂A domain, and contrasts with the head-like protrusion surrounded by a negatively charged "neck" typically found in other C(2) domains. Isothermal titration calorimetry and circular dichroism spectroscopy studies confirmed that Otof C₂A is unable to bind Ca(2+), while the synaptotagmin-1 C₂A domain exhibited Ca(2+) binding under the same conditions. Furthermore, floatation assays revealed a failure of Otof C(2)A to bind to phospholipid membranes. Accordingly, no positively charged β-groove-like surface structure, which is known to bind phosphatidylinositol-4,5-bisphosphate in other C(2) domains, was found at the respective position in Otof C₂A. Taken together, these data demonstrate that the Otof C₂A domain differs structurally and functionally from other C(2) domains.