Key calcium ion sensor involved in the Ca(2+)-triggered synaptic vesicle-plasma membrane fusion and in the control of neurotransmitter release at these output synapses. Detected at basolateral cell membrane with synaptic vesicles surrounding the ribbon and at the presynaptic plasma membrane in the inner hair cells (IHCs) at postnatal day 30 (P30). Colocalizes with GPR25 and RAB8B in inner hair cells. The N-terminal first 124 residues can be classified as C2 domain, based on their 3D-structure. They are not sufficient for calcium ion or phospholipid binding (By similarity). Belongs to the ferlin family.