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. Interacts in a calcium-dependent manner to the presynaptic SNARE proteins at ribbon synapses of cochlear inner hair cells (IHCs) to trigger exocytosis of neurotransmitter. Also essential to synaptic exocytosis in immature outer hair cells (OHCs). May also play a role within the recycling of endosomes (By similarity). Binds Ca(2+). The ions are bound to the C2 1 domain. Interacts with SNAP25; the interaction is direct. Interacts with STX1; the interaction is direct. Interacts with RAB8B (By similarity). 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. Isoform 1 is expressed in the cochlea and brain. Expressed in cerebellum (Purkinje cells), hippocampus (granule cells of the dentate gyrus and in pyramidal cells of the CA1-CA3 region) and cortex (stellate and pyramidal cells). Expressed in hair cells of vestibular organs such as the saccule, utricle and crista ampullari. Expressed in the cochlear inner and outer cells (IHCs and OHCs) (at protein level). Expressed in brain: brainstem, cerebellum (granules cells and Purkinje cell layer), cortex (layers IV and V), inferior colliculus, superior colliculus and hippocampus (granule cells of the dentate gyrus and in pyramidal cells of the CA1-CA3 region). 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. Belongs to the ferlin family.