Synaptic vesicle phosphoprotein that enhances spontaneous neurotransmitter release but does not effect induced neurotransmitter release (By similarity). Unlike other synaptotagmins, it does not bind Ca(2+) or phospholipids (By similarity). Essential for mossy-fiber long-term potentiation in the hippocampus (PubMed:23739973). Homodimer (By similarity). Can also form heterodimers (By similarity). Interacts with SYT1 (By similarity). Expressed in the brain, specifically by neurons in the hippocampus, and in the adrenal medulla (at protein level). Detected at low levels in the brain at postnatal day 5 (at protein level) (PubMed:17190793). Expression in the brain is increased by postnatal days 9 and 10, and continues to increase at postnatal days 15 and 20 (at protein level) (PubMed:17190793). Phosphorylation of Ser-97 is required for mossy-fiber long-term potentiation. No measurable change in basal synaptic strength or short-term neuronal plasticity (PubMed:23739973). Neurons from the CA3 hippocampal region exhibit an impairment of cAMP-dependent long-term potentiation in mossy-fiber synapses (PubMed:23739973). Belongs to the synaptotagmin family. Unlike classical synaptotagmins, lacks Ca(2+)-binding sequences and does not bind phospholipids.