Synaptic vesicle phosphoprotein that enhances spontaneous neurotransmitter release but does not effect induced neurotransmitter release (PubMed:17190793). Unlike other synaptotagmins, it does not bind Ca(2+) or phospholipids (PubMed:17190793). Essential for mossy-fiber long-term potentiation in the hippocampus (By similarity). Homodimer (By similarity). Can also form heterodimers (By similarity). Interacts with SYT1 (PubMed:17190793). Expressed in the brain, specifically in neurons of the cerebellum, cortex, hippocampus, olfactory bulb, brainstem and spinal cord (at protein level). Phosphorylation of Ser-97 is required for mossy-fiber long-term potentiation. Belongs to the synaptotagmin family. Unlike classical synaptotagmins, lacks Ca(2+)-binding sequences and does not bind phospholipids.