SYT3 (Synaptotagmin 3) encodes a member of the synaptotagmin family of calcium-binding proteins that function as calcium sensors for synaptic vesicle exocytosis. While most synaptotagmins (including SYT1, SYT2, SYT5) are predominantly presynaptic, SYT3 exhibits unique expression patterns including both pre- and postsynaptic localization, suggesting distinct functional roles in synaptic transmission and plasticity[@jackman2020][@chen2023].
SYT3 is primarily expressed in the brain and functions as a calcium sensor for synaptic vesicle exocytosis, particularly at excitatory synapses. It plays important roles in neurotransmitter release, synaptic plasticity, and may be implicated in Alzheimer's disease and other neurological disorders[@david1996][@matsuda1999][@baur2023].
Gene Information
Protein Structure and Domains
SYT3 contains several conserved domains that mediate its function as a calcium sensor:
Domain Architecture
N-terminal Transmembrane Region (TMD): A single transmembrane helix (residues 1-60) that anchors the protein to synaptic vesicles and plasma membrane. Unlike other synaptotagmins, SYT3 has an extended linker region between the TMD and C2 domains.
C2A Domain (Synaptotagmin-like C2 Domain 1):
Residues 150-270
High-affinity calcium-binding site (Kd ~ 1-10 μM)
Binds 2 Ca²⁺ ions per domain
Mediates interaction with SNARE proteins
Linker Region:
Variable length (residues 270-320)
Contains regulatory phosphorylation sites
May contribute to isoform-specific localization
C2B Domain (Synaptotagmin-like C2 Domain 2):
Residues 320-480
Lower calcium affinity (Kd ~ 10-100 μM)
Mediates protein-protein interactions
Required for synaptotagmin function
Calcium Binding Properties
The C2 domains of SYT3 exhibit distinct calcium-binding characteristics compared to SYT1:
C2A domain has higher affinity for Ca²⁺
C2B domain participates in interactions with PIP2 and synaptotagmin dimerization
Calcium binding induces conformational changes that promote SNARE interaction[@jackman2016]
Normal Neuronal Function
Synaptic Transmission
SYT3 functions as a calcium sensor for synaptic vesicle fusion with several distinctive features:
Calcium Binding: The C2 domains bind Ca²⁺ with different affinities, allowing graded responses to calcium signals
SNARE Interaction: Interacts with SNARE complex proteins (SNAP-25, Syntaxin) in a calcium-dependent manner
Vesicle Docking: Facilitates synaptic vesicle positioning at the active zone
Fusion Triggering: Triggers fusion upon calcium influx, though with slower kinetics than SYT1
Asynchronous Release: Contributes to asynchronous neurotransmitter release following synaptic stimulation
The unique kinetic properties of SYT3 suggest it may regulate specific aspects of synaptic transmission, particularly during sustained activity or under conditions of high-frequency stimulation[@robinson2022].
Synaptic Plasticity
SYT3 plays important roles in synaptic plasticity mechanisms:
Long-term Potentiation (LTP):
SYT3 is involved in LTP at hippocampal synapses
Postsynaptic SYT3 may regulate AMPA receptor trafficking
Activity-dependent modulation of SYT3 expression during LTP induction[@matsuda1999][@mendelsohn2022]