Syt4 (rat, UniProt P50232) - manual literature synthesis

No deep-research API was available for this review. This file is a manual synthesis
written by the reviewer from the cached publications in publications/ and the UniProt
record (Syt4-uniprot.txt). Every claim carries its source.

Identity and architecture

• Synaptotagmin-4 (Syt IV), 425 aa, single-pass membrane protein with a short
  intravesicular N-terminus, one TM helix (17-37) and a cytoplasmic region carrying two C2
  domains (C2A 153-274, C2B 287-420) [UniProt P50232 FT TRANSMEM/DOMAIN].
• Belongs to the synaptotagmin family; PANTHER PTHR10024:SF114 (SYNAPTOTAGMIN-4).
• Immediate early gene: mRNA induced in PC12 cells by depolarization, Ca2+ ionophore, ATP and
  forskolin, and in hippocampus/piriform cortex after kainate seizures PMID:7892240. Protein is labile (t1/2 about 2 h) PMID:10217258.

Ca2+ binding: lost in mammals

• C2A carries an Asp-to-Ser substitution (Ser244) of a Ca2+ ligand; wild-type rat Syt4 C2A has
  no Ca2+-dependent phospholipid binding, and S244D restores it PMID:9162066.
• C2A of Syt IV unable to bind Ca2+ PMID:7993622; not among the C2As that bind phospholipids Ca2+-dependently PMID:7791877.
• Crystallography: rat Syt4 C2B also cannot form full Ca2+-binding sites; neither rat C2 domain
  binds Ca2+ or phospholipids efficiently, while fly Syt4 does PMID:15311271.
• Contrary older data: Thomas et al. reported Ca2+-dependent C2B-mediated homo- and
  hetero-oligomerization with Syt I PMID:10397765, and Chapman et al. reported Ca2+-promoted
  Syt I/IV hetero-oligomerization PMID:9830048.

Molecular function: Ca2+-independent SNARE binding; inhibitory isoform in fusion

• Syt4 C2A binds syntaxin isoforms (1A, 2, 3, 4) Ca2+-independently PMID:10397765.
• Syt4 binds syntaxin 6 on immature secretory granules PMID:16618809.
• Reconstituted fusion: Syt IV binds t-SNAREs and PS without Ca2+ enhancement, fails to stimulate
  fusion, inhibits SNARE-only fusion and inhibits Syt I-stimulated fusion partly by displacing
  Syt I from t-SNAREs PMID:18508778.
• Rat versus fly Syt4: rat protein inhibits fusion with or without Ca2+, binds t-SNARE
  heterodimers strongly and Ca2+-independently, and in a co-sedimentation assay fails to bind PS;
  reversing S244D converts it into a Ca2+ sensor PMID:20688915.
• The two Chapman-lab papers disagree on PS binding (some Ca2+-independent PS binding in
  co-flotation in PMID:18508778, none in co-sedimentation in PMID:20688915).
• Syt IV inhibits liposome fusion driven by Syt I, VII or IX with different efficacies
  PMID:21551071.

Cellular roles

• Negative regulator of dense-core vesicle exocytosis in PC12 cells: overexpression reduces the
  basal rate of Ca2+-induced fusion and alters fusion and fission pores PMID:20303854; inhibits
  catecholamine release, more strongly when Syt I is knocked down PMID:20735850; inhibits
  exocytosis of Syt I- and Syt IX-bearing DCVs PMID:21551071. A microinjected C2A fragment
  inhibits Ca2+-triggered release PMID:10397765.
• Positive role in immature secretory granule (ISG) homotypic fusion and granule maturation in
  PC12 cells (siRNA) PMID:16618809.
• Mouse: Syt4 localises to BDNF-containing vesicles and inhibits BDNF release in axons and dendrites;
  effects on presynaptic SV exocytosis are indirect, via BDNF PMID:19448629. In posterior pituitary
  terminals Syt IV is on micro- and dense-core vesicles; knockout changes exocytosis in a
  Ca2+-dependent way and speeds endocytosis PMID:19136969. Knockout mice show enhanced CA1
  short-term plasticity and memory deficits PMID:15390175.
• DCV trafficking: Syt4 links DCVs to KIF1A; JNK phosphorylation of Ser135 weakens Syt4-KIF1A
  binding and leads to DCV capture at synapses PMID:29166604. KIF1A pulls down
  Syt4 in a Ca2+-dependent way PMID:30021165.
• NGF/JNK-dependent phosphorylation at Ser135 moves Syt4 from immature to mature secretory vesicles
  and is needed for NGF enhancement of Ca2+-evoked release in PC12 cells PMID:18046461.
• Astrocytes: Syt IV in astrocyte processes; RNAi reduces Ca2+-dependent glutamate release
  PMID:15197251.
• Pancreatic beta cells: SYT4 silencing reduced GSIS in INS1-832/13 cells [PMID:22939844,
  abstract only].

Localisation

• Golgi and tips of neurites in PC12 cells and hippocampal neurons PMID:10646502; mouse neurons:
  dendrites, axons and Golgi, not synaptic vesicles PMID:12135783.
• Synaptic-like microvesicles and secretory granules in PC12 cells (fractionation) PMID:10217258;
  ISG and Golgi fractions PMID:16618809.
• Neuronal DCV membrane, co-localised with chromogranin A PMID:29166604.