TAMALIN

Tamalin (also called GRASP) is a postsynaptic PDZ-domain scaffold protein that bridges metabotropic glutamate receptors and neurotrophin receptors to ARF-family GEF/Rac signaling cascades, thereby controlling receptor surface trafficking, cytoskeletal remodeling, and neuroplasticity. Its PDZ domain directly binds the C-terminal PDZ-binding motifs of group 1 mGluRs (mGluR1a, mGluR5) and other synaptic scaffolds (SAPAP, S-SCAM, PSD-95, Mint2), while its C-terminal region recruits cytohesins (ARNO/cytohesin-2, GRP1) to form ternary complexes that promote mGluR surface delivery; this trafficking function is gated by an autoinhibitory intramolecular PDZ–ligand interaction that is competitively relieved upon mGluR binding, as revealed by crystal structures (PMID:17396155, PMID:11850456, PMID:12586822). Tamalin also contains a functional ITAM that, upon phosphorylation by Src-family kinases (c-Src, Fyn), recruits and activates Syk kinase (PMID:15173175), and it links ARNO to the Rac-GEF Dock180 to couple ARF signaling to Rac1 activation and cell migration (PMID:20016009, PMID:16636148). In vivo, tamalin is required for drug-of-abuse behavioral responses and for electroconvulsive shock–induced adult hippocampal neurogenesis and synaptic remodeling (PMID:17766434, PMID:22396401).

1. 2000 Medium

   Identification of tamalin as a retinoic acid–induced scaffold that binds the ARF-GEF GRP1 and promotes its plasma-membrane association established tamalin as a link between intracellular scaffolding and ARF-dependent membrane trafficking.

   Evidence Yeast two-hybrid, in vitro binding, co-localization and subcellular fractionation in transfected mammalian cells

   PMID:10828067

   Open questions at the time

   • No receptor or upstream signal identified for tamalin at this stage
   • GRP1 interaction not validated from native tissue
2. 2002 High

   Demonstration that tamalin's PDZ domain directly binds group 1 mGluRs and that native mGluR1a/tamalin/cytohesin-2 ternary complexes exist in brain established tamalin as a trafficking scaffold for metabotropic glutamate receptors.

   Evidence Co-immunoprecipitation from rat brain, yeast two-hybrid, COS-7 overexpression and dominant-negative trafficking assays, adenovirus-mediated neuronal expression

   PMID:11850456

   Open questions at the time

   • Structural basis for the PDZ–mGluR interaction not yet resolved
   • Mechanism by which cytohesin linkage promotes surface trafficking unknown
3. 2003 High

   Mapping of tamalin's multi-domain interactions with SAPAP, S-SCAM, PSD-95, and Mint2 — including self-association via its own PDZ domain and C-terminal ligand — revealed tamalin as a promiscuous postsynaptic hub connecting multiple scaffold networks.

   Evidence Yeast two-hybrid, pull-down, co-immunoprecipitation from rat brain, deletion mapping

   PMID:12586822

   Open questions at the time

   • Functional consequence of each specific scaffold interaction on synaptic signaling not tested
   • Whether self-association is regulatory or simply oligomerization was unclear
4. 2004 High

   Discovery of a functional ITAM within tamalin that is phosphorylated by Src/Fyn and recruits/activates Syk kinase revealed an unexpected tyrosine-kinase signaling branch downstream of the mGluR1–tamalin complex.

   Evidence Co-immunoprecipitation from rat brain membranes and COS-7 cells, tyrosine-to-phenylalanine mutagenesis, kinase activity assays

   PMID:15173175

   Open questions at the time

   • Downstream targets of Syk activation in this complex not identified
   • Physiological stimulus triggering ITAM phosphorylation in neurons not defined
5. 2006 High

   Showing that the kinase-dead neurotrophin receptor TrkCT1 signals through tamalin to activate ARF6 and Rac1 in an NT3-dependent manner broadened tamalin's receptor repertoire beyond mGluRs and placed it in a growth-factor–induced cytoskeletal remodeling pathway.

   Evidence Co-immunoprecipitation, GTPase activation assays, dominant-negative and siRNA knockdown, live imaging of membrane ruffling

   PMID:16636148

   Open questions at the time

   • How a kinase-dead receptor triggers tamalin complex assembly remains unclear
   • Relevance of TrkCT1–tamalin signaling in vivo not tested
6. 2007 High

   Crystal structures of the tamalin PDZ domain resolved the autoinhibitory mechanism: intramolecular PDZ–ligand interaction locks tamalin in an inactive oligomer, and mGluR binding competitively relieves autoinhibition to expose the C-terminal motif for motor-protein recruitment and receptor surface trafficking.

   Evidence X-ray crystallography, gel filtration, competitive binding assays, cell-based trafficking assays

   PMID:17396155

   Open questions at the time

   • Identity of the motor-protein receptor engaged by the liberated C-terminal motif not fully characterized
   • Whether autoinhibition regulates the ITAM or cytohesin-binding functions was not addressed
7. 2007 Medium

   Tamalin knockout mice exhibited selective deficits in morphine- and cocaine-driven behavioral responses without gross brain anatomical abnormalities, establishing an in vivo role for tamalin in reward-circuit plasticity.

   Evidence Constitutive knockout mouse, behavioral pharmacology (hot-plate, CPP, locomotor sensitization), in vivo microdialysis

   PMID:17766434

   Open questions at the time

   • Molecular mechanism linking tamalin scaffolding to drug-of-abuse circuitry not resolved
   • Contribution of individual tamalin interaction partners to the behavioral phenotype unknown
8. 2009 High

   Demonstration that tamalin and IPCEF jointly scaffold ARNO to Dock180, coupling ARF activation to Rac1 and cell migration, defined the complete ARF→Rac relay and positioned tamalin as its critical linker.

   Evidence Co-immunoprecipitation, siRNA knockdown, Rac activation and cell migration assays

   PMID:20016009

   Open questions at the time

   • Whether this ARNO–tamalin–Dock180 complex operates in neurons or only in non-neuronal migration contexts is unresolved
   • Structural basis for the tripartite complex assembly unknown
9. 2012 Medium

   Requirement of tamalin for electroconvulsive shock–induced hippocampal neurogenesis, mossy fiber sprouting, and dendritic remodeling demonstrated that tamalin's scaffolding function is essential for activity-dependent adult neuroplasticity.

   Evidence Constitutive knockout mouse, BrdU/EdU neurogenesis assay, immunohistochemistry, qPCR

   PMID:22396401

   Open questions at the time

   • Upstream signal that upregulates tamalin after ECS not identified
   • Which tamalin interaction partner(s) mediate the neuroplasticity phenotype is unknown

• It remains unknown how tamalin's multiple signaling branches — mGluR trafficking, ITAM/Syk signaling, and ARF-to-Rac relay — are coordinately regulated in neurons, and whether specific post-translational modifications or activity states switch tamalin between these functions.
• No integrated model connecting ITAM phosphorylation state to trafficking or GTPase scaffolding functions
• Conditional or cell-type-specific knockout studies have not been reported
• No disease-associated mutations in tamalin have been identified in human genetics