CASKIN2

CASKIN2 is a multidomain scaffold protein that organizes glutamatergic synaptic transmission presynaptically and also operates in cell migration machinery outside the synapse (PMID:41223222, PMID:38587458). Its tandem SAM domains (SAM1-SAM2) mediate oligomerization through a distinctive mode whose minimal repeating unit is a dimer, generating cellular punctae morphologically distinct from the related CASKIN1 (PMID:27549312), while its SH3 domain, though correctly folded, carries non-canonical basic residues (K290/R319) in the binding cleft that abolish polyproline-ligand binding, rendering it functionally vestigial (PMID:27619958). At Schaffer collateral synapses, presynaptic CASKIN2 — but not CASKIN1 — is required for synaptic strength, active zone protein arrangement, and regulation of postsynaptic NMDAR surface expression and LTP; PTPσ-mediated tyrosine dephosphorylation and multimerization of CASKIN2 are necessary for these functions and for activity-dependent presynaptic F-actin rearrangement and object-location memory (PMID:41223222). Beyond the synapse, CASKIN2 binds the talin R8 domain via a C-terminal LD motif and associates with the WAVE regulatory complex through Abi1 to promote cell migration, an interaction controlled by growth factor-induced phosphorylation on serine 878 (PMID:38587458).

1. 2011 Low

   Whether the CASKIN2 SAM domain could polymerize was unknown; an initial screen placed it among candidate self-associating SAM modules, framing oligomerization as a structural feature to test.

   Evidence negGFP-SAM fusion native gel screen with electron microscopy verification

   PMID:21805519

   Open questions at the time

   • Single screening method, no CASKIN2-specific quantitative biophysics
   • Did not define the oligomerization mode or interface
   • No functional consequence established
2. 2016 High

   Resolved how the CASKIN2 SAM tandem self-associates, establishing a dimer-based oligomerization mode distinct from CASKIN1 and providing a structural basis for distinct cellular assemblies.

   Evidence X-ray crystallography of SAM1-SAM2, analytical ultracentrifugation, structure-directed mutagenesis, and fluorescence imaging in Neuro2a cells

   PMID:27549312

   Open questions at the time

   • Physiological binding partners recruited to SAM oligomers not identified
   • Functional role of dimeric versus monomeric state in vivo undefined
3. 2016 High

   Addressed whether the CASKIN2 SH3 domain mediates canonical interactions, showing the fold is intact but its binding cleft cannot engage polyproline ligands, marking the domain as vestigial.

   Evidence NMR structure determination, site-directed mutagenesis restoring canonical aromatic residues, and thermal stability assay

   PMID:27619958

   Open questions at the time

   • Whether the SH3 domain has any non-canonical function is untested
   • No alternative ligand identified
4. 2019 Medium

   Tested the role of CASKIN proteins in synaptic plasticity, showing combined Caskin1/Caskin2 loss impairs hippocampal LTP and spine structure and that CASKIN proteins reside in postsynaptic complexes with Shank2 and influence AMPAR phosphorylation.

   Evidence Caskin1/Caskin2 double-knockout mice with electrophysiology, EM ultrastructure, immunocytochemistry, co-immunoprecipitation, and overexpression rescue

   PMID:31727973

   Open questions at the time

   • Phenotypes attributed to both paralogs jointly; CASKIN2-specific contribution not isolated
   • Shank2 co-IP not reciprocally validated or mapped to a domain
5. 2024 Medium

   Defined a non-synaptic role for CASKIN2 in cell migration, identifying direct talin R8 binding via an LD motif and a phosphorylation-regulated link to the WAVE complex through Abi1.

   Evidence Direct binding assay, co-immunoprecipitation, growth factor-induced phosphorylation analysis, Abi1-dependent migration assay, and localization in CASKIN2-amplified cancer cell lines

   PMID:38587458

   Open questions at the time

   • Kinase responsible for S878 phosphorylation not identified
   • Migration role established in cancer cell lines only; in vivo relevance unaddressed
6. 2025 High

   Distinguished CASKIN2-specific synaptic function from CASKIN1, establishing presynaptic CASKIN2 as a transsynaptic regulator of NMDAR responses requiring PTPσ-mediated dephosphorylation and multimerization.

   Evidence Conditional CASKIN1/CASKIN2 knockout mice with electrophysiology, NMDAR surface expression and F-actin rearrangement assays, genetic epistasis with PTPσ, and behavioral memory testing

   PMID:41223222

   Open questions at the time

   • Molecular link between PTPσ dephosphorylation, CASKIN2 multimerization, and active zone F-actin not structurally resolved
   • Tyrosine residues dephosphorylated by PTPσ not mapped
   • Presynaptic effectors transducing CASKIN2 state to postsynaptic NMDARs unidentified

• How CASKIN2's structurally defined SAM-mediated multimerization is mechanistically coupled to PTPσ-dependent dephosphorylation to control active zone organization and transsynaptic NMDAR signaling remains unresolved.
• No structure of CASKIN2 multimers in the dephosphorylated active state
• Direct presynaptic-to-postsynaptic signaling intermediary unknown