MAPK8IP2

MAPK8IP2 (JIP2/IB2) is a cytoplasmic scaffold protein that organizes the JNK signaling cascade and governs whether JNK activity is restrained in the cytosol or licensed to drive downstream transcription (PMID:39013462, PMID:29570727). It homodimerizes through its SH3 domain and heterodimerizes with JIP1 through their SH3 domains with comparable affinity; the JIP2-SH3 homodimer crystal structure reveals interface charge differences forming compensatory hydrogen bonds and salt bridges, and disrupting these dimer contacts impairs activation of the JNK pathway, establishing dimerization as a requirement for pathway output (PMID:39013462). In neurons, JIP2 acts upstream of JNK to sustain JNK protein levels and is required for the generation of mature neurons, with its loss linked to defective semaphorin/NRP1 signaling (PMID:30456368), while in cerebellar granule cells JIP2 loss raises NMDA receptor-mediated currents, intrinsic excitability, and long-term plasticity, distorting spatial signal transfer in the granular layer (PMID:30696733). In non-neuronal settings JIP2 retains JNK1 in the cytosol to suppress AP-1/c-Jun-driven MMP9 transcription, a brake that is released when miR-126-5p ablates JIP2 expression (PMID:29570727). JIP2 is also recruited into a CTRP3–LAMP1–JIP2–JNK axis that confers cardioprotection during myocardial ischemia/reperfusion (PMID:35278832, PMID:35114641), and its translation is selectively upregulated by HPV E6 to activate noncanonical WNT/PCP/JNK signaling and promote cervical cancer cell proliferation (PMID:31637011).

1. 2018 Medium

   Established JIP2 as a determinant of JNK subcellular localization, answering whether JIP2 acts by spatially controlling JNK rather than merely binding it.

   Evidence RNAseq, miRNA manipulation, and JNK1 localization analysis in Theileria-infected macrophages

   PMID:29570727

   Open questions at the time

   • Direct demonstration that JIP2 physically tethers JNK1 in the cytosol not shown
   • Generality beyond the infection context untested
2. 2018 Medium

   Placed JIP2 upstream of JNK in human neuronal maturation, addressing whether JIP2 controls JNK abundance during neural differentiation.

   Evidence iPSC genome editing haploinsufficiency, neural differentiation, JNK western blotting, NRP1 agonist rescue

   PMID:30456368

   Open questions at the time

   • Mechanism by which JIP2 loss lowers JNK protein levels unresolved
   • Direct biochemical link between JIP2 and semaphorin/NRP1 signaling not shown
3. 2019 High

   Defined a physiological neuronal consequence of JIP2 loss, showing it shapes cerebellar granule cell excitability and plasticity rather than acting only in development.

   Evidence Whole-cell patch-clamp, field potential recordings, and LTP assays in IB2 knockout mouse cerebellar slices

   PMID:30696733

   Open questions at the time

   • Molecular link between JIP2 scaffolding and NMDA receptor current changes not established
   • Whether the phenotype is JNK-dependent untested
4. 2019 Medium

   Identified JIP2 as a translationally regulated effector downstream of HPV E6, answering how E6 engages noncanonical WNT/JNK signaling to drive proliferation.

   Evidence Polysome profiling, RNAseq, siRNA silencing, ectopic rescue, in vitro and in vivo tumor growth assays

   PMID:31637011

   Open questions at the time

   • Mechanism of selective JIP2 mRNA translation by E6 not defined
   • Direct scaffolding role of JIP2 in WNT/PCP/JNK output not biochemically dissected
5. 2022 Medium

   Embedded JIP2 in a cardioprotective signaling axis, showing it physically associates with CTRP3 and LAMP1 to modulate JNK during I/R injury.

   Evidence Co-immunoprecipitation, siRNA knockdown, overexpression, JNK inhibitor treatment, in vivo mouse I/R model

   PMID:35114641 PMID:35278832

   Open questions at the time

   • Co-IP without reciprocal or structural validation of the JIP2–LAMP1 interaction
   • Direct vs indirect nature of CTRP3–JIP2 contact unresolved
6. 2024 High

   Provided the structural and functional basis for JIP2 dimerization, resolving how SH3-mediated homo- and heterodimers control JNK pathway activation.

   Evidence NMR, X-ray crystallography of the JIP2-SH3 homodimer, site-directed mutagenesis, cell-based JNK activation assays

   PMID:39013462

   Open questions at the time

   • Stoichiometry and architecture of full-length JIP1/JIP2 scaffolds with kinases not resolved
   • How dimerization couples to specific JNK substrate selection unknown

• How JIP2 scaffolding switches between cytosolic JNK retention and active JNK pathway assembly across cell types remains unresolved.
• No unified model linking SH3 dimerization state to JNK localization
• Full set of kinase partners assembled on the JIP2 scaffold not mapped