DOK4

DOK4 (IRS5) is a PH- and PTB-domain adapter protein that couples activated tyrosine kinases to downstream signaling and predominantly restrains those pathways (PMID:16820412, PMID:28275114). It is a substrate of multiple kinases, becoming tyrosine-phosphorylated downstream of the c-Ret receptor, the insulin/IGF-1 receptors, and cytosolic Src, Fyn and Jak2, and its PTB domain docks directly onto the phospho-Y1062 NPXY motif of c-Ret via a C-terminally extended α-helix and also binds Ship1 (PMID:12730241, PMID:11470823, PMID:14963042, PMID:22982678). Through its PH domain it is constitutively positioned at the plasma membrane, a localization required for its inhibitory output (PMID:14963042). In neuronal cells DOK4 acts downstream of GDNF/RET to activate the small GTPase Rap1, driving sustained ERK1/2 signaling and neurite outgrowth dependent on tyrosines Y187, Y220 and Y270 (PMID:11470823, PMID:16820412), and it is similarly required for early Schwann cell myelination (PMID:21264944). In T cells the same Rap1 axis instead operates negatively, with DOK4 relocalizing to the immunological-synapse MTOC to dampen ERK phosphorylation, IL-2 promoter activity and proliferation (PMID:19494292). DOK4 also localizes to mitochondria via an N-terminal targeting sequence where it anchors c-Src, lowers complex I expression, and promotes TNF-α-driven ROS production and NF-κB activation (PMID:15855164). Finally, DOK4 inhibits the transcription factor Elk-4/Sap1 by direct PTB-mediated binding, nuclear export, and proteasome-dependent degradation, thereby reducing Egr-1, Fos and cyclin D1 expression and proliferation independently of ERK (PMID:28275114); an alternatively spliced isoform (Dok-4b) and the opposing activities of its N- and C-termini further tune this inhibitory balance (PMID:17258175).

1. 2001 Medium

   Established DOK4 as a direct effector of an activated receptor tyrosine kinase by showing it docks onto c-Ret and can drive a differentiation output.

   Evidence Co-IP and c-Ret/Dok-4 fusion constructs with neurite outgrowth and MAPK assays in PC12 cells

   PMID:11470823

   Open questions at the time

   • Did not define which DOK4 domain mediates Ret binding
   • Downstream effectors beyond MAPK not identified
2. 2003 Medium

   Extended the kinase inputs to DOK4 to the insulin/IGF-1 receptors and mapped its phospho-dependent SH2 partners, framing it as a signaling adapter.

   Evidence Transfection, tyrosine phosphorylation, and SH2-domain pull-down/MAPK assays

   PMID:12730241

   Open questions at the time

   • Binding partner panel from overexpression, not endogenous
   • Functional consequence beyond MAPK activation unclear
3. 2004 Medium

   Defined the kinase substrate spectrum and showed PH-domain-dependent membrane localization is required for DOK4's inhibitory effect on Elk-1, recasting it as a negative regulator.

   Evidence PH/PTB deletion and myristoylation-rescue constructs, kinase panel, and Elk-1 reporter in COS cells

   PMID:14963042

   Open questions at the time

   • Mechanism of Elk-1 inhibition not resolved
   • Single overexpression system
4. 2005 High

   Revealed a distinct mitochondrial pool of DOK4 that anchors c-Src to regulate complex I, ROS and NF-κB, expanding its roles beyond plasma-membrane signaling.

   Evidence Mitochondrial targeting-sequence mapping, fractionation, Co-IP, siRNA, dominant-negative Src, PP2, ROS and NF-κB reporters

   PMID:15855164

   Open questions at the time

   • How complex I expression is lowered mechanistically not defined
   • Physiological context of mitochondrial pool versus membrane pool unclear
5. 2006 High

   Placed DOK4 upstream of a Rap1–ERK pathway required for GDNF/RET-driven neurite outgrowth and identified the essential phosphotyrosines.

   Evidence siRNA, Y-to-F mutants, dominant-active/negative Rap1 epistasis and rescue in neuroblastoma and hippocampal neurons

   PMID:16820412

   Open questions at the time

   • Direct Rap1 GEF linkage not identified
   • How phosphotyrosines couple to Rap1 activation unknown
6. 2007 Medium

   Showed alternative splicing and opposing N-/C-terminal activities tune DOK4's net inhibitory strength, introducing a regulatory dimension to its output.

   Evidence Identification of Dok-4b splice variant, truncation/isolated-domain constructs, ERK and Elk-1 reporters in epithelial cells

   PMID:17258175

   Open questions at the time

   • Endogenous relevance of Dok-4b isoform not established
   • Molecular basis of C-terminal stimulatory effect unknown
7. 2009 Medium

   Demonstrated DOK4 as a negative regulator in T cells, relocalizing to the immunological-synapse MTOC and suppressing ERK, IL-2 and proliferation via Rap1.

   Evidence siRNA, PH-domain mutant, TCR stimulation, MTOC imaging, ERK/IL-2 reporter and proliferation assays

   PMID:19494292

   Open questions at the time

   • MTOC recruitment mechanism not defined
   • Reconciliation of positive (neuron) versus negative (T cell) Rap1 outputs unexplained
8. 2010 Medium

   Identified a requirement for DOK4 in early Schwann cell myelination, extending its neuronal RTK roles to glial development.

   Evidence siRNA knockdown in Schwann cell–neuron co-cultures with myelination, migration and proliferation assays

   PMID:21264944

   Open questions at the time

   • Upstream kinase driving the myelination role not identified
   • Single in vitro system
9. 2012 Medium

   Defined the structural requirements of the DOK4 PTB domain for Ret and Ship1 binding and identified a human SNP that abolishes Ret-mediated phosphorylation.

   Evidence PTB deletion/extension constructs, NPXY peptide binding assays, R186H mutant and phosphorylation assay

   PMID:22982678

   Open questions at the time

   • Functional/phenotypic consequence of R186H in vivo unknown
   • Selectivity for Ship1 over Ship2 not mechanistically explained
10. 2017 High

    Established a direct, ERK-independent route by which DOK4 silences gene expression—binding, exporting and degrading the transcription factor Elk-4.

    Evidence Co-IP, NES mapping, localization imaging, proteasome inhibition, siRNA, ubiquitination-site mutants, transactivation/mRNA/proliferation assays in MDCK cells

    PMID:28275114

    Open questions at the time

    • Identity of the E3 ligase mediating Elk-4 degradation unknown
    • Whether DOK4 enters the nucleus to capture Elk-4 not fully resolved

• How DOK4 switches between positive (neuronal differentiation) and negative (lymphocyte, epithelial) signaling outputs through the same Rap1/ERK machinery remains unresolved.
• No in vivo loss-of-function model defining physiological function
• Context-determining cofactors that flip DOK4 output unidentified
• No structural model of full-length DOK4 with its partners