STAP1

STAP1 (BRDG1) is a PH- and SH2-domain-containing adaptor/scaffold protein that operates downstream of tyrosine kinases and immune receptors in hematopoietic and immune cells, becoming tyrosine-phosphorylated upon receptor engagement and assembling signaling complexes that amplify kinase output (PMID:10518561, PMID:10679268). It was first defined as a substrate and positive-feedback amplifier of the Tec tyrosine kinase, phosphorylated directly by Tec in a manner requiring both its own PH and SH2 domains, and undergoing phosphorylation in B cells in response to BCR stimulation (PMID:10518561); in parallel it acts downstream of c-kit, with its SH2 domain engaging tyrosine-phosphorylated c-kit and STAT5 (PMID:10679268). Through these domains STAP1 nucleates distinct complexes: its SH2 domain binds BCR-ABL and stabilizes the BCR-ABL protein in CML cells while its PH domain binds STAT5a, with STAP1 itself transcriptionally driven by NFATc1 (PMID:33845308), and in T cells it engages LCK and phospholipase C-γ1 to potentiate TCR signaling, IL-2 production and proliferation (PMID:40288812). Downstream, STAP1 augments BCR-mediated CREB activation in an isoform- and PH-domain-dependent manner (PMID:11716489), and in microglia its overexpression remodels the cytoskeleton, engages the M-CSF receptor c-Fms, and reprograms activation, migration and phagocytosis (PMID:19100238, PMID:37462801). A STAP1-derived peptide blocking the STAP1–LCK interaction suppresses TCR signaling and ameliorates experimental autoimmune encephalomyelitis, establishing the scaffold as a tractable immunomodulatory target (PMID:40288812). Despite its identification as a candidate familial hypercholesterolemia gene, rigorous knockout, bone-marrow transplant, and human variant-carrier studies establish that STAP1 does not regulate plasma LDL cholesterol (PMID:31996024).

1. 1999 High

   Established STAP1/BRDG1 as a bona fide docking protein in tyrosine kinase signaling by showing it is a direct Tec substrate that feeds back to amplify Tec activity, defining its core adaptor function.

   Evidence Yeast two-hybrid, in vitro kinase assay, co-expression and phosphorylation assays in 293 cells with kinase specificity controls, and BCR stimulation of B cells

   PMID:10518561

   Open questions at the time

   • Physiological receptor context beyond Tec/BCR not defined
   • Phosphorylation sites and the structural basis of feedback amplification not mapped
2. 2000 Medium

   Extended STAP1 function to a second kinase axis by showing it acts downstream of c-kit in hematopoietic cells and that its SH2 domain binds phosphorylated c-kit and STAT5, indicating selective receptor coupling.

   Evidence Yeast two-hybrid with c-kit bait, in vitro SH2-domain binding assays, phosphorylation assay in 293 cells, RT-PCR expression analysis

   PMID:10679268

   Open questions at the time

   • Interactions shown largely in vitro/heterologous cells
   • Endogenous c-kit–STAP1 complex and downstream consequence not established
3. 2001 Medium

   Connected STAP1 to a transcriptional output by showing it selectively amplifies BCR-driven CREB activation, and assigned the PH domain a functional role via isoform-dependent activity.

   Evidence Promoter reporter assays (CREB, c-Jun, c-MYC, BCL-xL), overexpression in B cells, alternative splicing and tyrosine phosphorylation analysis

   PMID:11716489

   Open questions at the time

   • Mechanism linking STAP1 to CREB not resolved
   • Overexpression-based; endogenous requirement untested
4. 2008 Medium

   Implicated STAP1 in microglial biology by showing ectopic expression alters cytoskeleton, engages c-Fms, and shifts cells toward an activated, cytotoxic, low-migration phenotype.

   Evidence Ectopic expression in BV-2 microglia, co-IP with c-Fms, phosphorylation, morphology, migration and cytotoxicity assays

   PMID:19100238

   Open questions at the time

   • Relies on overexpression in a single cell line
   • Loss-of-function effect on endogenous microglia not tested
5. 2020 High

   Resolved the candidate disease hypothesis by demonstrating, across mouse and human systems, that STAP1 does not regulate plasma LDL cholesterol, redirecting attention to its immune/hematopoietic roles.

   Evidence Stap1 knockout mice, bone marrow transplant into Ldlr-/- mice, PBMC variant-carrier analysis, and PBMC–HepG2 coculture LDL-uptake assays

   PMID:31996024

   Open questions at the time

   • Does not address non-lipid phenotypes of STAP1 loss
   • Mechanism of original genetic association left unexplained
6. 2021 Medium

   Defined STAP1 as a domain-bipartite scaffold in CML, with SH2-mediated BCR-ABL binding stabilizing the oncoprotein and PH-mediated STAT5a binding, and identified NFATc1 as its transcriptional driver.

   Evidence Deletion-mutant co-immunoprecipitation, STAP1-promoter luciferase reporter, and western blots for BCR-ABL stability

   PMID:33845308

   Open questions at the time

   • Mechanism of BCR-ABL stabilization unresolved
   • Single-lab co-IP without reciprocal/endogenous confirmation
7. 2023 Medium

   Linked STAP1 to tumor-associated immune modulation by showing microglial overexpression promotes M2-like polarization and suppresses phagocytosis, aiding glioma progression.

   Evidence STAP1-overexpressing BV-2 cells, phagocytosis flow cytometry/microscopy, ARG1/cofilin readouts, and orthotopic/subcutaneous glioma mouse models

   PMID:37462801

   Open questions at the time

   • Causal molecular link to IL-6/STAT3 and cofilin not mechanistically dissected
   • Overexpression-based; endogenous loss not tested
8. 2025 Medium

   Established STAP1 as a positive regulator of TCR signaling through LCK and PLC-γ1 binding and provided proof-of-concept that disrupting the STAP1–LCK interaction is therapeutically actionable in autoimmunity.

   Evidence STAP1-derived inhibitory peptide (iSP1), binding-inhibition, IL-2 and proliferation assays, and an EAE mouse model with Th1/Th17 infiltration analysis

   PMID:40288812

   Open questions at the time

   • Direct structural definition of the STAP1–LCK interface not provided
   • Single-lab study; on-target specificity of iSP1 in vivo not fully delineated

• How STAP1 is recruited to specific receptors versus others, the structural basis of its PH/SH2 domain selectivity, and its endogenous loss-of-function physiology across immune cell types remain unresolved.
• No structural model of STAP1 complexes
• Endogenous knockout phenotype in immune cells largely uncharacterized
• Phosphorylation-site map and feedback mechanism undefined