FARP2

FARP2 (FRG) is a FERM domain-containing Dbl-family guanine nucleotide exchange factor for the Rho GTPases Rac1 and Cdc42 that converts multiple cell-surface signals into localized cytoskeletal remodeling (PMID:16286926, PMID:15277544). Its GEF activity is intrinsically held off: crystal structures of the DH and DH-PH-PH module show an autoinhibited conformation in which the substrate-binding site is occluded by the terminal DH helix and the two PH domains, and disrupting these elements relieves suppression in cells (PMID:23375260). This autoinhibited pool is recruited to signaling platforms and released by defined upstream cues. In semaphorin signaling, FARP2 basally associates with plexin-A1; Sema3A binding to neuropilin-1 triggers FARP2 dissociation, activating its Rac GEF activity, promoting Rnd1 recruitment and R-Ras downregulation, while its FERM domain sequesters PIPKIgamma661 from talin to drive axonal repulsion (PMID:16286926). Through a distinct interaction with the KRK motif of PlxnA4, FARP2 mediates Sema3A-induced Rac1 activation selectively for dendritic morphogenesis (PMID:32499377). At nectin-based adherens junctions, c-Src recruits and tyrosine-phosphorylates FARP2 to locally activate Cdc42 (PMID:15277544), and aPKCι binds and phosphorylates FARP2 to form a positive-feedback loop driving epithelial polarity and tight junction formation (PMID:30872454). Beyond neurons and epithelia, FARP2 supports localized Rac1 activation in osteoclast podosomes during bone resorption (PMID:20702777) and is required for TGF-β1-driven myofibroblast differentiation (PMID:41786192).

1. 2004 High

   Established FARP2 as a Cdc42-specific GEF that is recruited and activated at cell-cell adhesion sites, linking nectin/Src signaling to junction assembly.

   Evidence Co-IP, RNAi, GEF and tyrosine-phosphorylation assays at nectin-based junctions

   PMID:15277544

   Open questions at the time

   • Did not resolve how Src-mediated phosphorylation structurally relieves autoinhibition
   • Selectivity between Cdc42 and Rac in this context not fully defined
2. 2005 High

   Defined FARP2 as the Rac GEF released from plexin-A1 upon Sema3A/neuropilin-1 engagement, coupling a guidance receptor to bifunctional Rac activation and talin/PIPKIgamma sequestration in axon repulsion.

   Evidence Reciprocal Co-IP, biochemical dissociation and GEF assays, axon repulsion functional readouts

   PMID:16286926

   Open questions at the time

   • Mechanism triggering dissociation from plexin-A1 at atomic resolution unknown
   • Relative contribution of GEF activity versus PIPKIgamma sequestration not quantified
3. 2006 Medium

   Placed FARP2 downstream of CD47/Src as a contributor to Cdc42/Rac activation supporting neuronal process development.

   Evidence FRG inhibition, CD47 overexpression, autophosphorylation assays and neuronal morphology in hippocampal neurons

   PMID:17135401

   Open questions at the time

   • Relied on inhibition rather than clean loss-of-function
   • Functional redundancy with Vav2 not dissected
   • Single lab
4. 2010 High

   Extended FARP2's role beyond neurons by showing it drives localized Rac1 activation in osteoclast podosomes required for resorption and osteoclastogenesis.

   Evidence Live imaging, GTP-Rac1 pull-down, knockdown/knockout osteoclasts, resorption pit assay

   PMID:20702777

   Open questions at the time

   • Upstream receptor recruiting FARP2 to podosomes not identified
   • Link to integrin β3 activation mechanistically incomplete
5. 2013 High

   Provided the structural basis for FARP2 regulation, showing intramolecular autoinhibition of the GEF active site by the DH and tandem PH domains.

   Evidence X-ray crystallography of DH and DH-PH-PH modules with cell-based GEF activity validation

   PMID:23375260

   Open questions at the time

   • Structure of the activated, signal-released state not determined
   • How phosphorylation or receptor binding triggers the conformational switch unresolved
6. 2019 High

   Identified FARP2 as an aPKCι binding partner and phosphorylation substrate forming a positive-feedback loop that activates Cdc42 to drive epithelial polarity and tight junction formation.

   Evidence Co-IP, phosphorylation assay, siRNA, GTP-Cdc42 pull-down, polarity and junction assays

   PMID:30872454

   Open questions at the time

   • How phosphorylation-induced detachment couples to Cdc42 loading not structurally defined
   • In vivo relevance of the feedback loop not tested
7. 2020 High

   Resolved pathway specificity by showing FARP2 binds the PlxnA4 KRK motif to mediate Sema3A-induced Rac1 activation for dendrite morphogenesis while being dispensable for axon growth cone collapse.

   Evidence CRISPR/Cas9 KRK knock-in mice, FARP2 loss-of-function, Rac1 activity assays, dendritic and axon guidance readouts in vivo and in vitro

   PMID:32499377

   Open questions at the time

   • What distinguishes the plexin-A1 versus PlxnA4 FARP2 pools mechanistically unclear
   • Effector specificity for dendrite versus axon outcomes not fully explained
8. 2026 Medium

   Implicated FARP2 in pathological tissue remodeling, showing it is induced by TGF-β1 and required for myofibroblast differentiation and migration in corneal stromal fibroblasts.

   Evidence siRNA knockdown, qPCR, Western blot, α-SMA immunocytochemistry, scratch migration assay

   PMID:41786192

   Open questions at the time

   • No in vivo confirmation of a role in corneal fibrosis
   • GTPase effector mediating the fibrotic phenotype not directly demonstrated
   • Single lab

• How the structurally defined autoinhibited state is converted to the active state by the diverse upstream inputs (plexin release, Src and aPKCι phosphorylation) remains unresolved at the mechanistic level.
• No structure of an activated/receptor-bound FARP2 state
• Quantitative rules for Rac1 versus Cdc42 selectivity across contexts undefined