ACAP2

ACAP2 (centaurin-β2) is an Arf6 GTPase-activating protein that functions as an effector of GTP-bound Rab35, coupling Rab35-dependent membrane recruitment to local Arf6 inactivation at endosomes and phagocytic cups to regulate neurite outgrowth, Fcγ receptor–mediated phagocytosis, and oligodendrocyte differentiation/myelination (PMID:22344257, PMID:22045739, PMID:24600047). Specificity of the Rab35–ACAP2 interaction is determined by two threonine residues in Rab35 switch II (Thr-76, Thr-81) and two asparagine residues in the ACAP2 C-terminal domain (Asn-610, Asn-691), and disruption of this interface abolishes neurite outgrowth (PMID:25694427). ACAP2 also possesses a phosphoinositide-binding–dependent pro-apoptotic function, as its knockdown blocks 5-fluorouracil–induced apoptosis in cancer cells (PMID:25853217). ACAP2 protein levels are regulated by RNF126-mediated ubiquitination and proteasomal degradation, which in ovarian cancer cells reprograms lipid metabolism to promote proliferation and metastasis (PMID:40251363).

1. 2006 Medium

   ACAP2 was identified as a physical interactor of vaccinia virus K1L protein, establishing it as a GAP for ARF6 accessible to viral manipulation, though K1L's host-range function proved independent of this binding.

   Evidence Mutagenesis and co-immunoprecipitation with vaccinia K1L in cell culture

   PMID:16806385

   Open questions at the time

   • Only a single study; no independent confirmation of the K1L–ACAP2 interaction
   • Biological significance of the K1L–ACAP2 interaction remains undefined
   • No endogenous cellular phenotype linked to this interaction
2. 2011 High

   The upstream activator Rab35 was shown to recruit ACAP2 to phagocytic cups during Fcγ receptor–mediated phagocytosis, establishing the Rab35→ACAP2→Arf6 axis as a regulatory module controlling actin-dependent phagosome formation in macrophages.

   Evidence Live-cell imaging, RNAi knockdown, and expression of GDP/GTP-locked Rab35 mutants in macrophages

   PMID:22045739

   Open questions at the time

   • The precise step at which Arf6 inactivation blocks phagosome closure was not resolved
   • Whether other Rab35 effectors contribute in parallel was not tested
3. 2012 High

   The same Rab35→ACAP2→Arf6 cascade was extended to neuronal differentiation, showing that NGF-stimulated Rab35 recruits ACAP2 to Arf6-positive endosomes and that ACAP2 GAP activity is required for neurite outgrowth.

   Evidence Knockdown-rescue, live-cell imaging, and GTPase activity assays in PC12 cells

   PMID:22344257

   Open questions at the time

   • Downstream effectors of Arf6 inactivation that drive neurite extension were not identified
   • Whether ACAP2 acts at recycling endosomes versus the plasma membrane was not fully distinguished
4. 2014 High

   The Rab35/ACAP2 module was shown to negatively regulate oligodendrocyte differentiation and myelination by inactivating Arf6, broadening the pathway's physiological scope to CNS glia.

   Evidence siRNA knockdown epistasis and oligodendrocyte–neuronal co-culture with pharmacological validation

   PMID:24600047

   Open questions at the time

   • In vivo myelination phenotypes in knockout animals were not examined
   • The Arf6 effector(s) controlling morphological differentiation downstream were not identified
5. 2015 High

   The molecular basis of Rab35–ACAP2 specificity was resolved: Thr-76/Thr-81 in Rab35 switch II and Asn-610/Asn-691 in ACAP2's C-terminal domain are essential for binding, and binding-deficient mutants fail to support neurite outgrowth.

   Evidence Point mutagenesis, co-immunoprecipitation, and knockdown-rescue in PC12 cells

   PMID:25694427

   Open questions at the time

   • No structural model of the Rab35–ACAP2 complex exists
   • Whether post-translational modifications modulate this interface is unknown
6. 2015 Medium

   A GAP-independent pro-apoptotic role for ACAP2 was revealed: ACAP2 shares phosphoinositide-binding properties with its C. elegans ortholog CNT-1, and its knockdown blocks 5-FU–induced apoptosis, linking ACAP2 to programmed cell death.

   Evidence siRNA knockdown and phosphoinositide-binding assays in human cancer cell lines

   PMID:25853217

   Open questions at the time

   • Single-lab finding; independent replication in other cancer cell types is lacking
   • Whether the pro-apoptotic function requires GAP activity or only phosphoinositide binding is unresolved
   • Mechanism connecting phosphoinositide binding to apoptosis execution is unknown
7. 2025 Medium

   ACAP2 protein stability was shown to be controlled by RNF126-mediated ubiquitination and proteasomal degradation, and this degradation reprograms lipid metabolism to promote ovarian cancer progression.

   Evidence Co-immunoprecipitation, cycloheximide chase, siRNA knockdown, and in vivo xenograft in ovarian cancer cells

   PMID:40251363

   Open questions at the time

   • The specific ubiquitination sites on ACAP2 were not mapped
   • Mechanism linking ACAP2 loss to lipid metabolic reprogramming is not delineated
   • Whether RNF126-ACAP2 regulation occurs in normal physiology or is cancer-specific is unknown

• A high-resolution structural understanding of ACAP2's GAP domain in complex with Arf6 and of the Rab35–ACAP2 interface is lacking, and the in vivo physiological consequences of ACAP2 loss in mammalian development and myelination have not been characterized genetically.
• No crystal or cryo-EM structure of ACAP2 alone or in complex
• No conditional or constitutive ACAP2 knockout mouse phenotype reported
• Relationship between GAP-dependent and GAP-independent (apoptotic) functions is unresolved