GAPVD1

GAPVD1 is a cytoplasmic VPS9-domain guanine nucleotide exchange regulator for endocytic RAB GTPases that promotes early endosome formation and receptor internalization (PMID:29603841, PMID:32321936). Through its VPS9 domain it acts on RAB5, RAB21, and RAB22, and its ortholog is required for membrane recruitment of Rab5 and Rab21, placing GAPVD1 in the conserved Rabex5+GAPVD1 subfamily of endocytic Rab regulators (PMID:29603841). GAPVD1 physically associates with active RAB5 and with ANKFY1, and its activity supports internalization of the transferrin and EGF receptors (PMID:29959197, PMID:32321936). This endocytic function is gated by phosphorylation: CK1δ/ε directly phosphorylate GAPVD1 at numerous residues, and a phosphomimetic but not a phospho-ablating form rescues transferrin and EGF uptake in GAPVD1-deficient cells, establishing phosphorylation as a requirement for its trafficking role (PMID:32321936). GAPVD1 is also a substrate of AMPK at Ser902 (PMID:30772465), and it is a component of cytoplasmic PER complexes in which CK1δ/CSNK1D and PER2 control its phosphorylation state and consequent degradation kinetics, coupling its trafficking activity to circadian-clock machinery (PMID:28886335, PMID:33917494). In the kidney, GAPVD1 binds nephrin and partially colocalizes with it at the glomerular slit diaphragm, and loss-of-function missense mutations that weaken its binding to active RAB5 or to nephrin cause human nephrotic syndrome, with parallel endocytic and nephrin-trafficking defects in Drosophila nephrocytes (PMID:29959197).

1. 2017 Medium

   Established that GAPVD1 is not solely an endocytic factor but a stable cytoplasmic component of PER complexes, linking it to circadian-clock assembly machinery.

   Evidence Biochemical fractionation and single-particle EM of endogenous PER complexes from mouse liver

   PMID:28886335

   Open questions at the time

   • Functional role inferred from complex composition rather than direct loss-of-function
   • No mechanism for how a trafficking factor contributes to PER complex assembly
   • Human relevance not tested in this study
2. 2018 High

   Defined GAPVD1's disease relevance and molecular partners by showing it binds active RAB5 and nephrin and that patient mutations weaken these interactions to cause nephrotic syndrome.

   Evidence Reciprocal Co-IP, immunofluorescence colocalization in rat glomeruli, siRNA podocyte migration assay, Drosophila nephrocyte endocytosis assay, patient-mutant expression

   PMID:29959197

   Open questions at the time

   • Whether GAPVD1 acts catalytically as a GEF on RAB5 in this context not directly assayed
   • Mechanistic link between nephrin endocytosis and slit-diaphragm maintenance not fully resolved
3. 2018 Medium

   Placed GAPVD1 in the conserved Rabex5+GAPVD1 VPS9-domain subfamily and assigned it nucleotide exchange activity toward endocytic Rab GTPases.

   Evidence Molecular evolutionary analysis with T. brucei ortholog knockdown reading out Rab5/Rab21 membrane recruitment

   PMID:29603841

   Open questions at the time

   • GEF activity for mammalian GAPVD1 inferred by orthology rather than measured biochemically
   • Substrate Rab specificity in human cells not directly ranked
4. 2019 Medium

   Identified GAPVD1 as an AMPK substrate, connecting its trafficking function to energy-sensing signaling.

   Evidence Chemical genetic AMPK activation in primary hepatocytes, mass spectrometry, phospho-Ser902-specific immunoblotting

   PMID:30772465

   Open questions at the time

   • Functional consequence of Ser902 phosphorylation on endocytosis not established
   • Single lab, single cell system
5. 2020 High

   Demonstrated that CK1δ/ε directly phosphorylate GAPVD1 and that this phosphorylation is required for its receptor-internalization function.

   Evidence MS co-purification with endogenously tagged CK1δ/ε, in vitro kinase assay, phosphomimetic/phospho-ablating mutant rescue of transferrin and EGF endocytosis

   PMID:32321936

   Open questions at the time

   • Which of the up-to-38 sites are functionally critical not resolved
   • How phosphorylation alters GEF activity or partner binding not defined
6. 2021 Medium

   Extended the PER-complex finding to human cells and showed CSNK1D and PER2 control GAPVD1 phosphorylation, which in turn sets its degradation kinetics.

   Evidence Biochemical screen for PER2-interacting proteins, Co-IP, phospho-specific immunoblotting, degradation/pulse-chase assays

   PMID:33917494

   Open questions at the time

   • No in vitro reconstitution of the regulatory circuit
   • Whether circadian regulation of GAPVD1 feeds back on endocytosis untested
7. 2024 Low

   Proposed a signaling role for GAPVD1 downstream of NRP-1/VEGFA via Wnt/β-catenin in triple-negative breast cancer stemness.

   Evidence GAPVD1 knockdown/overexpression in TNBC cells, Western blotting, macrophage co-culture and stemness assays

   PMID:38169627

   Open questions at the time

   • Pathway placement inferred from KD/OE and Western blot without direct NRP-1–GAPVD1 interaction
   • Mechanism linking endocytic regulator to Wnt signaling unestablished
8. 2025 Low

   Associated GAPVD1 levels with TNBC proliferation, cell-cycle progression, and ERK/MAPK activity in vitro and tumor growth in vivo.

   Evidence CCK-8, colony formation, flow cytometry, Western blotting for PCNA/Cyclin A/ERK, xenograft model

   PMID:39021189

   Open questions at the time

   • No direct biochemical mechanism linking GAPVD1 to ERK pathway
   • Phenotypic correlation only; causal molecular step undefined

• It remains unresolved how GAPVD1 phosphorylation by CK1δ/ε and AMPK mechanistically modulates its GEF activity toward specific Rab GTPases, and whether its circadian and cancer-signaling roles share a common biochemical basis.
• No direct measurement of GAPVD1 GEF kinetics as a function of phosphorylation
• No structural model of the VPS9 domain engaging Rab5/Rab21/Rab22
• Connection between PER-complex regulation and endocytic output untested