Affinage

PITPNM2

Membrane-associated phosphatidylinositol transfer protein 2 · UniProt Q9BZ72

Length
1349 aa
Mass
148.9 kDa
Annotated
2026-06-10
28 papers in source corpus 11 papers cited in narrative 11 extracted findings
Cross-family judge vs UniProt: Affinage preferred faithfulness: 6/6 claims corpus-supported (100%)

Mechanistic narrative

Synthesis pass · prose summary of the discoveries below

PITPNM2 (Nir3) is a membrane-associated phosphatidylinositol (PI) transfer protein that maintains plasma-membrane PI(4,5)P2 homeostasis at ER–plasma-membrane junctions (PMID:25887399, PMID:37376972). It is not an integral membrane protein but is stably tethered to particulate membranes through protein–protein interactions (PMID:10460238), engaging the integral ER protein VAP-B via a conserved FFAT motif; this interaction remodels the ER and bundles microtubules along the altered membranes (PMID:15545272). Through its PA-binding and PI-transfer activities, Nir3 sustains resting-state PM PIP2 levels and acts in tandem with Nir2 (PITPNM1), which replenishes PIP2 after intense receptor stimulation (PMID:25887399). This lipid-homeostatic function underlies several physiological roles: in T cells, Nir3 accelerates PIP2 replenishment after TCR stimulation and supports calcium mobilization required for thymocyte β-selection, positive selection, and mature T cell fitness (PMID:36581712); during phagocytosis it accumulates on ER cisternae juxtaposed to phagocytic cups to sustain PI(4,5)P2, store-operated Ca2+ entry, actin contractility, and phagosome sealing (PMID:37376972). Nir3 was originally identified as a PYK2-binding protein that becomes tyrosine-phosphorylated upon calcium elevation (PMID:10022914), and it associates with Kv2.1/VAP complexes at neuronal ER–PM junctions linked to phosphoinositide composition (PMID:31594866). In the nervous system it is expressed in retinal synaptic terminals and GABAergic amacrine cells, where it is required for dim-light pupillary and circadian photoentrainment responses but is dispensable for photoreceptor survival and phototransduction (PMID:11744244, PMID:12037004, PMID:26269578); its Drosophila ortholog regulates nocturnal sleep through mushroom body neurons (PMID:36586155).

Mechanistic history

Synthesis pass · year-by-year structured walk · 11 steps
  1. 1999 Medium

    Established Nir3 as a calcium-responsive signaling protein with intrinsic PI transfer activity and a defined upstream kinase partner, framing it as a link between calcium signaling and phosphoinositide metabolism.

    Evidence Molecular cloning, reciprocal Co-IP with PYK2, in vivo PI transfer and calcium-binding assays in cells and brain tissue

    PMID:10022914

    Open questions at the time
    • Does not define where transfer occurs subcellularly
    • Functional consequence of PYK2-mediated phosphorylation unresolved
  2. 1999 Medium

    Determined that PITPNM2 is peripherally membrane-associated rather than integral and is functionally distinct from its paralog, addressing how the protein engages membranes.

    Evidence Subcellular fractionation and transgenic rescue in Drosophila rdgB2 null mutants

    PMID:10460238

    Open questions at the time
    • Tethering partner not identified at this stage
    • Why rescue of light response was incomplete unexplained
  3. 2001 High

    Tested whether PITPNM2 is essential for mammalian vision, showing it is dispensable for photoreceptor function and survival and redirecting attention to non-phototransduction roles.

    Evidence Gene-targeted knockout mouse with electroretinography, histology, and immunocytochemistry to 18 months

    PMID:11744244

    Open questions at the time
    • Negative result; does not reveal the protein's actual function
    • Possible paralog redundancy not excluded
  4. 2002 Medium

    Localized Nir3 to neuronal synaptic terminals and photoreceptor inner segments, indicating a role at synaptic membranes rather than the outer-segment phototransduction machinery.

    Evidence Confocal immunofluorescence with co-staining for the presynaptic marker SNAP-25 in developing and adult rat retina

    PMID:12037004

    Open questions at the time
    • Co-localization does not establish synaptic function
    • Molecular role at synapse not defined
  5. 2004 Medium

    Identified the FFAT-motif/VAP-B interaction as the molecular basis for ER membrane targeting, explaining how a non-integral protein is recruited to the ER.

    Evidence Co-IP, FFAT-motif mutant analysis, and fluorescence imaging of ER morphology

    PMID:15545272

    Open questions at the time
    • Whether ER remodeling occurs at physiological expression levels unclear
    • Functional output of microtubule bundling not established
  6. 2015 High

    Defined the core biochemical function: Nir3 maintains resting PM PIP2 at ER-PM junctions while Nir2 handles stimulated replenishment, distinguishing the two paralogs by PA-binding and transfer kinetics.

    Evidence Live-cell PIP2 biosensor imaging, PA-binding and PI transfer assays, siRNA knockdown and overexpression

    PMID:25887399

    Open questions at the time
    • Structural basis of differential PA binding not resolved
    • In vivo relevance across tissues not addressed here
  7. 2015 High

    Placed PITPNM2 in a specific retinal circuit, showing it acts in GABAergic amacrine cells to transmit dim-light input to ipRGCs for pupillary and circadian responses.

    Evidence Knockout mice with ipRGC electrophysiology, pupillary light reflex, circadian photoentrainment assays, and immunofluorescence

    PMID:26269578

    Open questions at the time
    • Link between PIP2 homeostasis and circuit phenotype not directly demonstrated
    • Cell-autonomous mechanism in amacrine cells unresolved
  8. 2019 Medium

    Showed Nir3 is recruited to neuronal Kv2.1-organized ER-PM junctions through VAP, connecting a channel-scaffolded junction to phosphoinositide composition.

    Evidence Kv2.1 complex proteomics, Co-IP, FRAP turnover analysis, and Kv2.1 knockout mouse lipid profiling

    PMID:31594866

    Open questions at the time
    • Direct vs VAP-bridged Nir3-Kv2.1 contact not distinguished
    • Functional consequence for channel activity untested
  9. 2022 High

    Established a physiological role in adaptive immunity, demonstrating that Nir3-dependent PIP2 replenishment is required for TCR-driven calcium signaling and thymocyte selection.

    Evidence Nir3 knockout mice with PIP2 biosensor imaging, calcium mobilization, and flow-cytometric thymocyte development analysis

    PMID:36581712

    Open questions at the time
    • Whether Nir2 compensates in peripheral T cells unclear
    • Downstream effectors of attenuated Ca2+ signaling not mapped
  10. 2022 Medium

    Demonstrated a conserved neuronal behavioral role, showing the Drosophila ortholog regulates sleep specifically through mushroom body neurons.

    Evidence Pan-neuronal and MB-specific RNAi knockdown with sleep quantification and tissue-specific rescue in Drosophila

    PMID:36586155

    Open questions at the time
    • Lipid/PIP2 mechanism in sleep circuit not tested
    • Mammalian relevance of sleep role not established
  11. 2023 High

    Extended the lipid-homeostatic function to innate immunity, showing Nir3 sustains PI(4,5)P2 at phagocytic cups to drive actin contractility and phagosome sealing.

    Evidence CRISPR-Cas9 double knockout with PI(4,5)P2 biosensor imaging, SOCE measurement, phagocytosis and actin assays, and rescue by re-expression

    PMID:37376972

    Open questions at the time
    • Relative contribution of Nir2 vs Nir3 not separated
    • How ER cisternae are positioned at cups not defined

Open questions

Synthesis pass · forward-looking unresolved questions
  • How Nir3-specific lipid transfer at ER-PM junctions is regulated and mechanistically coupled to its diverse physiological outputs (immune selection, phagocytosis, retinal circuits, sleep) remains unresolved.
  • No structural model of the Nir3 transfer cycle
  • Regulatory role of PYK2 phosphorylation undefined
  • Unifying mechanism linking PIP2 homeostasis to tissue-specific phenotypes not established

Mechanism profile

Synthesis pass · controlled-vocabulary classification · explore literature graph →
Molecular activity
GO:0140104 molecular carrier activity 3 GO:0008289 lipid binding 2 GO:0060089 molecular transducer activity 2
Localization
GO:0005783 endoplasmic reticulum 3 GO:0005886 plasma membrane 3
Pathway
R-HSA-162582 Signal Transduction 2 R-HSA-168256 Immune System 2 GO:0005856 cytoskeleton 1
Partners
KCNB1PITPNM1PYK2VAPB
Complex memberships
Kv2.1-VAP ER-PM junction complex

Evidence

Reading pass · 11 per-paper findings extracted from the source corpus
Year Finding Method Journal Conf PMIDs
1999 PITPNM2 (Nir3) was identified as a PYK2-binding protein that interacts with the amino-terminal domain of PYK2 via a conserved sequence motif in its carboxy terminus. Nir proteins (including Nir3) are calcium-binding proteins that exhibit phosphatidylinositol (PI) transfer activity in vivo, and activation of PYK2 by agents that elevate intracellular calcium or phorbol ester induces tyrosine phosphorylation of Nir proteins. PYK2-Nir complexes were detected in lysates from cultured cells and brain tissues. Molecular cloning, Co-immunoprecipitation (Co-IP), in vivo PI transfer assay, calcium-binding assay, tyrosine phosphorylation assay Molecular and cellular biology Medium 10022914
1999 M-RdgB2 (PITPNM2) is not an integral membrane protein but is stably associated with a particulate fraction through protein-protein interactions, as demonstrated by subcellular fractionation. Transgenic expression of M-RdgB2 in rdgB2 null mutant flies suppressed retinal degeneration but failed to fully restore the electrophysiological light response, indicating functional differences from M-RdgB1. Subcellular fractionation, antibody-based detection, transgenic rescue in Drosophila rdgB2 null mutants The Journal of neuroscience Medium 10460238
2001 Homozygous knockout of m-rdgB2 (PITPNM2) in mice produced no detectable defects in photoreceptor function or survival (normal electroretinograms, normal photoreceptor morphology by light microscopy, normal immunocytochemistry) up to 18 months, establishing that PITPNM2 is not required for mammalian phototransduction or photoreceptor survival. Gene targeting knockout, electroretinography, light microscopy, immunocytochemistry Neuroscience High 11744244
2004 PITPNM2 (Nir3) interacts with the integral ER-membrane protein VAP-B via a conserved FFAT motif present in Nir proteins. The Nir3-VAP-B interaction leads to gross remodeling of the ER and bundling of thick microtubules along the altered ER membranes, an effect distinct from that of Nir2-VAP-B interaction. Co-immunoprecipitation, overexpression/fluorescence microscopy of ER morphology, FFAT-motif mutant analysis The Journal of biological chemistry Medium 15545272
2002 In the developing and adult rat retina, Nir3 (PITPNM2) is highly expressed in synaptic terminals of neuronal cells and co-localizes with the presynaptic protein SNAP-25, as demonstrated by confocal immunofluorescence with co-immunostaining. In photoreceptor cells, Nir3 is expressed in the inner segments but not the outer segments. Indirect immunofluorescence, confocal microscopy, co-immunostaining with subcellular markers including SNAP-25 Investigative ophthalmology & visual science Medium 12037004
2015 Nir3 (PITPNM2) differentially regulates PIP2 homeostasis compared to Nir2: Nir3 has distinct phosphatidic acid (PA) binding ability and PI transfer protein activity, and functions to maintain resting-state PM PIP2 levels at ER-PM junctions, while Nir2 replenishes PM PIP2 in response to intense receptor stimulation. The two proteins work in tandem to achieve different levels of feedback on PIP2 homeostasis. Live-cell imaging of PIP2 biosensors, PA-binding assay, PI transfer protein activity assay, siRNA knockdown, overexpression The Journal of biological chemistry High 25887399
2015 RdgB2 (PITPNM2) is expressed in GABAergic amacrine cells (not in intrinsically photosensitive retinal ganglion cells/ipRGCs), and RdgB2 knockout mice show normal ipRGC intrinsic light responses and normal pupillary light reflex/circadian photoentrainment under high light conditions, but display defects in both under low-light conditions. This places PITPNM2 in a cellular circuit transducing dim-light (rod) input via bipolar and GABAergic amacrine cells to ipRGCs. RdgB2 knockout mice, electrophysiology of ipRGCs, pupillary light reflex assay, circadian photoentrainment behavioral assay, immunofluorescence localization Molecular biology of the cell High 26269578
2019 Nir3 (PITPNM2) specifically associates with Kv2.1 complexes at ER-PM junctions organized by Kv2/VAP pairing in neurons. This association is mediated via VAP proteins. FRAP experiments showed that Kv2.1, VAPA, and Nir2 have comparable turnover rates at ER-PM junctions, suggesting they form stable complexes at these sites. Kv2.1 knockout mouse brains show altered PtdIns lipid composition, linking the Kv2-VAP-Nir3 ER-PM junction complex to PtdIns lipid homeostasis. Proteomics (mass spectrometry) of Kv2.1 complexes, co-immunoprecipitation, co-localization by immunofluorescence, FRAP, Kv2.1 knockout mouse lipid analysis The Journal of biological chemistry Medium 31594866
2022 Nir3 (PITPNM2) promotes PIP2 replenishment following TCR stimulation in T cells. In Nir3-/- T lineage cells, PIP2 replenishment following TCR stimulation is slower, calcium mobilization in double-positive (DP) thymocytes is attenuated in response to weak TCR stimulation, leading to impaired thymocyte development at TCRβ selection and positive selection, and diminished mature T cell fitness. Nir3 knockout mice, live-cell PIP2 biosensor imaging, calcium mobilization assay, flow cytometric analysis of thymocyte development Nature immunology High 36581712
2023 PITPNM2 (Nir3) maintains PI(4,5)P2 homeostasis at phagocytic cups, thereby promoting actin contractility and phagosome sealing during phagocytosis. Nir3 accumulates on ER cisternae juxtaposed to phagocytic cups. CRISPR-Cas9 double knockout of Nir2 and Nir3 decreased PM PI(4,5)P2 levels, store-operated Ca2+ entry (SOCE), and receptor-mediated phagocytosis, stalling particle capture at the cup stage; re-expression of Nir3 (or Nir2) restored phagocytosis proportionally to PM PI(4,5)P2 levels. CRISPR-Cas9 knockout, live-cell PI(4,5)P2 biosensor imaging, SOCE measurement, phagocytosis assay, actin dynamics imaging, rescue by re-expression Journal of cell science High 37376972
2022 Drosophila rdgB (ortholog of PITPNM2) knockdown specifically in mushroom body (MB) neurons reduced nocturnal sleep; re-expression of rdgB only in MB neurons in the pan-neuronal knockdown background reversed the sleep-reducing effect, placing rdgB/PITPNM2 in a sleep-regulatory circuit in MB neurons. Pan-neuronal and MB-specific RNAi knockdown in Drosophila, sleep behavior quantification, tissue-specific rescue Biochemical and biophysical research communications Medium 36586155

Source papers

Stage 0 corpus · 28 papers · ranked by NIH iCite citations
Year Title Journal Citations PMID
2004 Differential regulation of endoplasmic reticulum structure through VAP-Nir protein interaction. The Journal of biological chemistry 165 15545272
1999 Identification of a novel family of targets of PYK2 related to Drosophila retinal degeneration B (rdgB) protein. Molecular and cellular biology 133 10022914
1997 The phosphatidylinositol transfer protein domain of Drosophila retinal degeneration B protein is essential for photoreceptor cell survival and recovery from light stimulation. The Journal of cell biology 128 9334340
2015 Phosphatidylinositol 4,5-Bisphosphate Homeostasis Regulated by Nir2 and Nir3 Proteins at Endoplasmic Reticulum-Plasma Membrane Junctions. The Journal of biological chemistry 118 25887399
2006 Phospholipid transfer proteins in perspective. FEBS letters 47 16828756
1999 Cloning and characterization of a novel human phosphatidylinositol transfer protein, rdgBbeta. The Journal of biological chemistry 44 10531358
1999 A neuronal-specific mammalian homolog of the Drosophila retinal degeneration B gene with expression restricted to the retina and dentate gyrus. The Journal of neuroscience : the official journal of the Society for Neuroscience 41 10460238
2001 The mammalian retinal degeneration B2 gene is not required for photoreceptor function and survival. Neuroscience 37 11744244
2019 Neuronal ER-plasma membrane junctions organized by Kv2-VAP pairing recruit Nir proteins and affect phosphoinositide homeostasis. The Journal of biological chemistry 36 31594866
2012 The diverse functions of phosphatidylinositol transfer proteins. Current topics in microbiology and immunology 36 23086419
2020 Integrated Analysis of Summary Statistics to Identify Pleiotropic Genes and Pathways for the Comorbidity of Schizophrenia and Cardiometabolic Disease. Frontiers in psychiatry 31 32425817
2017 Whole-exome sequencing of individuals from an isolated population implicates rare risk variants in bipolar disorder. Translational psychiatry 22 28195573
2000 Cloning and tissue localization of a novel zebrafish RdgB homolog that lacks a phospholipid transfer domain. Visual neuroscience 22 10824684
2022 Nir1 constitutively localizes at ER-PM junctions and promotes Nir2 recruitment for PIP2 homeostasis. Molecular biology of the cell 19 35020418
2023 ELF4 is a critical component of a miRNA-transcription factor network and is a bridge regulator of glioblastoma receptor signaling and lipid dynamics. Neuro-oncology 17 35862252
2022 The phosphatidylinositol-transfer protein Nir3 promotes PI(4,5)P2 replenishment in response to TCR signaling during T cell development and survival. Nature immunology 17 36581712
2013 Negative regulation of the novel norpA(P24) suppressor, diehard4, in the endo-lysosomal trafficking underlies photoreceptor cell degeneration. PLoS genetics 17 23754968
2005 Chromosomal localization, genomic organization and evolution of the genes encoding human phosphatidylinositol transfer protein membrane-associated (PITPNM) 1, 2 and 3. Cytogenetic and genome research 17 15627748
2003 Cloning and characterization of a novel variant (mM-rdgBbeta1) of mouse M-rdgBs, mammalian homologs of Drosophila retinal degeneration B gene proteins, and its mRNA localization in mouse brain in comparison with other M-rdgBs. Journal of neurochemistry 17 12562526
2013 Pitpnm1 is expressed in hair cells during development but is not required for hearing. Neuroscience 11 23820044
2002 Cellular and developmental distribution of human homologues of the Drosophilia rdgB protein in the rat retina. Investigative ophthalmology & visual science 11 12037004
2019 Next-Generation Sequencing Profiles of the Methylome and Transcriptome in Peripheral Blood Mononuclear Cells of Rheumatoid Arthritis. Journal of clinical medicine 9 31443559
2023 The lipid transfer proteins Nir2 and Nir3 sustain phosphoinositide signaling and actin dynamics during phagocytosis. Journal of cell science 8 37376972
2023 Gene co-expression network construction and analysis for identification of genetic biomarkers associated with glioblastoma multiforme using topological findings. Journal of the Egyptian National Cancer Institute 6 37482563
2015 RdgB2 is required for dim-light input into intrinsically photosensitive retinal ganglion cells. Molecular biology of the cell 6 26269578
2022 rdgB knockdown in neurons reduced nocturnal sleep in Drosophila melanogaster. Biochemical and biophysical research communications 5 36586155
2024 Adipocytes reprogram the proteome of breast cancer cells in organotypic three-dimensional cell cultures. Scientific reports 4 39505903
2026 Differential expression of cancer-related genes supports prediction of poor response to first-line treatments in T-ALL pediatric patients with high minimal residual disease. Molecular oncology 0 41954065

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