Grpel2

id: O88396
gene_symbol: Grpel2
product_type: PROTEIN
status: IN_PROGRESS
taxon:
  id: NCBITaxon:10090
  label: Mus musculus
description: >-
  GrpE protein homolog 2 (mt-GrpE#2) is a mitochondrial nucleotide exchange
  factor (NEF) for mitochondrial Hsp70 (mortalin/HSPA9). It is a component
  of the PAM complex (presequence translocase-associated motor) required for
  ATP-dependent translocation of transit peptide-containing proteins into the
  mitochondrial matrix. Grpel2 stimulates ADP release from mt-Hsp70, thereby
  accelerating the Hsp70 ATPase cycle. It may also modulate the interconversion
  of oligomeric (inactive) and monomeric (active) forms of mt-Hsp70. A paralog
  of Grpel1 (mt-GrpE#1), both are ubiquitously expressed but may be subject
  to different post-transcriptional regulation (PMID:9694873).
existing_annotations:
- term:
    id: GO:0000774
    label: adenyl-nucleotide exchange factor activity
  evidence_type: IBA
  original_reference_id: GO_REF:0000033
  review:
    summary: >-
      Grpel2 is a GrpE-family nucleotide exchange factor for mitochondrial Hsp70.
      Naylor et al. (PMID:9694873) demonstrated that mt-GrpE#2 specifically
      interacts with and stimulates the ATPase activity of mt-Hsp70. The IBA
      annotation from phylogenetic inference (PANTHER/SGD) is well-supported
      by the conserved GrpE domain and experimental evidence for the mammalian
      protein. Adenyl-nucleotide exchange factor activity is the core molecular
      function of GrpE proteins.
    action: ACCEPT
    reason: >-
      This is the primary molecular function of Grpel2. GrpE proteins catalyze
      nucleotide exchange on Hsp70 chaperones, which is the rate-limiting step
      in the Hsp70 ATPase cycle. The IBA inference is phylogenetically sound
      and consistent with direct experimental evidence (PMID:9694873).
    supported_by:
      - reference_id: PMID:9694873
        supporting_text: >-
          The functional integrity of mt-GrpE#1 and #2 was verified by their
          ability to specifically interact with and stimulate the ATPase activity
          of mammalian mitochondrial Hsp70 (mt-Hsp70).
- term:
    id: GO:0001405
    label: PAM complex, Tim23 associated import motor
  evidence_type: IBA
  original_reference_id: GO_REF:0000033
  review:
    summary: >-
      Grpel2 is annotated as a component of the PAM complex by phylogenetic
      inference. UniProt states (by similarity) that it is a probable component
      of the PAM complex composed of mt-HSP70, GRPEL1 or GRPEL2, TIMM44,
      PAM16, and DNAJC19. The yeast ortholog (MGE1/YOR232W) is a well-established
      PAM complex component. This annotation is consistent with the known biology.
    action: ACCEPT
    reason: >-
      GrpE proteins function as nucleotide exchange factors within the PAM
      import motor complex. The IBA annotation based on phylogenetic inference
      from the yeast ortholog is well-supported by structural and functional
      conservation of the PAM complex across eukaryotes. UniProt confirms
      this localization by similarity.
- term:
    id: GO:0030150
    label: protein import into mitochondrial matrix
  evidence_type: IBA
  original_reference_id: GO_REF:0000033
  review:
    summary: >-
      As a component of the PAM complex, Grpel2 participates in protein import
      into the mitochondrial matrix. The nucleotide exchange activity of GrpE
      is essential for the mt-Hsp70 ATPase cycle that drives translocation of
      preproteins through the TIM23 channel. This is a core biological process
      for this protein, well-supported by IBA from phylogenetic inference.
    action: ACCEPT
    reason: >-
      Protein import into the mitochondrial matrix is the primary biological
      process function of the PAM complex. Grpel2 as a NEF for mt-Hsp70 plays
      an essential role in this process. The IBA annotation is phylogenetically
      sound and consistent with UniProt functional description.
- term:
    id: GO:0051082
    label: unfolded protein binding
  evidence_type: IBA
  original_reference_id: GO_REF:0000033
  review:
    summary: >-
      GO:0051082 is proposed for obsoletion. Grpel2 is a nucleotide exchange
      factor (NEF), not a chaperone that directly binds unfolded proteins.
      Its function is to accelerate nucleotide exchange on mt-Hsp70, thereby
      facilitating the Hsp70 chaperone cycle. While NEFs participate in the
      protein folding machinery, they are co-chaperones rather than direct
      unfolded protein binders.
    action: MODIFY
    reason: >-
      GO:0051082 (unfolded protein binding) is slated for obsoletion and is
      not the correct term for a nucleotide exchange factor. Grpel2 functions
      as a co-chaperone by stimulating nucleotide exchange on mt-Hsp70. The
      appropriate replacement is GO:0044183 (protein folding chaperone), which
      captures the co-chaperone role in the protein folding machinery without
      implying direct unfolded substrate binding.
    proposed_replacement_terms:
      - id: GO:0044183
        label: protein folding chaperone
- term:
    id: GO:0000774
    label: adenyl-nucleotide exchange factor activity
  evidence_type: IEA
  original_reference_id: GO_REF:0000002
  review:
    summary: >-
      IEA annotation from InterPro (IPR000740, GrpE domain) mapping to
      adenyl-nucleotide exchange factor activity. Consistent with the IBA
      annotation and experimental evidence.
    action: ACCEPT
    reason: >-
      This IEA annotation from InterPro domain mapping is correct and
      consistent with the IBA annotation for the same term. The GrpE domain
      (IPR000740) is the defining feature of nucleotide exchange factors
      for Hsp70 chaperones.
- term:
    id: GO:0005759
    label: mitochondrial matrix
  evidence_type: IEA
  original_reference_id: GO_REF:0000044
  review:
    summary: >-
      IEA annotation based on UniProt subcellular location mapping. UniProt
      states Grpel2 localizes to the mitochondrion matrix. The protein has
      a mitochondrial transit peptide (residues 1-31) and functions within
      the matrix as part of the PAM complex.
    action: ACCEPT
    reason: >-
      Mitochondrial matrix localization is well-supported by the presence
      of a mitochondrial transit peptide, the protein's function as a NEF
      for matrix-localized mt-Hsp70, and its role in the PAM complex. More
      specific than GO:0005739 (mitochondrion) and accurately reflects
      the sub-organellar localization.
- term:
    id: GO:0006457
    label: protein folding
  evidence_type: IEA
  original_reference_id: GO_REF:0000002
  review:
    summary: >-
      IEA annotation from InterPro mapping. Grpel2 participates in protein
      folding indirectly as a co-chaperone for mt-Hsp70. However, its more
      specific biological process role is in protein import into the
      mitochondrial matrix (GO:0030150). Protein folding is a broader
      acceptable annotation but not the most informative.
    action: ACCEPT
    reason: >-
      While protein import into the mitochondrial matrix is the more specific
      process, Grpel2 does participate in protein folding through its role
      in regulating mt-Hsp70. The IEA annotation is not incorrect and provides
      complementary biological context to GO:0030150. As an IEA it is
      acceptably general.
- term:
    id: GO:0042803
    label: protein homodimerization activity
  evidence_type: IEA
  original_reference_id: GO_REF:0000002
  review:
    summary: >-
      IEA annotation from InterPro mapping. Bacterial GrpE forms homodimers,
      and this is a conserved feature of GrpE-family proteins. However,
      homodimerization is a structural property rather than a functional
      molecular activity of interest, and it has not been directly
      demonstrated for mouse Grpel2.
    action: MARK_AS_OVER_ANNOTATED
    reason: >-
      While GrpE-family proteins are known to form homodimers, this
      annotation describes a structural feature rather than the functional
      molecular activity. Homodimerization is not experimentally verified
      for Grpel2 specifically, and this IEA may be an over-extension from
      bacterial GrpE structural data. The core molecular function is
      GO:0000774 (adenyl-nucleotide exchange factor activity).
- term:
    id: GO:0051087
    label: protein-folding chaperone binding
  evidence_type: IEA
  original_reference_id: GO_REF:0000002
  review:
    summary: >-
      IEA annotation from InterPro mapping. Grpel2 does bind mt-Hsp70
      (a protein-folding chaperone) as part of its nucleotide exchange
      function. Naylor et al. (PMID:9694873) showed that the GrpE-like
      proteins bound to E. coli DnaK and to mammalian mt-Hsp70.
    action: ACCEPT
    reason: >-
      This annotation correctly captures the binding interaction between
      Grpel2 and mt-Hsp70. As a nucleotide exchange factor, Grpel2 must
      physically bind to Hsp70 to catalyze ADP release. This is
      experimentally supported (PMID:9694873).
    supported_by:
      - reference_id: PMID:9694873
        supporting_text: >-
          Investigation of the microsomal and two mitochondrial GrpE-like
          proteins revealed that they bound specifically to Escherichia coli
          DnaK, and the complexes formed were not disrupted in the presence
          of 0.5 M salt but were readily dissociated in the presence of 5 mM ATP.
- term:
    id: GO:0005739
    label: mitochondrion
  evidence_type: ISO
  original_reference_id: GO_REF:0000119
  review:
    summary: >-
      ISO annotation transferred from the human ortholog (UniProtKB:Q8TAA5,
      GRPEL2). Mitochondrial localization is well-established for this protein
      family and supported by multiple lines of evidence including transit
      peptide, proteomics, and functional data.
    action: ACCEPT
    reason: >-
      Mitochondrial localization is strongly supported by the mitochondrial
      transit peptide (residues 1-31), proteomic detection in mitochondrial
      preparations (PMID:18614015, PMID:14651853), and functional role in
      the mitochondrial PAM complex. The ISO transfer from human ortholog
      is well-justified.
- term:
    id: GO:0005739
    label: mitochondrion
  evidence_type: IEA
  original_reference_id: GO_REF:0000107
  review:
    summary: >-
      IEA annotation from Ensembl Compara ortholog transfer (from human
      GRPEL2, UniProtKB:Q8TAA5). Consistent with all other evidence for
      mitochondrial localization.
    action: ACCEPT
    reason: >-
      Redundant with the ISO annotation for the same term but from a
      different evidence pipeline. Mitochondrial localization is
      well-established.
- term:
    id: GO:0005739
    label: mitochondrion
  evidence_type: HDA
  original_reference_id: PMID:18614015
  review:
    summary: >-
      HDA annotation from the MitoCarta mitochondrial protein compendium
      (Pagliarini et al. 2008). This large-scale proteomics study used mass
      spectrometry, GFP tagging, and machine learning to create a compendium
      of 1098 mitochondrial genes across 14 mouse tissues.
    action: ACCEPT
    reason: >-
      The MitoCarta compendium is a high-quality proteomics dataset that
      provides strong experimental support for mitochondrial localization.
      Detection by mass spectrometry in purified mitochondrial preparations
      is direct evidence for this cellular component annotation.
    supported_by:
      - reference_id: PMID:18614015
        supporting_text: >-
          We performed mass spectrometry, GFP tagging, and machine learning
          to create a mitochondrial compendium of 1098 genes and their protein
          expression across 14 mouse tissues.
- term:
    id: GO:0005739
    label: mitochondrion
  evidence_type: HDA
  original_reference_id: PMID:14651853
  review:
    summary: >-
      HDA annotation from Mootha et al. (2003), an integrated proteomic
      survey of mitochondria from mouse brain, heart, kidney, and liver.
      This study produced a list of 591 mitochondrial proteins.
    action: ACCEPT
    reason: >-
      Detection in a large-scale mitochondrial proteomics study provides
      direct evidence for mitochondrial localization. Consistent with all
      other localization evidence.
    supported_by:
      - reference_id: PMID:14651853
        supporting_text: >-
          To explore its molecular composition, we performed a proteomic survey
          of mitochondria from mouse brain, heart, kidney, and liver and combined
          the results with existing gene annotations to produce a list of 591
          mitochondrial proteins, including 163 proteins not previously associated
          with this organelle.
- term:
    id: GO:0005743
    label: mitochondrial inner membrane
  evidence_type: HDA
  original_reference_id: PMID:12865426
  review:
    summary: >-
      HDA annotation from Da Cruz et al. (2003), a proteomic analysis of
      purified mouse liver mitochondrial inner membrane. The study identified
      182 proteins in the inner membrane fraction. However, Grpel2 is a
      soluble matrix protein that associates with the PAM complex at the
      inner membrane. Detection in the inner membrane fraction may reflect
      its association with the membrane-bound TIM23 complex rather than
      intrinsic membrane localization.
    action: ACCEPT
    reason: >-
      While Grpel2 is primarily a soluble matrix protein, it functions as
      part of the PAM complex which is associated with the inner membrane
      TIM23 translocase. Detection in the inner membrane proteome is
      consistent with this peripheral membrane association. The HDA
      evidence from a purified inner membrane proteomics study supports
      this annotation.
    supported_by:
      - reference_id: PMID:12865426
        supporting_text: >-
          We have focused our study on the identification of proteins of the
          mitochondrial inner membrane (MIM)... This procedure allowed us to
          identify 182 proteins that are involved in several biochemical
          processes, such as the electron transport machinery, the protein
          import machinery, protein synthesis, lipid metabolism, and ion or
          substrate transport.
- term:
    id: GO:0051082
    label: unfolded protein binding
  evidence_type: IDA
  original_reference_id: PMID:9694873
  review:
    summary: >-
      IDA annotation from Naylor et al. (1998). The paper demonstrated that
      GrpE-like proteins bind specifically to DnaK (an Hsp70 chaperone) in
      a salt-resistant, ATP-dissociable manner, and stimulate the ATPase
      activity of mt-Hsp70. However, the paper does not demonstrate direct
      binding to unfolded protein substrates. The interaction characterized
      is between the NEF and the Hsp70 chaperone, not between the NEF and
      unfolded substrates. GO:0051082 is proposed for obsoletion and is
      not the correct term for a nucleotide exchange factor.
    action: MODIFY
    reason: >-
      The experimental evidence in PMID:9694873 demonstrates that Grpel2
      interacts with Hsp70 chaperones and stimulates their ATPase activity,
      which is the function of a co-chaperone/NEF. The paper does not show
      direct binding of Grpel2 to unfolded protein substrates. The appropriate
      replacement term is GO:0044183 (protein folding chaperone), which
      captures the co-chaperone role without implying direct substrate binding.
      GO:0051082 is also proposed for obsoletion.
    proposed_replacement_terms:
      - id: GO:0044183
        label: protein folding chaperone
    supported_by:
      - reference_id: PMID:9694873
        supporting_text: >-
          The functional integrity of mt-GrpE#1 and #2 was verified by their
          ability to specifically interact with and stimulate the ATPase activity
          of mammalian mitochondrial Hsp70 (mt-Hsp70).
core_functions:
- description: >-
    Grpel2 functions as a mitochondrial nucleotide exchange factor (NEF) within
    the PAM complex, stimulating ADP release from mt-Hsp70 to drive the
    ATP-dependent import of transit peptide-containing proteins into the
    mitochondrial matrix.
  molecular_function:
    id: GO:0000774
    label: adenyl-nucleotide exchange factor activity
  directly_involved_in:
    - id: GO:0030150
      label: protein import into mitochondrial matrix
  locations:
    - id: GO:0005759
      label: mitochondrial matrix
  in_complex:
    id: GO:0001405
    label: PAM complex, Tim23 associated import motor
references:
- id: GO_REF:0000002
  title: Gene Ontology annotation through association of InterPro records with GO
    terms
  findings: []
- id: GO_REF:0000033
  title: Annotation inferences using phylogenetic trees
  findings: []
- id: GO_REF:0000044
  title: Gene Ontology annotation based on UniProtKB/Swiss-Prot Subcellular Location
    vocabulary mapping, accompanied by conservative changes to GO terms applied by
    UniProt
  findings: []
- id: GO_REF:0000107
  title: Automatic transfer of experimentally verified manual GO annotation data to
    orthologs using Ensembl Compara
  findings: []
- id: GO_REF:0000119
  title: Automated transfer of experimentally-verified manual GO annotation data to
    mouse-human orthologs
  findings: []
- id: PMID:12865426
  title: Proteomic analysis of the mouse liver mitochondrial inner membrane.
  findings: []
- id: PMID:14651853
  title: Integrated analysis of protein composition, tissue diversity, and gene regulation
    in mouse mitochondria.
  findings: []
- id: PMID:18614015
  title: A mitochondrial protein compendium elucidates complex I disease biology.
  findings: []
- id: PMID:9694873
  title: Evidence for the existence of distinct mammalian cytosolic, microsomal, and
    two mitochondrial GrpE-like proteins, the Co-chaperones of specific Hsp70 members.
  findings: []