id: Q96L12 gene_symbol: CALR3 product_type: PROTEIN status: COMPLETE taxon: id: NCBITaxon:9606 label: Homo sapiens description: Calreticulin-3 (also known as calreticulin-2, calsperin, or CRT2) is a testis-specific member of the calreticulin/calnexin family of endoplasmic reticulum (ER) lumen chaperones and a paralog of calreticulin-1 (CALR). It has the canonical calreticulin architecture - an N-terminal globular lectin domain, a proline-rich P-domain, and a C-terminal acidic domain - together with an N-terminal signal peptide and a C-terminal ER-retention motif. During spermatogenesis it acts as a molecular chaperone that assists the folding and maturation of specific client proteins such as ADAM3 and is required for normal sperm fertility. Unlike calreticulin-1, which is a major ER calcium-buffering protein, calreticulin-3 does not bind calcium (or binds it with much lower capacity), indicating that its chaperone role rather than calcium handling is its principal function. It is localized to the lumen of the endoplasmic reticulum. existing_annotations: - term: id: GO:0006457 label: protein folding evidence_type: IBA original_reference_id: GO_REF:0000033 qualifier: involved_in review: summary: Protein folding is a core process for calreticulin-family chaperones, and CALR3 acts as a chaperone for client proteins such as ADAM3 during spermatogenesis. Deep research confirms CALR3 selectively assists folding/quality control of the sperm fertilization protein ADAM3 rather than acting as a broad nascent-glycoprotein chaperone. action: ACCEPT reason: The phylogenetic transfer is consistent with the documented chaperone function of CALR3 and with its membership in the calreticulin family; assisting protein folding is a core function of this protein. supported_by: - reference_id: file:human/CALR3/CALR3-uniprot.txt supporting_text: During spermatogenesis, may act as a lectin-independent chaperone for specific client proteins such as ADAM3. - reference_id: file:human/CALR3/CALR3-deep-research-falcon.md supporting_text: CALR3 functions as a testis-specific molecular chaperone localized to the endoplasmic reticulum lumen during spermatogenesis - term: id: GO:0036503 label: ERAD pathway evidence_type: IBA original_reference_id: GO_REF:0000033 qualifier: involved_in review: summary: ERAD-pathway involvement is a generic calreticulin-family phylogenetic transfer; CALR3's specifically documented role is client chaperoning in spermatogenesis rather than ER-associated degradation per se. action: KEEP_AS_NON_CORE reason: ERAD participation is plausible by family membership but has not been specifically demonstrated for CALR3, whose characterized function is folding/maturation of sperm client proteins; retain as a non-core, lower-confidence annotation. supported_by: - reference_id: file:human/CALR3/CALR3-uniprot.txt supporting_text: Belongs to the calreticulin family. - term: id: GO:0005783 label: endoplasmic reticulum evidence_type: IEA original_reference_id: GO_REF:0000002 qualifier: located_in review: summary: CALR3 is an ER-resident protein, so localization to the endoplasmic reticulum is correct, but this is a less specific parent of the experimentally supported ER lumen location. action: KEEP_AS_NON_CORE reason: The annotation is accurate but generic; the more precise ER lumen term (GO:0005788) better captures the localization, so this broad term is retained as non-core. supported_by: - reference_id: file:human/CALR3/CALR3-uniprot.txt supporting_text: SUBCELLULAR LOCATION; Endoplasmic reticulum lumen - term: id: GO:0005788 label: endoplasmic reticulum lumen evidence_type: IEA original_reference_id: GO_REF:0000044 qualifier: located_in review: summary: ER lumen is the experimentally confirmed, core localization of CALR3, consistent with its C-terminal ER-retention motif and lumenal chaperone role. action: ACCEPT reason: Immunoelectron microscopy demonstrated CALR3 in the ER lumen, colocalizing with calnexin and PDI; this matches the IEA subcellular-location mapping. supported_by: - reference_id: PMID:21590275 supporting_text: Immunoelectron microscopy confirmed that HA-CRT-2 was localized in the lumen of the endoplasmic reticulum. - term: id: GO:0006457 label: protein folding evidence_type: IEA original_reference_id: GO_REF:0000002 qualifier: involved_in review: summary: Protein folding is a core function of CALR3 as a calreticulin-family chaperone; this InterPro-based transfer agrees with the IBA annotation of the same term. action: ACCEPT reason: The InterPro2GO transfer is consistent with the documented chaperone role of CALR3 in folding sperm client proteins such as ADAM3. supported_by: - reference_id: file:human/CALR3/CALR3-uniprot.txt supporting_text: During spermatogenesis, may act as a lectin-independent chaperone for specific client proteins such as ADAM3. - term: id: GO:0044183 label: protein folding chaperone evidence_type: IEA original_reference_id: GO_REF:0000107 qualifier: enables review: summary: Molecular chaperone (protein folding chaperone) activity is the core molecular function of CALR3, transferred from its experimentally characterized mouse ortholog (calsperin). The mouse ortholog is a lectin-deficient, client-selective chaperone whose best-characterized client is ADAM3, contrasting with the broad lectin-chaperone activity of ubiquitous CALR/CANX. action: ACCEPT reason: CALR3 functions as a chaperone for client proteins such as ADAM3 during spermatogenesis; the ortholog-based transfer captures the core molecular function. supported_by: - reference_id: file:human/CALR3/CALR3-uniprot.txt supporting_text: During spermatogenesis, may act as a lectin-independent chaperone for specific client proteins such as ADAM3. Required for sperm fertility. - reference_id: file:human/CALR3/CALR3-deep-research-falcon.md supporting_text: Unlike the ubiquitous calreticulin (CALR) and calnexin (CANX), which serve as broad-spectrum lectin chaperones for nascent glycoproteins, CALR3 exhibits a remarkably narrow substrate specificity - term: id: GO:0005509 label: calcium ion binding evidence_type: IDA original_reference_id: PMID:21590275 qualifier: enables negated: true review: summary: Unlike calreticulin-1, CALR3 does not bind calcium; the experimental data support this negated annotation. action: ACCEPT reason: Stains-all staining (which detects Ca2+-binding proteins) failed to stain CALR3 (CRT-2) while it stained calreticulin-1 (CRT-1), demonstrating that CALR3 lacks (or has much lower) calcium-binding capacity; the NOT qualifier is correct. supported_by: - reference_id: PMID:21590275 supporting_text: Stains-all staining, a method to detect Ca(2+)-binding proteins, could not stain the immunoprecipitate of HA-CRT-2, although HA-CRT-1 immunoprecipitate was stained blue. - term: id: GO:0005635 label: nuclear envelope evidence_type: IDA original_reference_id: PMID:21590275 qualifier: located_in review: summary: The nuclear-envelope signal reflects the nuclear-envelope component of the continuous ER network (the outer nuclear membrane is contiguous with the ER) seen on immunofluorescence, rather than a distinct nuclear-envelope function. action: KEEP_AS_NON_CORE reason: The reticular/nuclear-envelope staining pattern colocalizes with the ER markers calnexin and PDI, and immunoEM places CALR3 in the ER lumen; the ER lumen is the core location, so the nuclear-envelope term is retained as non-core. supported_by: - reference_id: PMID:21590275 supporting_text: labeling for HA-CRT-2 was seen as a reticular network with a nuclear envelope pattern that colocalized with calnexin and protein disulfide isomerase. - term: id: GO:0005788 label: endoplasmic reticulum lumen evidence_type: IDA original_reference_id: PMID:21590275 qualifier: located_in review: summary: ER lumen is the experimentally confirmed, core localization of CALR3, demonstrated directly by immunoelectron microscopy. action: ACCEPT reason: Immunoelectron microscopy directly localized CALR3 to the ER lumen, and it colocalized with the ER-lumen/membrane markers calnexin and PDI; this is the core subcellular location. supported_by: - reference_id: PMID:21590275 supporting_text: Immunoelectron microscopy confirmed that HA-CRT-2 was localized in the lumen of the endoplasmic reticulum. core_functions: - description: Endoplasmic reticulum lumen molecular chaperone of the calreticulin family that assists the folding and maturation of specific client glycoproteins (such as ADAM3) during spermatogenesis, contributing to sperm protein maturation and fertility. molecular_function: id: GO:0044183 label: protein folding chaperone directly_involved_in: - id: GO:0006457 label: protein folding - id: GO:0007283 label: spermatogenesis locations: - id: GO:0005788 label: endoplasmic reticulum lumen supported_by: - reference_id: file:human/CALR3/CALR3-uniprot.txt supporting_text: During spermatogenesis, may act as a lectin-independent chaperone for specific client proteins such as ADAM3. Required for sperm fertility. - reference_id: PMID:21590275 supporting_text: Immunoelectron microscopy confirmed that HA-CRT-2 was localized in the lumen of the endoplasmic reticulum. - reference_id: file:human/CALR3/CALR3-deep-research-falcon.md supporting_text: CALR3 functions as a testis-specific molecular chaperone localized to the endoplasmic reticulum lumen during spermatogenesis proposed_new_terms: [] suggested_questions: - question: Which sperm client proteins besides ADAM3 depend on CALR3 for folding/maturation, and does CALR3 act through lectin-dependent or lectin-independent recognition in humans? - question: Given that CALR3 does not bind calcium, how is its chaperone cycle regulated in the absence of the calcium-dependent conformational switching used by calreticulin-1? - question: Does CALR3 cooperate with calnexin, ERp57/PDI, or other ER quality-control machinery during spermatogenesis, and does it participate in ERAD? suggested_experiments: - description: Affinity purification / proximity labeling of CALR3 in human or mouse testis to define its physiological client and interaction network beyond ADAM3. - description: Quantitative calcium-binding assays (e.g., 45Ca overlay, isothermal titration calorimetry) on purified recombinant CALR3 versus CALR to rigorously confirm and quantify the reduced/absent calcium-binding capacity. - description: In vitro chaperone (aggregation-suppression / refolding) assays with CALR3 and candidate clients to directly measure its folding-assistance activity and lectin dependence. 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: PMID:21590275 title: Calreticulin-2 is localized in the lumen of the endoplasmic reticulum but is not a Ca2+ -binding protein. findings: - statement: CALR3 (CRT-2) is localized in the lumen of the endoplasmic reticulum, colocalizing with calnexin and protein disulfide isomerase, confirmed by immunoelectron microscopy. reference_section_type: ABSTRACT supporting_text: Immunoelectron microscopy confirmed that HA-CRT-2 was localized in the lumen of the endoplasmic reticulum. - statement: Unlike calreticulin-1, CALR3 does not bind calcium (or binds it with much lower capacity), as shown by Stains-all staining. reference_section_type: ABSTRACT supporting_text: CRT-2 capacity for Ca(2+)-binding may be absent or much lower than that of CRT-1. - id: file:human/CALR3/CALR3-deep-research-falcon.md title: Falcon deep research report for CALR3 reference_review: relevance: HIGH correctness: UNVERIFIED review_notes: 'LLM-synthesized deep research report focused on the correct gene (human CALR3 / calsperin, Q96L12); it correctly distinguishes CALR3 from the ubiquitous paralog CALR and centers its functional account on the Ikawa et al. 2011 J Biol Chem "Calsperin" study (testis-specific ER-lumen chaperone, lectin-deficient client-selective folding of ADAM3, CALR3/PDILT complex, male-fertility role) and Nomura et al. 2011 (no calcium binding). Marked UNVERIFIED because the primary calsperin paper (Ikawa 2011, PMID:21343304 / doi:10.1074/jbc.M110.140152) is not in the local publications cache and its full text could not be independently checked here; the falcon synthesis is internally consistent with the cached UniProt record and PMID:21590275, and is used only as a verbatim supporting source for claims already corroborated by those records.' findings: - statement: CALR3 is a testis-specific ER-lumen molecular chaperone that, unlike the broad-spectrum lectin chaperones CALR/CANX, has narrow substrate specificity and selectively supports folding/maturation of the sperm protein ADAM3. supporting_text: Unlike the ubiquitous calreticulin (CALR) and calnexin (CANX), which serve as broad-spectrum lectin chaperones for nascent glycoproteins, CALR3 exhibits a remarkably narrow substrate specificity - id: file:human/CALR3/CALR3-uniprot.txt title: UniProt entry Q96L12 (CALR3_HUMAN) findings: - statement: CALR3 acts during spermatogenesis as a chaperone for client proteins such as ADAM3 and is required for sperm fertility; calcium-binding capacity may be absent or much lower than that of CALR. supporting_text: During spermatogenesis, may act as a lectin-independent chaperone for specific client proteins such as ADAM3. Required for sperm fertility (By similarity). CALR3 capacity for calcium-binding may be absent or much lower than that of CALR.