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.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0006457 protein folding | IBA GO_REF:0000033 | ACCEPT | 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. 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. Supporting Evidence: file:human/CALR3/CALR3-uniprot.txt During spermatogenesis, may act as a lectin-independent chaperone for specific client proteins such as ADAM3. file:human/CALR3/CALR3-deep-research-falcon.md CALR3 functions as a testis-specific molecular chaperone localized to the endoplasmic reticulum lumen during spermatogenesis |
| GO:0036503 ERAD pathway | IBA GO_REF:0000033 | KEEP AS NON CORE | 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. 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. Supporting Evidence: file:human/CALR3/CALR3-uniprot.txt Belongs to the calreticulin family. |
| GO:0005783 endoplasmic reticulum | IEA GO_REF:0000002 | KEEP AS NON CORE | 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. 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. Supporting Evidence: file:human/CALR3/CALR3-uniprot.txt SUBCELLULAR LOCATION; Endoplasmic reticulum lumen |
| GO:0005788 endoplasmic reticulum lumen | IEA GO_REF:0000044 | ACCEPT | Summary: ER lumen is the experimentally confirmed, core localization of CALR3, consistent with its C-terminal ER-retention motif and lumenal chaperone role. Reason: Immunoelectron microscopy demonstrated CALR3 in the ER lumen, colocalizing with calnexin and PDI; this matches the IEA subcellular-location mapping. Supporting Evidence: PMID:21590275 Immunoelectron microscopy confirmed that HA-CRT-2 was localized in the lumen of the endoplasmic reticulum. |
| GO:0006457 protein folding | IEA GO_REF:0000002 | ACCEPT | 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. Reason: The InterPro2GO transfer is consistent with the documented chaperone role of CALR3 in folding sperm client proteins such as ADAM3. Supporting Evidence: file:human/CALR3/CALR3-uniprot.txt During spermatogenesis, may act as a lectin-independent chaperone for specific client proteins such as ADAM3. |
| GO:0044183 protein folding chaperone | IEA GO_REF:0000107 | ACCEPT | 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. Reason: CALR3 functions as a chaperone for client proteins such as ADAM3 during spermatogenesis; the ortholog-based transfer captures the core molecular function. Supporting Evidence: file:human/CALR3/CALR3-uniprot.txt During spermatogenesis, may act as a lectin-independent chaperone for specific client proteins such as ADAM3. Required for sperm fertility. file:human/CALR3/CALR3-deep-research-falcon.md 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 |
| GO:0005509 calcium ion binding | IDA NOT PMID:21590275 Calreticulin-2 is localized in the lumen of the endoplasmic ... | ACCEPT | Summary: Unlike calreticulin-1, CALR3 does not bind calcium; the experimental data support this negated annotation. 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. Supporting Evidence: PMID:21590275 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. |
| GO:0005635 nuclear envelope | IDA PMID:21590275 Calreticulin-2 is localized in the lumen of the endoplasmic ... | KEEP AS NON CORE | 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. 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. Supporting Evidence: PMID:21590275 labeling for HA-CRT-2 was seen as a reticular network with a nuclear envelope pattern that colocalized with calnexin and protein disulfide isomerase. |
| GO:0005788 endoplasmic reticulum lumen | IDA PMID:21590275 Calreticulin-2 is localized in the lumen of the endoplasmic ... | ACCEPT | Summary: ER lumen is the experimentally confirmed, core localization of CALR3, demonstrated directly by immunoelectron microscopy. 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. Supporting Evidence: PMID:21590275 Immunoelectron microscopy confirmed that HA-CRT-2 was localized in the lumen of the endoplasmic reticulum. |
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Download this section (compressed HTML)Q: 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?
Q: 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?
Q: Does CALR3 cooperate with calnexin, ERp57/PDI, or other ER quality-control machinery during spermatogenesis, and does it participate in ERAD?
Experiment: Affinity purification / proximity labeling of CALR3 in human or mouse testis to define its physiological client and interaction network beyond ADAM3.
Experiment: 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.
Experiment: In vitro chaperone (aggregation-suppression / refolding) assays with CALR3 and candidate clients to directly measure its folding-assistance activity and lectin dependence.
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