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.
|
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.
The research report should be a detailed narrative explaining the function, biological processes, and localization of the gene product. Citations should be given for all claims.
You should prioritize authoritative reviews and primary scientific literature when conducting research. You can supplement
this with annotations you find in gene/protein databases, but these can be outdated or inaccurate.
We are specifically interested in the primary function of the gene - for enzymes, what reaction is catalyzed, and what is the substrate specificity? For transporters, what is the substrate? For structural proteins or adapters, what is the broader structural role? For signaling molecules, what is the role in the pathway.
We are interested in where in or outside the cell the gene product carries out its function.
We are also interested in the signaling or biochemical pathways in which the gene functions. We are less interested in broad pleiotropic effects, except where these elucidate the precise role.
Include evidence where possible. We are interested in both experimental evidence as well as inference from structure, evolution, or bioinformatic analysis. Precise studies should be prioritized over high-throughput, where available.
CALR3 (UniProt: Q96L12) encodes calreticulin-3, also known as calsperin or CRT2, a member of the calreticulin protein family in humans (ikawa2011calsperinisa pages 1-2). This gene belongs to the calreticulin family and contains characteristic calreticulin/calnexin domains including the Calret/calnex domain (IPR001580), Calret/calnex_CS (IPR018124), and the ConA-like_dom_sf (IPR013320) (michalak2024calreticulinendoplasmicreticulum pages 1-2, varricchio2017calreticulinchallengesposed pages 1-2). CALR3 represents a testis-specific counterpart of the ubiquitously expressed endoplasmic reticulum (ER) chaperone calreticulin (CALR), distinguishing it functionally and spatially from its somatic homolog (ikawa2011calsperinisa pages 1-2, ikawa2011calsperinisa pages 2-4).
CALR3 functions as a testis-specific molecular chaperone localized to the endoplasmic reticulum lumen during spermatogenesis (ikawa2011calsperinisa pages 1-2, ikawa2011calsperinisa pages 2-4). 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 (ikawa2011calsperinisa pages 1-2, ikawa2011calsperinisa pages 6-7). The primary and best-characterized substrate of CALR3 is ADAM3 (A Disintegrin and Metalloproteinase Domain 3), a sperm membrane protein essential for male fertility (ikawa2011calsperinisa pages 1-2, ikawa2011calsperinisa pages 6-7, ikawa2011calsperinisa pages 5-6).
Despite possessing amino acid sequences homologous to the carbohydrate-binding domains found in canonical calreticulin, CALR3 does not function as a general lectin chaperone for nascent N-glycoproteins (ikawa2011calsperinisa pages 6-7, ikawa2011calsperinisa pages 5-6). Biochemical studies using the glucosidase inhibitor castanospermine demonstrated that while CALR clearly interacted with putative client proteins in a lectin-dependent manner, CALR3 did not exhibit such broad lectin chaperone activity (ikawa2011calsperinisa pages 6-7). Furthermore, endoglycosidase H treatment of ADAM3 did not interfere with the CALR3/ADAM3 interaction, confirming that ADAM3 is a lectin-independent client of CALR3 (ikawa2011calsperinisa pages 6-7). This lectin deficiency likely results from differences in the divergent P-domain despite conservation of essential amino acids required for oligosaccharide binding (ikawa2011calsperinisa pages 6-7, michalak2024calreticulinendoplasmicreticulum pages 1-2).
CALR3 cooperates with PDILT (Protein Disulfide Isomerase-Like protein of the Testis) to form a specialized CALR3/PDILT quality control complex in the ER of testicular germ cells (ikawa2011calsperinisa pages 6-7, xiong2019anupdateof pages 7-8). This complex is crucial for proper disulfide bond formation and ADAM3 maturation (xiong2019anupdateof pages 7-8). The CALR3/PDILT partnership represents a testis-specific quality control hub analogous to the CANX/PDIA3 and HSPA5/PDI partnerships found in somatic cells (xiong2019anupdateof pages 7-8). PDILT cooperates with CALR3 but not with CLGN (calmegin, the testis-specific calnexin homolog) in this quality control process (xiong2019anupdateof pages 7-8).
As a member of the calreticulin family, CALR3 retains calcium-binding capacity, though recent studies suggest that the calcium-binding capacity of calreticulin-3 may be absent or very low compared to canonical calreticulin (verhagen2018lackofevidence pages 1-2, verhagen2018lackofevidence pages 2-3). The protein contains the characteristic P-domain with calcium-binding motifs, though its specialized function appears to prioritize client-specific protein folding over general calcium buffering (michalak2024calreticulinendoplasmicreticulum pages 1-2).
CALR3 is localized exclusively to the endoplasmic reticulum lumen in testicular germ cells during spermatogenesis (ikawa2011calsperinisa pages 1-2, ikawa2011calsperinisa pages 2-4, ikawa2011calsperinisa pages 6-7). The protein is soluble within the ER lumen, contrasting with the membrane-tethered topology of CLGN (calmegin) (ikawa2011calsperinisa pages 6-7, ikawa2011calsperinisa pages 7-8). Immunofluorescence studies on testicular sections demonstrate that CALR3 is detected specifically in elongating spermatids, the post-meiotic haploid germ cells undergoing spermiogenesis (ikawa2011calsperinisa pages 2-4). Importantly, CALR3 is absent from mature spermatozoa, indicating that its function is restricted to the biosynthetic phase during spermatid development rather than in the final gamete (ikawa2011calsperinisa pages 2-4, ikawa2011calsperinisa pages 7-8).
CALR3 expression is strictly restricted to the testis with no detectable expression in other tissues by Western blot analysis (ikawa2011calsperinisa pages 1-2, ikawa2011calsperinisa pages 2-4). This testis-specificity has been comprehensively validated, and importantly, immunohistochemical studies have demonstrated no calreticulin-3 protein expression in myocardial tissues at various developmental ages (verhagen2018lackofevidence pages 1-2, verhagen2018lackofevidence pages 2-3). This finding refutes earlier suggestions that CALR3 might be involved in cardiomyopathy, with a 2018 comprehensive study concluding it is highly questionable that variants in CALR3 are a monogenic cause of cardiomyopathy (verhagen2018lackofevidence pages 1-2, verhagen2018lackofevidence pages 2-3).
Within the testis, CALR3 expression exhibits precise temporal regulation. In mouse postnatal development, CALR3 appears at approximately 3 weeks of age, corresponding to the post-meiotic stage when elongating spermatids are present (ikawa2011calsperinisa pages 2-4). This timing is notably later than the appearance of CLGN, which is detected at approximately 2 weeks of age during the meiotic pachytene spermatocyte stage (ikawa2011calsperinisa pages 2-4). The offset expression of CLGN preceding CALR3 mirrors the sequential expression of their respective major substrates: CLGN's clients ADAM1B/ADAM2 appear before ADAM3 (ikawa2011calsperinisa pages 2-4, ikawa2011calsperinisa pages 6-7). This temporal coordination suggests a hierarchical quality control system during male germ cell differentiation.
CALR3 participates in a specialized endoplasmic reticulum protein quality control pathway that is essential for the maturation of proteins required for sperm fertilizing ability (ikawa2011calsperinisa pages 1-2, ikawa2011calsperinisa pages 6-7, xiong2019anupdateof pages 7-8). This pathway differs from the canonical CANX/CALR quality control system found in somatic cells and represents an evolutionarily conserved mechanism specific to mammalian spermatogenesis (xiong2019anupdateof pages 7-8).
The pathway involves a coordinated sequence of molecular events:
CLGN-mediated ADAM1/ADAM2 heterodimerization: Calmegin (CLGN), the membrane-tethered testis-specific calnexin homolog, is required for the formation of ADAM1A/ADAM2 and ADAM1B/ADAM2 heterodimers (ikawa2011calsperinisa pages 6-7, xiong2019anupdateof pages 7-8). The ADAM1A/ADAM2 complex is essential for subsequent ADAM3 maturation (ikawa2011calsperinisa pages 6-7).
CALR3/PDILT-mediated ADAM3 maturation: CALR3, in complex with PDILT, directly associates with ADAM3 and regulates its proper folding and disulfide bond formation (ikawa2011calsperinisa pages 6-7, xiong2019anupdateof pages 7-8). This step is independent of CLGN but requires the prior CLGN-dependent formation of ADAM1A/ADAM2 complexes (ikawa2011calsperinisa pages 6-7).
ADAM3 transport and surface expression: Properly matured ADAM3 is transported from the ER to the sperm surface, where it becomes essential for fertilization (ikawa2011calsperinisa pages 6-7, ikawa2011calsperinisa pages 5-6).
Trypsin sensitivity assays demonstrate that in the absence of CALR3, ADAM3 remains in the secretory pathway and is not properly exported to the sperm surface, whereas in the absence of CLGN, ADAM3 is transported to the surface but in an altered conformation (ikawa2011calsperinisa pages 6-7, ikawa2011calsperinisa pages 5-6).
The CALR3-dependent maturation of ADAM3 is critical for two key fertilization events (ikawa2011calsperinisa pages 1-2, ikawa2011calsperinisa pages 5-6, ikawa2011calsperinisa pages 4-5):
Sperm migration through the uterotubal junction: ADAM3-deficient sperm, including those from Calr3-knockout mice, fail to migrate from the uterus into the oviduct (ikawa2011calsperinisa pages 4-5, xiong2019anupdateof pages 7-8, fujihara2019identificationofmultiple pages 1-2). This defect appears to be the primary cause of infertility in Calr3-null males.
Zona pellucida binding: ADAM3 is required for sperm binding to the zona pellucida, the glycoprotein matrix surrounding the oocyte (ikawa2011calsperinisa pages 5-6, ikawa2011calsperinisa pages 4-5). ADAM3 has been shown to directly bind ZP3, a major zona pellucida glycoprotein (ikawa2011calsperinisa pages 7-8).
Recent proteomic and genetic studies from 2023-2025 have expanded our understanding of the CALR3 interaction network. CALR3 has been found in complexes with additional proteins including PRSS37, TEX38, ARRDC5, CLGN, and PDILT, all of which influence ADAM3 maturation and sperm function (yuan2025disruptionoftex38 pages 1-2, xiong2019anupdateof pages 7-8). These studies suggest that CALR3 functions within broader multi-protein complexes that regulate not only ADAM3 maturation but also sperm morphogenesis, energy metabolism, and fertilization competence (yuan2025disruptionoftex38 pages 1-2).
Genetic ablation of Calr3 in mice results in male sterility despite normal spermatogenesis, sperm morphology, motility, and mating behavior (ikawa2011calsperinisa pages 1-2, ikawa2011calsperinisa pages 2-4, ikawa2011calsperinisa pages 5-6, ikawa2011calsperinisa pages 4-5). Calr3-knockout males copulate normally and produce vaginal plugs, but pregnancy rates are dramatically reduced (ikawa2011calsperinisa pages 4-5). The specific defects observed include:
Impaired sperm migration: Calr3-null sperm fail to migrate from the uterus into the oviduct through the uterotubal junction (ikawa2011calsperinisa pages 4-5).
Defective zona pellucida binding: In vitro fertilization experiments demonstrate that Calr3-deficient sperm cannot bind to the zona pellucida despite frequent collisions with eggs (ikawa2011calsperinisa pages 5-6).
Loss of ADAM3 from mature sperm: Western blot and immunofluorescence analyses show that ADAM3 is absent from Calr3-null mature spermatozoa, while other sperm proteins remain unaffected (ikawa2011calsperinisa pages 6-7, ikawa2011calsperinisa pages 5-6).
Retained fusion competence: When the zona pellucida is removed or bypassed through partial zona dissection, Calr3-deficient sperm can successfully fuse with eggs and produce viable, fertile offspring (ikawa2011calsperinisa pages 5-6, ikawa2011calsperinisa pages 4-5). This demonstrates the genomic integrity of Calr3-null sperm and confirms that the fertility defect is specifically related to migration and zona binding rather than a general sperm dysfunction.
Transgenic expression of Calr3 on a Calr3-knockout background successfully rescues male fertility, with restored litter sizes comparable to wild-type mice (ikawa2011calsperinisa pages 4-5). This genetic rescue confirms that the infertility phenotype is directly caused by CALR3 deficiency rather than secondary effects or off-target mutations.
The Calr3-knockout phenotype is remarkably similar to that observed in mice lacking CLGN, ADAM1A, ADAM2, ADAM3, or other components of the ADAM maturation pathway (ikawa2011calsperinisa pages 1-2, xiong2019anupdateof pages 7-8, ikawa2011calsperinisa pages 7-8). All these models exhibit defective sperm migration and zona binding, reinforcing the concept that ADAM3 plays a central role in these processes and that both CLGN and CALR3, though functioning through distinct mechanisms, are essential components of the ADAM3 maturation pathway.
Recent knockout studies of interacting proteins such as PDILT, TEX38, and ARRDC5 have revealed similar male infertility phenotypes with defective ADAM3 maturation and sperm migration, further validating the importance of this quality control network (yuan2025disruptionoftex38 pages 1-2, xiong2019anupdateof pages 7-8, wang2020lypd4mousehomolog pages 1-2).
CALR3 is conserved in mammals including humans, and the findings from mouse models have direct relevance to human male infertility (ikawa2011calsperinisa pages 1-2, dun2012theroleof pages 1-2, xiong2019anupdateof pages 7-8). Recent studies from 2024 have confirmed that CALR3 defects can disrupt sperm-zona pellucida binding in humans, providing new insights into male factor fertilization failure and relevant clinical therapeutic approaches (yuan2025disruptionoftex38 pages 1-2). The identification of CALR3 as essential for sperm function suggests it could serve as:
A diagnostic biomarker for cases of unexplained male infertility, particularly those involving defective sperm migration or zona binding.
A target for male contraceptive development, as specific inhibition of the CALR3-PDILT-ADAM3 pathway could reversibly prevent fertilization without affecting spermatogenesis or other physiological processes (ikawa2011calsperinisa pages 1-2, dun2012theroleof pages 1-2, ikawa2011calsperinisa pages 7-8).
A therapeutic target for improving outcomes in assisted reproductive technologies by ensuring proper ADAM3 maturation (wang2020lypd4mousehomolog pages 1-2, gahlay2020theenigmaticsperm pages 22-24).
Early reports suggested CALR3 might be associated with cardiomyopathy based on rare variants identified in patient cohorts. However, a comprehensive 2018 study examining 6,154 cardiomyopathy patients found no convincing evidence that CALR3 is a monogenic cause of cardiomyopathy (verhagen2018lackofevidence pages 1-2, verhagen2018lackofevidence pages 2-3). Critically, immunohistochemical analysis demonstrated no detectable calreticulin-3 protein expression in myocardial tissues from patients with cardiomyopathy, non-cardiac death controls, or samples from various developmental ages including fetuses, neonates, children, and adults (verhagen2018lackofevidence pages 1-2, verhagen2018lackofevidence pages 2-3). Furthermore, CALR3 variants identified in cardiomyopathy patients often did not segregate with disease and were frequently accompanied by pathogenic variants in established cardiomyopathy genes (verhagen2018lackofevidence pages 1-2, verhagen2018lackofevidence pages 2-3). These findings strongly argue against a primary cardiac role for CALR3.
The existence of testis-specific calreticulin and calnexin homologs (CALR3 and CLGN) represents a remarkable evolutionary adaptation in mammalian reproduction (ikawa2011calsperinisa pages 1-2, ikawa2011calsperinisa pages 6-7, ikawa2011calsperinisa pages 7-8). This specialization suggests that during spermatogenesis, the ER requires quality control mechanisms distinct from those in somatic cells, possibly due to:
Specialized client proteins: Sperm-specific proteins like ADAM3 may require chaperones with unique substrate specificities.
Temporal coordination: The sequential expression of CLGN and CALR3 allows for stage-specific quality control during the complex morphological transformations of spermiogenesis.
Functional redundancy avoidance: The narrow substrate specificity of CALR3 for ADAM3 contrasts with the broader specificity of CLGN for multiple ADAM proteins, suggesting a division of labor that may prevent inappropriate interactions and ensure precise control.
Similar evolutionary convergence has been observed in plants, where a plant-specific calreticulin homolog (CRT3) is expressed mainly in leaves and functions in specialized quality control of the leucine-rich repeat receptor protein EFR, involved in innate immunity (ikawa2011calsperinisa pages 7-8). Both plant leaves and mammalian testis have evolved tissue-specific calreticulin homologs released from essential calcium homeostasis roles to mediate distinct quality control processes in the ER.
| Property | Summary | Key evidence/citation |
|---|---|---|
| Protein names | CALR3 encodes calreticulin-3, also called calsperin, and has been described as a testis-specific homolog/counterpart of canonical calreticulin (CALR). | (ikawa2011calsperinisa pages 1-2, verhagen2018lackofevidence pages 1-2) |
| Primary molecular function and mechanism | CALR3 is a testis-specific ER luminal molecular chaperone. Unlike ubiquitous CALR/CANX, it behaves as a lectin-deficient, client-selective chaperone that directly supports maturation of sperm fertilization proteins, especially ADAM3. Its mechanism is not broad nascent glycoprotein lectin chaperoning, but selective assistance in folding/quality control of specific clients during spermatogenesis. | (ikawa2011calsperinisa pages 1-2, ikawa2011calsperinisa pages 6-7, ikawa2011calsperinisa pages 7-8) |
| Substrate specificity | The strongest experimentally supported client is ADAM3; CALR3 co-immunoprecipitates with ADAM3 but not broadly with other nascent glycoproteins. In contrast, CLGN associates with ADAM1B, ADAM2, and ADAM3, whereas CALR3 shows much narrower specificity centered on ADAM3. | (ikawa2011calsperinisa pages 6-7, ikawa2011calsperinisa pages 5-6) |
| Subcellular localization | CALR3 functions in the endoplasmic reticulum lumen of testicular germ cells and is soluble rather than membrane-anchored, contrasting with the membrane-tethered chaperone CLGN. It is not retained in mature sperm, indicating its action is during biosynthesis/maturation rather than in the terminal gamete. | (ikawa2011calsperinisa pages 1-2, ikawa2011calsperinisa pages 7-8, xiong2019anupdateof pages 7-8) |
| Tissue/cell type expression | Expression is testis-specific by Western blot and restricted to germ cells. Within the seminiferous epithelium, CALR3 is detected in post-meiotic elongating spermatids; studies evaluating myocardium found no detectable cardiac CALR3 protein expression, arguing against a primary cardiac role. | (ikawa2011calsperinisa pages 2-4, verhagen2018lackofevidence pages 1-2, verhagen2018lackofevidence pages 2-3) |
| Temporal expression pattern during development | In mouse postnatal testis, CALR3 appears at about 3 weeks of age, later than CLGN (~2 weeks), consistent with post-meiotic/haploid expression during spermiogenesis. This timing parallels its specialized role after meiosis in elongating spermatids. | (ikawa2011calsperinisa pages 2-4) |
| Key interacting partners in the pathway | Core partners include ADAM3 (direct client), PDILT (forms a CALR3/PDILT quality-control module for ADAM3 folding/disulfide maturation), and functionally linked factors such as CLGN and ADAM1A/ADAM2 upstream in the broader ADAM3 maturation pathway. Recent work also places CALR3 in reproductive protein networks with PRSS37, and proteomic/genetic studies implicate linkage with TEX38/ARRDC5/PDILT/CLGN modules affecting ADAM3 maturation. | (xiong2019anupdateof pages 7-8, ikawa2011calsperinisa pages 7-8, yuan2025disruptionoftex38 pages 1-2) |
| Biological processes/pathways involved | CALR3 acts in a specialized ER protein quality-control pathway during spermatogenesis that enables proper ADAM3 maturation, sperm surface competence, migration through the uterotubal junction/oviduct, and binding to the zona pellucida. This pathway is part of the molecular program that generates sperm fertilizing ability. | (ikawa2011calsperinisa pages 4-5, xiong2019anupdateof pages 7-8, gahlay2020theenigmaticsperm pages 22-24) |
| Phenotype when disrupted (knockout studies) | Calr3-null males are infertile or nearly sterile despite apparently normal spermatogenesis, normal sperm morphology/motility, and normal mating behavior. The main defects are failure of sperm migration from uterus to oviduct/UTJ, impaired zona pellucida binding, and loss of ADAM3 from mature sperm. Fertility can be rescued transgenically, supporting on-target causality. | (ikawa2011calsperinisa pages 4-5, ikawa2011calsperinisa pages 5-6, ikawa2011calsperinisa pages 6-7) |
| Clinical relevance to human disease | Current evidence supports CALR3 primarily as a male fertility gene/protein, relevant to unexplained fertilization failure and potentially to contraceptive target discovery. By contrast, a large 2018 human genetic/pathology study found no convincing evidence that CALR3 is a monogenic cardiomyopathy gene, noting lack of myocardial protein expression and non-supportive segregation data. | (verhagen2018lackofevidence pages 1-2, verhagen2018lackofevidence pages 2-3, yuan2025disruptionoftex38 pages 1-2) |
Table: This table summarizes the key biochemical, cellular, developmental, and clinical properties of human CALR3/calsperin. It emphasizes the experimentally supported role of CALR3 as a testis-specific ER chaperone in ADAM3 maturation and male fertility, while noting evidence against a primary cardiac role.
CALR3 (calreticulin-3, calsperin) is a highly specialized endoplasmic reticulum chaperone with testis-specific expression and a remarkably narrow substrate specificity for ADAM3, a protein essential for sperm fertilization. Unlike ubiquitous calreticulin, CALR3 functions as a lectin-deficient, client-selective chaperone that cooperates with PDILT to ensure proper ADAM3 maturation during spermatogenesis. The CALR3-PDILT-ADAM3 pathway is critical for sperm migration through the female reproductive tract and zona pellucida binding, making CALR3 essential for male fertility. Genetic evidence from knockout studies in mice and emerging human data confirm that CALR3 deficiency causes male infertility without affecting other physiological processes. The protein is localized to the ER lumen of elongating spermatids during the post-meiotic phase of spermatogenesis and is absent from mature sperm and all non-testicular tissues. Current evidence strongly supports CALR3 as a male fertility gene with potential applications in diagnosing unexplained infertility and developing novel contraceptive approaches, while refuting earlier suggestions of a role in cardiac function.
References
(ikawa2011calsperinisa pages 1-2): Masahito Ikawa, Keizo Tokuhiro, Ryo Yamaguchi, Adam M. Benham, Taku Tamura, Ikuo Wada, Yuhkoh Satouh, Naokazu Inoue, and Masaru Okabe. Calsperin is a testis-specific chaperone required for sperm fertility. Journal of Biological Chemistry, 286:5639-5646, Feb 2011. URL: https://doi.org/10.1074/jbc.m110.140152, doi:10.1074/jbc.m110.140152. This article has 198 citations and is from a domain leading peer-reviewed journal.
(michalak2024calreticulinendoplasmicreticulum pages 1-2): Marek Michalak. Calreticulin: endoplasmic reticulum ca2+ gatekeeper. Journal of Cellular and Molecular Medicine, Jul 2024. URL: https://doi.org/10.1111/jcmm.17839, doi:10.1111/jcmm.17839. This article has 60 citations and is from a peer-reviewed journal.
(varricchio2017calreticulinchallengesposed pages 1-2): Lilian Varricchio, Mario Falchi, Massimiliano Dall'Ora, Caterina De Benedittis, Alessandra Ruggeri, Vladimir N. Uversky, and Anna Rita Migliaccio. Calreticulin: challenges posed by the intrinsically disordered nature of calreticulin to the study of its function. Frontiers in Cell and Developmental Biology, Nov 2017. URL: https://doi.org/10.3389/fcell.2017.00096, doi:10.3389/fcell.2017.00096. This article has 45 citations.
(ikawa2011calsperinisa pages 2-4): Masahito Ikawa, Keizo Tokuhiro, Ryo Yamaguchi, Adam M. Benham, Taku Tamura, Ikuo Wada, Yuhkoh Satouh, Naokazu Inoue, and Masaru Okabe. Calsperin is a testis-specific chaperone required for sperm fertility. Journal of Biological Chemistry, 286:5639-5646, Feb 2011. URL: https://doi.org/10.1074/jbc.m110.140152, doi:10.1074/jbc.m110.140152. This article has 198 citations and is from a domain leading peer-reviewed journal.
(ikawa2011calsperinisa pages 6-7): Masahito Ikawa, Keizo Tokuhiro, Ryo Yamaguchi, Adam M. Benham, Taku Tamura, Ikuo Wada, Yuhkoh Satouh, Naokazu Inoue, and Masaru Okabe. Calsperin is a testis-specific chaperone required for sperm fertility. Journal of Biological Chemistry, 286:5639-5646, Feb 2011. URL: https://doi.org/10.1074/jbc.m110.140152, doi:10.1074/jbc.m110.140152. This article has 198 citations and is from a domain leading peer-reviewed journal.
(ikawa2011calsperinisa pages 5-6): Masahito Ikawa, Keizo Tokuhiro, Ryo Yamaguchi, Adam M. Benham, Taku Tamura, Ikuo Wada, Yuhkoh Satouh, Naokazu Inoue, and Masaru Okabe. Calsperin is a testis-specific chaperone required for sperm fertility. Journal of Biological Chemistry, 286:5639-5646, Feb 2011. URL: https://doi.org/10.1074/jbc.m110.140152, doi:10.1074/jbc.m110.140152. This article has 198 citations and is from a domain leading peer-reviewed journal.
(xiong2019anupdateof pages 7-8): Wenfeng Xiong, Zhugang Wang, and Chunling Shen. An update of the regulatory factors of sperm migration from the uterus into the oviduct by genetically manipulated mice. Molecular Reproduction and Development, 86:935-955, Aug 2019. URL: https://doi.org/10.1002/mrd.23180, doi:10.1002/mrd.23180. This article has 35 citations and is from a peer-reviewed journal.
(verhagen2018lackofevidence pages 1-2): Judith M. A. Verhagen, Job H. Veldman, Paul A. van der Zwaag, Jan H. von der Thüsen, Erwin Brosens, Imke Christiaans, Dennis Dooijes, Apollonia T. J. M. Helderman-van den Enden, Ronald H. Lekanne Deprez, Michelle Michels, Anneke M. van Mil, Rogier A. Oldenburg, Jasper J. van der Smagt, Arthur van den Wijngaard, Marja W. Wessels, Robert M. W. Hofstra, Marjon A. van Slegtenhorst, Jan D. H. Jongbloed, and Ingrid M. B. H. van de Laar. Lack of evidence for a causal role of calr3 in monogenic cardiomyopathy. European Journal of Human Genetics, 26:1603-1610, Jul 2018. URL: https://doi.org/10.1038/s41431-018-0208-1, doi:10.1038/s41431-018-0208-1. This article has 10 citations and is from a domain leading peer-reviewed journal.
(verhagen2018lackofevidence pages 2-3): Judith M. A. Verhagen, Job H. Veldman, Paul A. van der Zwaag, Jan H. von der Thüsen, Erwin Brosens, Imke Christiaans, Dennis Dooijes, Apollonia T. J. M. Helderman-van den Enden, Ronald H. Lekanne Deprez, Michelle Michels, Anneke M. van Mil, Rogier A. Oldenburg, Jasper J. van der Smagt, Arthur van den Wijngaard, Marja W. Wessels, Robert M. W. Hofstra, Marjon A. van Slegtenhorst, Jan D. H. Jongbloed, and Ingrid M. B. H. van de Laar. Lack of evidence for a causal role of calr3 in monogenic cardiomyopathy. European Journal of Human Genetics, 26:1603-1610, Jul 2018. URL: https://doi.org/10.1038/s41431-018-0208-1, doi:10.1038/s41431-018-0208-1. This article has 10 citations and is from a domain leading peer-reviewed journal.
(ikawa2011calsperinisa pages 7-8): Masahito Ikawa, Keizo Tokuhiro, Ryo Yamaguchi, Adam M. Benham, Taku Tamura, Ikuo Wada, Yuhkoh Satouh, Naokazu Inoue, and Masaru Okabe. Calsperin is a testis-specific chaperone required for sperm fertility. Journal of Biological Chemistry, 286:5639-5646, Feb 2011. URL: https://doi.org/10.1074/jbc.m110.140152, doi:10.1074/jbc.m110.140152. This article has 198 citations and is from a domain leading peer-reviewed journal.
(ikawa2011calsperinisa pages 4-5): Masahito Ikawa, Keizo Tokuhiro, Ryo Yamaguchi, Adam M. Benham, Taku Tamura, Ikuo Wada, Yuhkoh Satouh, Naokazu Inoue, and Masaru Okabe. Calsperin is a testis-specific chaperone required for sperm fertility. Journal of Biological Chemistry, 286:5639-5646, Feb 2011. URL: https://doi.org/10.1074/jbc.m110.140152, doi:10.1074/jbc.m110.140152. This article has 198 citations and is from a domain leading peer-reviewed journal.
(fujihara2019identificationofmultiple pages 1-2): Yoshitaka Fujihara, Taichi Noda, Kiyonori Kobayashi, Asami Oji, Sumire Kobayashi, Takafumi Matsumura, Tamara Larasati, Seiya Oura, Kanako Kojima-Kita, Zhifeng Yu, Martin M. Matzuk, and Masahito Ikawa. Identification of multiple male reproductive tract-specific proteins that regulate sperm migration through the oviduct in mice. Proceedings of the National Academy of Sciences of the United States of America, 116:18498-18506, Aug 2019. URL: https://doi.org/10.1073/pnas.1908736116, doi:10.1073/pnas.1908736116. This article has 87 citations and is from a highest quality peer-reviewed journal.
(yuan2025disruptionoftex38 pages 1-2): Lu Yuan, Tingting Ge, Ling Yang, Wenhua Xu, Guanghua Li, Linwei Xu, Yichun Zhao, Xu Cheng, Wenting Lu, Shiqi Meng, Jieyu Zhao, Fan Yang, Changmin Niu, and Ying Zheng. Disruption of tex38 impairs sperm morphogenesis and the migration of sperm into the oviduct. Communications Biology, Aug 2025. URL: https://doi.org/10.1038/s42003-025-08644-1, doi:10.1038/s42003-025-08644-1. This article has 4 citations and is from a peer-reviewed journal.
(wang2020lypd4mousehomolog pages 1-2): Dan Wang, Liping Cheng, Wenjuan Xia, Xiaofei Liu, Yueshuai Guo, Xiaoyu Yang, Xuejiang Guo, and Eugene Yujun Xu. Lypd4, mouse homolog of a human acrosome protein, is essential for sperm fertilizing ability and male fertility†. Biology of Reproduction, 102:1033-1044, Feb 2020. URL: https://doi.org/10.1093/biolre/ioaa018, doi:10.1093/biolre/ioaa018. This article has 27 citations and is from a peer-reviewed journal.
(dun2012theroleof pages 1-2): Matthew D. Dun, R. John Aitken, and Brett Nixon. The role of molecular chaperones in spermatogenesis and the post-testicular maturation of mammalian spermatozoa. Human reproduction update, 18 4:420-35, Jul 2012. URL: https://doi.org/10.1093/humupd/dms009, doi:10.1093/humupd/dms009. This article has 168 citations and is from a highest quality peer-reviewed journal.
(gahlay2020theenigmaticsperm pages 22-24): Gagandeep Kaur Gahlay and Neha Rajput. The enigmatic sperm proteins in mammalian fertilization: an overview†. Biology of Reproduction, 103(6):1171-1185, Aug 2020. URL: https://doi.org/10.1093/biolre/ioaa140, doi:10.1093/biolre/ioaa140. This article has 22 citations and is from a peer-reviewed journal.
UniProt: Q96L12 (CALR3_HUMAN), 384 aa, gene CALR3 (synonym CRT2), HGNC:20407, chromosome 19.
*-deep-research*.md file found in this gene directory.Lectin chaperone label sits awkwardly with CALR3's lectin-independent, non-Ca2+ profile.ER proteostasis|Glycoproteostasis|N-glycosylation system|Lectin chaperone ; PN-node mapping: identical to CLGN — leaf [type] Lectin chaperone no_mapping; [group] N-glycosylation system → mapped GO:0006487 protein N-linked glycosylation (new_to_goa); class/branch unmapped.N-glycosylation system group projects a biosynthetic term onto a chaperone. Additionally, the Lectin chaperone leaf label conflicts with CALR3's documented lectin-independent mode, so even a lectin-specific mapping would be shaky. Recommend not propagating GO:0006487 to CALR3 and revisiting the leaf classification.Lectin chaperone label sits awkwardly with CALR3's lectin-independent, non-Ca2+ profile.Recommended edits: [MAP] Do not propagate GO:0006487 to CALR3; remap N-glycosylation system group to a folding/glycoprotein-QC term or leave chaperone members unmapped. [MAP] Reconsider the Lectin chaperone leaf for CALR3 (documented lectin-independent). [YAML] No glycosylation annotation for CALR3.
This file is generated from the current PROTEOSTASIS phase-1 dossier and local gene-review artifacts. Edit the source review, PN mapping, or dossier rather than this generated note when correcting the underlying curation.
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.