Loricrin is a glycine-, serine-, and cysteine-rich structural protein produced by terminally differentiating keratinocytes. It accumulates in the cytoplasm before becoming covalently incorporated into the cornified envelope, where epidermal transglutaminases create intra- and intermolecular glutamine-lysine isopeptide cross-links and disulfide bonds provide additional stabilization. Cross-linked loricrin forms a major mechanically resilient part of the human epidermal envelope. Loricrin loss in mice delays barrier formation and weakens isolated envelopes, but other envelope proteins compensate sufficiently to preserve steady-state permeability. Disease-associated C-terminal frameshift proteins instead accumulate in keratinocyte nuclei and disrupt late differentiation; this mutant gain of function is distinct from the normal cornified-envelope role of wild-type loricrin.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0030280 structural constituent of skin epidermis | IBA GO_REF:0000033 | ACCEPT | Summary: Phylogenetic assignment of loricrin's epidermal structural activity. Direct biochemical analysis independently identifies loricrin as the predominant cross-linked protein in the mature human cornified envelope. Reason: This is the most specific available GO molecular-function term for loricrin's core role as a mechanically resilient epidermal envelope constituent. Propagation Review Root cause: NO FAILURE CORE Sources checked: PANTHER:PTN002217128 SUPPORTS TRANSFER The loricrin-family ancestral node supports the conserved epidermal structural role. UniProtKB:P23490 SUPPORTS TRANSFER The target sequence is explicitly included in the PAINT evidence trace, and direct human evidence independently confirms the term. Supporting Evidence: PMID:7543090 most (94% molar mass) of the outer third of CE structure consists of intra- and interchain cross-linked loricrin |
| GO:0001533 cornified envelope | IBA GO_REF:0000033 | ACCEPT | Summary: Phylogenetic assignment placing loricrin activity in the cornified envelope, the mature structure in which cross-linked loricrin is a major protein component. Reason: Cornified-envelope localization is integral to loricrin's core structural function and is independently established in purified human envelopes. Propagation Review Root cause: NO FAILURE CORE Sources checked: PANTHER:PTN002217128 SUPPORTS TRANSFER The family node propagates the conserved mature-envelope localization. UniProtKB:P23490 SUPPORTS TRANSFER Direct biochemical evidence for human loricrin independently confirms the transferred location. Supporting Evidence: PMID:7543090 The data establish for the first time that these several proteins are indeed cross-linked protein components of the CE structure. |
| GO:0005737 cytoplasm | IBA GO_REF:0000033 | KEEP AS NON CORE | Summary: Phylogenetic assignment of a cytoplasmic site for loricrin before its incorporation into the cornified envelope. Reason: Cytoplasmic loricrin is biologically credible as a precursor pool, but this broad staging location is secondary to the mature cornified-envelope localization. Propagation Review Root cause: NO FAILURE NON CORE Sources checked: PANTHER:PTN002217128 SUPPORTS TRANSFER The family node supports a conserved pre-envelope cytoplasmic phase. UniProtKB:P23490 SUPPORTS TRANSFER The reviewed human record also reports cytoplasmic localization. |
| GO:0001533 cornified envelope | IEA GO_REF:0000002 | ACCEPT | Summary: InterPro-to-GO assignment of loricrin to the cornified envelope based on the loricrin family signature. Reason: The family mapping is highly specific and agrees with direct recovery of extensively cross-linked loricrin from purified human cornified envelopes. Propagation Review Root cause: NO FAILURE CORE Sources checked: InterPro:IPR031700 SUPPORTS TRANSFER This loricrin-specific family entry maps appropriately to the defining mature-envelope localization. Supporting Evidence: PMID:7543090 Likewise, the middle third of CE structure consists largely of cross-linked loricrin and SPR proteins |
| GO:0005654 nucleoplasm | IEA GO_REF:0000044 | MARK AS OVER ANNOTATED | Summary: Electronic mapping of an unattributed UniProt nucleoplasm statement that is not supported by direct evidence for wild-type loricrin. Reason: Normal loricrin is established as a cytoplasmic precursor that becomes incorporated into the cornified envelope. The strongest nuclear evidence instead concerns disease-associated frameshift protein with a gained C-terminal nuclear localization signal. Because UniProt does not attribute its nucleoplasm statement, this mapping may conflate mutant localization with wild-type biology and should not be treated as a normal LORICRIN location without direct validation. Propagation Review Root cause: SOURCE BAD Failure modes: SOURCE EVIDENCE WEAK Sources checked: UniProtKB-SubCell:SL-0190 SOURCE BAD The vocabulary mapping is mechanically faithful, but its underlying UniProt location statement is unattributed and lacks direct wild-type support; published nuclear targeting is specific to a disease frameshift protein. Supporting Evidence: PMID:11038186 Transfection experiments indicated that the COOH-terminal domain of the mutant loricrin contains a nuclear localization signal. |
| GO:0005737 cytoplasm | IEA GO_REF:0000044 | KEEP AS NON CORE | Summary: Electronic mapping of the reviewed UniProt cytoplasm statement. Reason: Cytoplasmic localization is credible for loricrin before envelope assembly, but it is a broad precursor-compartment annotation rather than the mature site of its core structural role. Propagation Review Root cause: NO FAILURE NON CORE Sources checked: UniProtKB-SubCell:SL-0086 SUPPORTS TRANSFER The vocabulary term correctly maps the reviewed UniProt cytoplasm statement. |
| GO:0007010 cytoskeleton organization | IEA GO_REF:0000108 | REMOVE | Summary: Logical inference of cytoskeleton organization from the legacy molecular-function annotation structural constituent of cytoskeleton (GO:0005200). Reason: Loricrin is a structural constituent of the cornified envelope, a modified plasma-membrane structure, rather than a constituent that assembles or organizes a cytoskeletal structure. The inference inherits the scope error of GO:0005200. Propagation Review Root cause: TERM SCOPING PROBLEM Failure modes: ROLE CONFLATION Sources checked: GO:0005200 SOURCE BAD The legacy source term conflates the cornified envelope with the cytoskeleton and should instead be GO:0030280. Supporting Evidence: PMID:7543090 most (94% molar mass) of the outer third of CE structure consists of intra- and interchain cross-linked loricrin |
| GO:0030280 structural constituent of skin epidermis | IEA GO_REF:0000002 | ACCEPT | Summary: InterPro-to-GO assignment of loricrin's epidermal structural activity from the loricrin family signature. Reason: The mapping is specific to the loricrin family and matches direct human biochemical evidence for a predominant structural role in the epidermal cornified envelope. Propagation Review Root cause: NO FAILURE CORE Sources checked: InterPro:IPR031700 SUPPORTS TRANSFER The loricrin-specific family signature supports this defining molecular function. Supporting Evidence: PMID:7543090 most (94% molar mass) of the outer third of CE structure consists of intra- and interchain cross-linked loricrin |
| GO:0001533 cornified envelope | TAS Reactome:R-HSA-6814764 | ACCEPT | Summary: Reactome places loricrin in the cornified envelope during plasma-membrane resorption in terminally differentiating keratinocytes. Reason: The modeled location agrees with the directly demonstrated incorporation of loricrin into the mature human cornified envelope and represents its core site of action. |
| GO:0005829 cytosol | TAS Reactome:R-HSA-6810937 | KEEP AS NON CORE | Summary: Reactome assigns cytosolic loricrin in the lamellar-body/early-envelope stage of cornified-envelope formation. Reason: A cytosolic precursor pool is compatible with envelope assembly, but the broad soluble compartment is secondary to loricrin's mature envelope location. |
| GO:0005829 cytosol | TAS Reactome:R-HSA-6811539 | KEEP AS NON CORE | Summary: Reactome assigns cytosolic loricrin during reinforcement of the cornified envelope. Reason: This staging location is credible within the assembly model but is non-core relative to the final cross-linked cornified-envelope localization. |
| GO:0005829 cytosol | TAS Reactome:R-HSA-6814298 | KEEP AS NON CORE | Summary: Reactome assigns loricrin to the cytosol in a late-envelope-protein binding event within cornified-envelope formation. Reason: The pathway model supports a cytosolic precursor context, but the event is not specific to loricrin's defining structural activity and the location is non-core. |
| GO:0005829 cytosol | TAS Reactome:R-HSA-6814387 | KEEP AS NON CORE | Summary: Reactome assigns cytosolic loricrin in the cornification event describing CASP14-dependent filaggrin cleavage. Reason: Cytosol is a defensible pathway compartment for precursor loricrin, but this particular event centers on filaggrin processing and does not define loricrin's core function. |
| GO:0005829 cytosol | TAS Reactome:R-HSA-6814734 | KEEP AS NON CORE | Summary: Reactome assigns cytosolic loricrin in the event linking corneodesmosin to the cornified envelope. Reason: The compartment is compatible with the broader envelope-assembly model, but this CDSN-centered event is contextual rather than a defining loricrin activity. |
| GO:0005829 cytosol | TAS Reactome:R-HSA-6814764 | KEEP AS NON CORE | Summary: Reactome assigns a cytosolic pool of loricrin during plasma-membrane resorption in differentiating keratinocytes. Reason: Cytosolic precursor localization is plausible, while the mature cornified envelope—not the generic cytosol—is the core structural destination. |
| GO:0018149 peptide cross-linking | IDA PMID:10908733 Small proline-rich protein 1 is the major component of the c... | ACCEPT | Summary: Experimental annotation of loricrin participation in peptide cross-linking. The cached abstract for the cited oral-keratinocyte study does not mention loricrin, but independent human biochemical studies directly establish loricrin isopeptide cross-links. Reason: The process is a core part of loricrin incorporation and envelope reinforcement. Because the original paper is abstract-only in the cache, this decision does not claim details absent from that abstract and relies on independent direct evidence while deferring to the full-text curator assessment. Supporting Evidence: PMID:2007607 Furthermore, we have isolated four peptides from purified human cell envelopes that contain recognizable loricrin sequences and which are cross-linked by the N epsilon-(gamma-glutamyl)lysine isodipeptide bond. |
| GO:0001533 cornified envelope | IDA PMID:7543090 The proteins elafin, filaggrin, keratin intermediate filamen... | ACCEPT | Summary: Direct biochemical recovery of extensively cross-linked loricrin from purified human epidermal cornified envelopes. Reason: The paper directly establishes the mature cornified envelope as a core location of human loricrin. Supporting Evidence: PMID:7543090 The data establish for the first time that these several proteins are indeed cross-linked protein components of the CE structure. |
| GO:0030280 structural constituent of skin epidermis | IDA PMID:7543090 The proteins elafin, filaggrin, keratin intermediate filamen... | ACCEPT | Summary: Direct biochemical evidence that cross-linked loricrin forms most of the outer portion and much of the middle portion of the human epidermal cornified envelope. Reason: The abundance and placement of cross-linked loricrin directly support its core molecular function as a structural constituent of skin epidermis. Supporting Evidence: PMID:7543090 most (94% molar mass) of the outer third of CE structure consists of intra- and interchain cross-linked loricrin |
| GO:0005737 cytoplasm | IDA PMID:11698679 Differentially expressed late constituents of the epidermal ... | KEEP AS NON CORE | Summary: Experimental cytoplasmic localization attributed to a study of late cornified-envelope constituents. The cached abstract does not describe a loricrin localization experiment. Reason: Cytoplasmic precursor localization is corroborated by independent UniProt and phylogenetic annotations, but the cited paper is abstract-only in the cache; this review therefore retains the curator's experimental call without asserting unverified assay details and treats the broad location as non-core. |
| GO:0001533 cornified envelope | IDA PMID:11698679 Differentially expressed late constituents of the epidermal ... | ACCEPT | Summary: Experimental cornified-envelope localization attributed to a study of late envelope constituents. Although the cached abstract does not mention loricrin, independent human biochemical evidence directly confirms the location. Reason: Cornified-envelope localization is unquestionably correct for loricrin. The original reference is abstract-only in the cache, so the decision defers to the curator while relying on separately quote-verifiable direct evidence. Supporting Evidence: PMID:7543090 Likewise, the middle third of CE structure consists largely of cross-linked loricrin and SPR proteins |
| GO:0001533 cornified envelope | IDA PMID:10908733 Small proline-rich protein 1 is the major component of the c... | ACCEPT | Summary: Experimental cornified-envelope localization attributed to a study of human oral keratinocyte envelopes. The cached abstract foregrounds SPR1 and does not mention loricrin, whereas independent studies directly establish loricrin in human epidermal envelopes. Reason: The location is a well-established core property. Because the cited paper is abstract-only in the cache, this review neither rejects the curator's full-text assessment nor invents paper-specific loricrin findings; it relies on independent direct evidence. Supporting Evidence: PMID:7543090 The data establish for the first time that these several proteins are indeed cross-linked protein components of the CE structure. |
| GO:0030216 keratinocyte differentiation | IDA PMID:10908733 Small proline-rich protein 1 is the major component of the c... | UNDECIDED | Summary: Experimental annotation linking loricrin to keratinocyte differentiation in an oral-keratinocyte study whose cached abstract does not mention loricrin. Reason: Loricrin is expressed during terminal differentiation, so the process is biologically plausible, but the cached abstract provides no LORICRIN-specific evidence and the full text is unavailable. Curator deference therefore requires leaving this experimental annotation undecided rather than treating generic envelope-development background as positive support. |
| GO:0005515 protein binding | IPI PMID:7592852 Biochemical, structural, and transglutaminase substrate prop... | MARK AS OVER ANNOTATED | Summary: Interaction with transglutaminase 1 (UniProtKB:P22735) detected while characterizing loricrin as a transglutaminase substrate. Reason: Generic protein binding is uninformative and converts a biologically meaningful enzyme-substrate relationship into a vague molecular function. The evidence supports loricrin's role as a cross-linking substrate, already captured by peptide cross-linking, but no suitably specific binding term exists. Supporting Evidence: PMID:7592852 The transglutaminase (TGase) 1, 2, and 3 enzymes expressed during epidermal differentiation utilized loricrin in vitro as a complete substrate, but the types of cross-linking were different. |
| GO:0005200 structural constituent of cytoskeleton | TAS PMID:2007607 Characterization of human loricrin. Structure and function o... | MODIFY | Summary: Legacy statement-level annotation describing loricrin as a structural constituent of cytoskeleton based on its biochemical characterization in epidermal cell envelopes. Reason: The structural essence is correct, but loricrin is incorporated into the cornified envelope rather than being a constituent of a cytoskeletal structure. GO:0030280 precisely captures the demonstrated epidermal structural role. Proposed replacements: structural constituent of skin epidermis Supporting Evidence: PMID:2007607 The presence of such bonds thus affords an explanation for the extraordinary insolubility of loricrin by cross-linking to the cell envelope and can also explain the low steady-state levels of monomeric loricrin in cytoskeletal extracts of epidermis. |
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Download this section (compressed HTML)Q: Which human cornified-envelope proteins compensate quantitatively for reduced or absent loricrin, and do those substitutions preserve permeability and mechanical resilience equally across anatomical sites and environmental stresses?
Q: Does wild-type loricrin have a reproducible physiological nucleoplasmic pool in differentiating human keratinocytes, or does the reviewed nucleoplasm statement reflect a transient or experimentally contingent localization without a nuclear function?
Experiment: Generate isogenic human epidermal organoids carrying LORICRIN knockout and rescue alleles, then combine quantitative cornified-envelope proteomics and cross-link-site mapping with transepidermal water-loss, rupture, abrasion, and humidity-stress assays.
Hypothesis: Human keratinocytes compensate for LORICRIN loss by increasing and redistributing small proline-rich proteins and other transglutaminase substrates, preserving permeability more effectively than mechanical resilience.
Type: genetic perturbation with proteomics and biomechanical assays
Experiment: Compare endogenous wild-type loricrin, disease frameshift alleles, and nuclear-targeting- signal-reverted mutants in differentiating primary human keratinocytes by live-cell imaging, biochemical fractionation, and cornified-envelope incorporation assays.
Hypothesis: Nuclear accumulation is specific to frameshifted loricrin with a novel C-terminal targeting signal and is not a normal functional state of wild-type loricrin.
Type: endogenous allele engineering and localization analysis
What is not known — curated, literature-grounded statements of the open unknowns (the inverse of core functions).
Gap: The extent and molecular composition of compensation for LORICRIN loss in human epidermis are unknown, including whether compensation preserves mechanical strength across body sites as effectively as it preserves permeability.
OPEN BIOLOGY RESIDUAL_SUBGAP
What is known: Human biochemical studies establish that loricrin is a predominant cross-linked envelope constituent. In mice, loricrin loss delays barrier acquisition and makes envelopes more fragile, yet increased expression of other envelope components permits apparently normal steady-state permeability; equivalent causal measurements have not been made in human epidermis.
Significance: Resolving this gap would distinguish loricrin's conserved load-bearing contribution from species- and site-specific redundancy and clarify why loss and dominant mutant alleles can have different phenotypic consequences.
What would resolve it: Quantify envelope composition, covalent cross-link topology, permeability, and mechanical failure in LORICRIN-null and rescued human epidermal organoids across relevant differentiation and stress conditions.
Provenance (the field's own admissions):
Gap: The physiological significance of the reported nucleoplasmic localization of wild-type loricrin is unresolved; no normal nuclear molecular function has been established.
OPEN BIOLOGYCURATION RESIDUAL_SUBGAP
What is known: The reviewed UniProt record reports both cytoplasmic and nucleoplasmic localization, whereas disease-associated frameshift proteins acquire a C-terminal nuclear-targeting signal, accumulate in nuclei instead of the cornified envelope, and disrupt late differentiation. The mutant mechanism therefore cannot establish a normal nuclear role for wild-type loricrin.
Significance: Clarification is needed to decide whether nucleoplasm should remain a contextual wild-type location and to prevent a pathogenic mutant gain of function from being generalized into the normal LORICRIN functional model.
What would resolve it: Use allele-specific endogenous tagging, nuclear-cytoplasmic fractionation, and envelope incorporation measurements in differentiating primary human keratinocytes, explicitly comparing wild-type protein with disease frameshift variants.
Provenance (the field's own admissions):
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