Affinage

SPPL2A

Signal peptide peptidase-like 2A · UniProt Q8TCT8

Length
520 aa
Mass
58.1 kDa
Annotated
2026-06-10
22 papers in source corpus 14 papers cited in narrative 14 extracted findings
Cross-family judge vs UniProt: Affinage preferred faithfulness: 7/7 claims corpus-supported (100%)

Mechanistic narrative

Synthesis pass · prose summary of the discoveries below

SPPL2a is a GxGD-type aspartyl intramembrane protease of the endosomal/lysosomal system that cleaves the membrane-anchored N-terminal fragments of type II transmembrane proteins after their ectodomains have been shed, liberating intracellular domains and turning over residual membrane stubs (PMID:16829952, PMID:17965014). Validated substrates include TNFα—whose released intracellular domain triggers IL-12 expression in dendritic cells (PMID:16829952)—FasL, whose liberated domain translocates to the nucleus to inhibit transcription (PMID:17557115), the Bri2/Itm2b fragment generated after ADAM10 shedding (PMID:17965014), TMEM106B (PMID:24872421), and the tail-anchored SNAREs VAMP1–4, whose accumulation in SPPL2a/b double-knockout tissues implicates the protease in SNARE turnover (PMID:36047592). Cleavage proceeds within the transmembrane segment, with substrate processing favored by helix-destabilizing residues and conformational flexibility; in CD74 the principal scission occurs between Y52 and F53 and does not require the substrate intracellular domain (PMID:26987812, PMID:33294784). Its most consequential physiological role is the clearance of the CD74 (invariant chain) N-terminal fragment in B cells and dendritic cells: failure to process CD74 NTF causes accumulation of the cathepsin S–derived p8 fragment, disrupts endosomal trafficking and MHC II compartment integrity, and arrests B cell maturation at the T1 stage, a defect rescued by CD74 ablation (PMID:23267015, PMID:23267013, PMID:23267016). Accumulated CD74 NTF engages the BCR and Syk to impair PI3K/Akt signaling and dysregulate FOXO1, placing SPPL2a-dependent CD74 turnover upstream of BCL2-dependent B cell survival (PMID:23267016, PMID:26157172). In humans, biallelic loss-of-function mutations in SPPL2A cause CD74 NTF accumulation that selectively depletes IL-12/IL-23–producing CD1c+ cDC2s and impairs IFN-γ–dependent antimycobacterial immunity (PMID:30127434). SPPL2a is additionally required for maturation-stage ameloblast homeostasis and enamel mineralization (PMID:23426979).

Mechanistic history

Synthesis pass · year-by-year structured walk · 8 steps
  1. 2006 High

    Established SPPL2a as a catalytically active endosomal intramembrane aspartyl protease with a defined substrate and a downstream signaling consequence, answering whether this GxGD family member is a functional protease.

    Evidence Subcellular sorting assays plus TNFα ICD release and IL-12 induction in activated human dendritic cells

    PMID:16829952

    Open questions at the time
    • Cleavage site within TNFα not defined
    • Physiological requirement in vivo not yet tested
  2. 2007 High

    Defined SPPL2a substrate specificity by showing it (and SPPL2b) process the Bri2 and FasL N-terminal fragments while other family members cannot, establishing it as a dedicated intramembrane protease acting after ADAM10 shedding.

    Evidence Family-wide overexpression with loss-of-function variants, Western blot of cleavage products, and nuclear translocation/transcription assays for FasL ICD

    PMID:17557115 PMID:17965014

    Open questions at the time
    • In vivo relevance of Bri2 and FasL processing not addressed
    • Mechanism of FasL ICD transcriptional inhibition unresolved
  3. 2012 High

    Identified CD74 as the critical in vivo substrate and explained the B cell phenotype, answering why SPPL2a loss disrupts humoral immunity.

    Evidence SPPL2a knockout and ENU-mutant mice, MS-based substrate identification, CD74 double-KO genetic rescue, and BCL2 vs BAFF epistasis

    PMID:23267013 PMID:23267015 PMID:23267016

    Open questions at the time
    • Molecular mechanism by which CD74 NTF blocks trafficking not yet defined
    • Effect on non-B-cell lineages incompletely mapped
  4. 2015 High

    Mechanistically linked CD74 NTF accumulation to defective BCR signaling, showing the fragment engages the BCR/Syk to impair PI3K/Akt and dysregulate FOXO1.

    Evidence SPPL2a KO mice with reciprocal Co-IP of CD74 NTF with BCR/Syk and phospho-signaling/gene-expression analysis

    PMID:26157172

    Open questions at the time
    • Structural basis of CD74 NTF–BCR interaction unknown
    • Quantitative contribution of FOXO1 dysregulation to apoptosis not isolated
  5. 2016 High

    Resolved the molecular determinants of cleavage by mapping the CD74 scission site and the TM/juxtamembrane features that promote processing, defining substrate recognition rules.

    Evidence IP-MS cleavage site identification (Y52/F53) plus domain-exchange and alanine-scanning mutagenesis

    PMID:26987812

    Open questions at the time
    • No structure of SPPL2a–substrate complex
    • Catalytic mechanism of TM scission not directly visualized
  6. 2018 High

    Extended the mechanism to human disease, showing SPPL2A loss-of-function selectively depletes cDC2s and impairs antimycobacterial immunity via CD74 NTF accumulation.

    Evidence Human patients with homozygous LOF mutations, DC subset flow cytometry, and Sppl2a KO mouse BCG/M. tuberculosis infection models

    PMID:30127434

    Open questions at the time
    • Why cDC2s are selectively vulnerable to CD74 NTF not fully explained
    • Therapeutic implications untested
  7. 2020 Medium

    Broadened understanding of substrate engagement and immune consequences, characterizing TNFα TM flexibility as a cleavage determinant and CD74-dependent rewiring of PRR responses.

    Evidence TNFα TM proline/leucine mutagenesis with CD/MD analysis; SPPL2a/CD74 double-KO BMDCs with Dectin-1 localization and cytokine profiling

    PMID:33239420 PMID:33294784

    Open questions at the time
    • TNFα shedding determinants studied biophysically without in vivo validation
    • Single-lab cytokine and localization data for PRR rewiring
  8. 2022 High

    Identified VAMP1–4 SNAREs as in vivo substrates, expanding SPPL2a's role to SNARE protein turnover in the endocytic/secretory pathway.

    Evidence Co-expression screen of 18 SNAREs, pharmacological inhibition, and Western blot in SPPL2a/b double-KO mouse tissues

    PMID:36047592

    Open questions at the time
    • Functional consequence of VAMP accumulation for membrane trafficking not defined
    • Relative SPPL2a vs SPPL2b contribution to each VAMP unresolved

Open questions

Synthesis pass · forward-looking unresolved questions
  • How SPPL2a substrate selection, cleavage-site choice, and tissue-specific activity are coordinated structurally and regulated remains unresolved.
  • No experimental structure of SPPL2a
  • Regulation of protease activity and substrate prioritization unknown
  • Mechanism of ameloblast requirement (#6) not connected to a defined substrate

Mechanism profile

Synthesis pass · controlled-vocabulary classification · explore literature graph →
Molecular activity
GO:0016787 hydrolase activity 5 GO:0140096 catalytic activity, acting on a protein 5
Localization
GO:0005764 lysosome 2 GO:0005768 endosome 2
Pathway
R-HSA-168256 Immune System 4 R-HSA-392499 Metabolism of proteins 4
Partners
CD74FASLGITM2BSYKTMEM106BTNFVAMP1VAMP2

Evidence

Reading pass · 14 per-paper findings extracted from the source corpus
Year Finding Method Journal Conf PMIDs
2006 SPPL2a localizes to endosomes and functions as an aspartyl intramembrane protease (GxGD type) that catalyzes intramembrane cleavage of TNFα, releasing the TNFα intracellular domain (ICD), which in turn triggers IL-12 expression in activated human dendritic cells. Subcellular localization by sorting assays; intramembrane proteolysis demonstrated by release of TNFα ICD; IL-12 induction measured in activated dendritic cells Nature cell biology High 16829952
2007 SPPL2a (and SPPL2b) mediate intramembrane proteolysis of the Bri2 (Itm2b) N-terminal fragment after ADAM10 ectodomain shedding, generating an intracellular domain; SPP and SPPL3 cannot process this substrate. Loss-of-function variants of SPPL2a/b abolish this cleavage. Overexpression of all SPP/SPPL family members and their loss-of-function variants; Western blot detection of cleavage products The Journal of biological chemistry High 17965014
2007 SPPL2a cleaves the membrane-anchored N-terminal fragment of FasL (generated by ADAM10 ectodomain shedding) via intramembrane proteolysis, liberating the FasL intracellular domain (ICD), which translocates to the nucleus and inhibits gene transcription. Co-expression, Western blot detection of FasL ICD fragment, nuclear translocation assay, transcription inhibition assay in T cells with endogenous FasL Cell death and differentiation High 17557115
2012 SPPL2a mediates intramembrane proteolysis of the invariant chain CD74 N-terminal fragment (NTF) in endosomes/lysosomes of B cells; SPPL2a deficiency causes accumulation of CD74 NTF, severely impairing endocytic membrane trafficking, B cell receptor signaling, BAFF-R surface expression, and MHC II compartment integrity, resulting in a B cell maturation arrest at the T1 stage and disrupted humoral immunity. The B cell defect is rescued by additional CD74 ablation. SPPL2a knockout mouse (genetic epistasis/rescue by CD74 double KO), Western blot for CD74 NTF accumulation, flow cytometry, endosomal trafficking assays, BCR signaling assays The Journal of experimental medicine High 23267013 23267015 23267016
2012 Proteomic analysis of SPPL2a-deficient (chompB) mice identified CD74 as a key substrate; Sppl2a deficiency blocks CD74 regulated intramembrane proteolysis in B cells and myeloid dendritic cells, causing accumulation of the p8 cathepsin S product of CD74 and interfering with endosomal retention and earlier CD74 processing steps. ENU mutagenesis screen, genetic mapping, proteomic (mass spectrometry) substrate identification, Western blot in primary B cells and DCs The Journal of experimental medicine High 23267013
2012 SPPL2a deficiency in B cells causes dramatic build-up of the CD74 p8 fragment (product of cathepsin S), low surface BAFF-R, IgM, and IgD BCR; BCL2 overexpression rescues B cell accumulation but BAFF overexpression does not, placing SPPL2a-mediated CD74 processing upstream of BCL2-dependent survival signaling. Inactivating point mutation knock-in mice, BCL2 transgenic rescue, BAFF transgenic rescue, flow cytometry, Western blot The Journal of experimental medicine High 23267016
2013 SPPL2a is expressed in enamel epithelium during amelogenesis and its genetic ablation in mice causes defective maturation-stage ameloblast function, incomplete resorption of proteinaceous enamel matrix, reduced mineral content, and enamel hypoplasia, establishing SPPL2a as essential for ameloblast cellular homeostasis. Sppl2a knockout mice, histology, micro-CT mineral quantification, immunohistochemistry of ameloblast morphology Journal of bone and mineral research Medium 23426979
2014 SPPL2a (and to a lesser extent SPPL2b) are responsible for intramembrane cleavage of the TMEM106B N-terminal fragment generated by lysosomal protease-dependent ectodomain processing, producing a small rapidly degraded ICD; TMEM106A, a paralog, is not a substrate of SPPL2a/b. Co-expression of GxGD protease family members, Western blot detection of cleavage products, pharmacological inhibition of lysosomal proteases, paralog comparison The Journal of biological chemistry Medium 24872421
2015 Accumulating CD74 NTF in SPPL2a-deficient B cells interacts with the BCR and Syk, impairs tonic and BCR-induced PI3K/Akt signaling, reduces surface IgM, and dysregulates FOXO1 transcription factor leading to elevated proapoptotic gene expression; SPPL2a-mediated CD74 NTF clearance is thus required to maintain appropriate BCR signaling for B cell maturation. SPPL2a KO mice, co-immunoprecipitation of CD74 NTF with BCR/Syk, phospho-flow/Western blot for PI3K/Akt pathway, FOXO1 localization, gene expression analysis Journal of immunology High 26157172
2016 The primary SPPL2a cleavage site in CD74 is between Y52 and F53 within the transmembrane segment, identified by IP-MS of cleavage products. The intracellular domain of CD74 is dispensable for SPPL2a cleavage; helix-destabilizing glycines within the TM segment and specific luminal membrane-proximal residues facilitate efficient intramembrane proteolysis, while none of these determinants individually are absolutely essential. Domain-exchange experiments, IP-MS cleavage site identification, systematic alanine-scanning mutagenesis of CD74 TM and juxtamembrane regions The Biochemical journal High 26987812
2018 In humans, loss-of-function mutations in SPPL2A cause accumulation of CD74 NTF in HLA class II+ myeloid and lymphoid cells, selectively depleting IL-12/IL-23-producing CD1c+ conventional dendritic cells (cDC2s) and their progenitors; SPPL2a-deficient mice recapitulate cDC2 loss and show impaired IFN-γ production after BCG infection with high susceptibility to mycobacterial disease. Human patients with homozygous SPPL2A loss-of-function mutations, flow cytometry of DC subsets, in vitro mycobacterial antigen stimulation, Sppl2a KO mice BCG/M. tuberculosis infection model Nature immunology High 30127434
2020 SPPL2a exhibits a non-canonical ectodomain shedding activity on TNFα; conformational flexibility in the center of the TNFα TM helix (promoted by proline insertions, reduced by leucine mutations) determines the efficiency of this non-canonical shedding, as shown by biophysical characterization and molecular dynamics simulations. Proline/leucine mutagenesis of TNFα TM helix, cell-based cleavage assays, biophysical analysis (CD spectroscopy), molecular dynamics simulations iScience Medium 33294784
2020 SPPL2a deficiency in dendritic cells alters pattern recognition receptor responses: Dectin-1 is redistributed to endosomal compartments, leading to enhanced IL-1β and reduced IL-10 and IFN-β secretion upon mycobacterial stimulation in a CD74-dependent manner. SPPL2a/CD74 double KO bone marrow-derived DCs, cytokine ELISA, selective PRR stimulation (TLR4, Dectin-1), confocal microscopy of Dectin-1 localization Journal of immunology Medium 33239420
2022 SPPL2a and SPPL2b cleave tail-anchored SNARE proteins VAMP1, VAMP2, VAMP3, and VAMP4 in the endocytic/late secretory pathway; loss of SPPL2a/b in double-KO mice causes tissue- and cell-type-dependent accumulation of VAMP1-4, identifying these SNAREs as in vivo substrates and implicating SPPL2a/b in SNARE protein turnover. Cellular co-expression screen of 18 SNARE proteins, pharmacological SPPL2a/b inhibition in cell lines, Western blot in SPPL2a/b double-KO mouse tissues and primary cells The FEBS journal High 36047592

Source papers

Stage 0 corpus · 22 papers · ranked by NIH iCite citations
Year Title Journal Citations PMID
2006 SPPL2a and SPPL2b promote intramembrane proteolysis of TNFalpha in activated dendritic cells to trigger IL-12 production. Nature cell biology 168 16829952
2007 Regulated intramembrane proteolysis of Bri2 (Itm2b) by ADAM10 and SPPL2a/SPPL2b. The Journal of biological chemistry 137 17965014
2007 The Fas ligand intracellular domain is released by ADAM10 and SPPL2a cleavage in T-cells. Cell death and differentiation 114 17557115
2012 The intramembrane protease SPPL2a promotes B cell development and controls endosomal traffic by cleavage of the invariant chain. The Journal of experimental medicine 107 23267015
2018 Disruption of an antimycobacterial circuit between dendritic and helper T cells in human SPPL2a deficiency. Nature immunology 93 30127434
2012 The intramembrane protease Sppl2a is required for B cell and DC development and survival via cleavage of the invariant chain. The Journal of experimental medicine 77 23267013
2012 B cell survival, surface BCR and BAFFR expression, CD74 metabolism, and CD8- dendritic cells require the intramembrane endopeptidase SPPL2A. The Journal of experimental medicine 73 23267016
2014 Regulated intramembrane proteolysis of the frontotemporal lobar degeneration risk factor, TMEM106B, by signal peptide peptidase-like 2a (SPPL2a). The Journal of biological chemistry 43 24872421
2015 Processing of CD74 by the Intramembrane Protease SPPL2a Is Critical for B Cell Receptor Signaling in Transitional B Cells. Journal of immunology (Baltimore, Md. : 1950) 29 26157172
1991 Identification, cloning and sequencing of the replication region of Lactococcus lactis ssp. lactis biovar. diacetylactis Bu2 citrate plasmid pSL2. FEMS microbiology letters 26 1884982
2016 Substrate determinants of signal peptide peptidase-like 2a (SPPL2a)-mediated intramembrane proteolysis of the invariant chain CD74. The Biochemical journal 24 26987812
2020 Non-canonical Shedding of TNFα by SPPL2a Is Determined by the Conformational Flexibility of Its Transmembrane Helix. iScience 22 33294784
1987 Human rheumatoid factor crossidiotypes. II. Primary structure-dependent crossreactive idiotype, PSL2-CRI, present on Wa monoclonal rheumatoid factors is present on Bla and other IgM kappa monoclonal autoantibodies. The Journal of experimental medicine 21 3098895
2013 The intramembrane protease SPPL2A is critical for tooth enamel formation. Journal of bone and mineral research : the official journal of the American Society for Bone and Mineral Research 16 23426979
2022 The intramembrane proteases SPPL2a and SPPL2b regulate the homeostasis of selected SNARE proteins. The FEBS journal 12 36047592
2020 Impaired proteolysis by SPPL2a causes CD74 fragment accumulation that can be recognized by anti-CD74 autoantibodies in human ankylosing spondylitis. European journal of immunology 9 32198923
2020 Deficiency of the Intramembrane Protease SPPL2a Alters Antimycobacterial Cytokine Responses of Dendritic Cells. Journal of immunology (Baltimore, Md. : 1950) 9 33239420
2017 Identification of SPPL2a Inhibitors by Multiparametric Analysis of a High-Content Ultra-High-Throughput Screen. SLAS discovery : advancing life sciences R & D 9 28731783
2023 Alleviation of Cadmium Toxicity in Thai Rice Cultivar (PSL2) Using Biofertilizer Containing Indigenous Cadmium-Resistant Microbial Consortia. Plants (Basel, Switzerland) 3 37896114
2022 On the track of intramembrane clippers: the SPPL2a/b proteases caught in the act in animal models. The FEBS journal 2 36310421
2018 Old vaccines, new diseases: when BCG meets SPPL2a. Nature immunology 1 30127437
2024 Mendelian Susceptibility to Mycobacterial Disease with Signal Peptide Peptidase-like 2A (SPPL2A) Deficiency: A Case Report. Iranian journal of allergy, asthma, and immunology 0 39586751

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