SEC24D

SEC24D is an isoform-specific cargo-adaptor subunit of the COPII coat that selects defined cargos for export from ER exit sites (ERES) and is essential for secretion of extracellular matrix proteins and for normal vertebrate development (PMID:20442775, PMID:23596517). It directly binds the C-terminal RL export motif of SLC6-family transporters such as GAT1 through its DD733-734 residues, and disrupting either side of this interaction blocks concentrative ER export of the cargo (PMID:17210573). Beyond membrane cargo, SEC24D is selectively required to capture secreted ECM proteins—type II collagen and matrilin—into COPII carriers, and its loss causes intracellular procollagen retention, ER dilation, and ER stress without impairing export of cargos such as β1-integrin and cadherins (PMID:20442775, PMID:20346938). SEC24D-positive ERES are distinguished by a cholesterol-rich, raft-like character and selectively recruit raft-preferring membrane proteins in a TMED2/10-dependent manner, while raft-excluded cargo is handled by SEC24A-positive ERES (PMID:41309618). Collagen export is further controlled by site-specific O-GlcNAcylation of SEC24D's N-terminal disordered region, which licenses an interaction with myoferlin to drive ERES–ERGIC fusion for transport of bulky collagen (PMID:42129160). SEC24D also functions in macroautophagy, where it is required for autophagosome closure and interacts with CK1δ and ATG9A under starvation (PMID:39056365). In osteogenic precursors, SEC24D loss or disease mutation triggers ER stress and an ATF6/TGF-β/Runx2 differentiation defect (PMID:40374976). Biallelic human SEC24D mutations cause a skeletal dysplasia (Cole-Carpenter syndrome/severe osteogenesis imperfecta) through inefficient procollagen ER export (PMID:25683121), and a 5'UTR uAUG-creating variant reduces SEC24D protein post-transcriptionally (PMID:41495099). Although SEC24D and SEC24C have distinct developmental requirements, their cargo-export functions are largely interchangeable, with isoform-specific expression rather than divergent cargo specificity driving their separate phenotypes (PMID:34702932).

1. 2007 High

   Established that SEC24D acts as a sequence-specific cargo adaptor, directly reading an export motif on transmembrane cargo to drive concentrative ER export.

   Evidence Reciprocal site-directed mutagenesis of GAT1 (RL motif) and SEC24D (DD733-734), RNAi, dominant-negative overexpression, and ER export assays

   PMID:17210573

   Open questions at the time

   • Did not address selection of secreted (non-transmembrane) ECM cargo
   • Structural basis of the DD/RL recognition not resolved
2. 2010 High

   Showed SEC24D is specifically required for export of secreted ECM cargo rather than all COPII cargo, defining its functional specialization in skeletal tissues.

   Evidence Genetic loss-of-function in zebrafish (bulldog) and medaka (vbi), with IF, EM, Golgi staining, and ER-stress readouts, plus paralog-specificity controls

   PMID:20346938 PMID:20442775

   Open questions at the time

   • Molecular mechanism by which large secreted cargo is captured not defined
   • Did not explain why membrane cargos are unaffected
3. 2013 High

   Demonstrated an absolute, non-redundant requirement for SEC24D in early mammalian development that the other three paralogs cannot compensate.

   Evidence Targeted knockout and hypomorphic mouse alleles with BAC transgene rescue and embryo staging

   PMID:23596517

   Open questions at the time

   • Identity of the essential pre-8-cell cargo unknown
   • Did not distinguish unique cargo specificity from expression timing
4. 2015 High

   Linked SEC24D directly to human disease, showing biallelic mutations cause procollagen export failure and skeletal dysplasia.

   Evidence Whole-exome sequencing and EM/IF of patient skin fibroblasts with structural domain mapping of mutations

   PMID:25683121

   Open questions at the time

   • Per-residue mechanistic effects on cargo binding not biochemically dissected
   • Tissue selectivity of the bone phenotype unexplained
5. 2017 Medium

   Placed SEC23A/SEC24D-mediated trafficking as a required step in a fibrogenic differentiation program controlled by CREB3L2.

   Evidence Expression analysis and siRNA knockdown of Sec24D/Sec23A during hepatic stellate cell activation with CREB3L2 loss-of-function

   PMID:28801610

   Open questions at the time

   • Specific cargos required for HSC activation not identified
   • Single-lab study without orthogonal validation
6. 2021 High

   Resolved whether paralog-specific phenotypes reflect distinct cargo specificity, showing SEC24C and SEC24D are functionally interchangeable and differ mainly in expression.

   Evidence Dual recombinase-mediated cassette exchange knock-in (Sec24c→d) rescuing Sec24c-null lethality in mice

   PMID:34702932

   Open questions at the time

   • Did not exclude subtle cargo-selectivity differences in specialized tissues
   • Mechanistic basis of expression-pattern divergence not addressed
7. 2024 Medium

   Extended SEC24D function beyond secretion, implicating it in autophagosome closure via new partners.

   Evidence siRNA depletion with IF/EM of unsealed isolation membranes and Co-IP of SEC24D with CK1δ and ATG9A under starvation

   PMID:39056365

   Open questions at the time

   • Direct vs. indirect nature of CK1δ/ATG9A interactions not established
   • How a COPII subunit mechanistically drives membrane sealing unresolved
8. 2025 High

   Defined a PTM-driven regulatory switch in which O-GlcNAcylation of SEC24D reshapes its interactome to enable bulky collagen transport.

   Evidence MS site mapping, crosslinking proteomics, glycosite mutagenesis, and collagen-trafficking assays in human cells and zebrafish identifying myoferlin

   PMID:42129160

   Open questions at the time

   • O-GlcNAc transferase/eraser controlling these sites not identified
   • Mechanism of myoferlin-mediated ERES-ERGIC fusion not structurally defined
9. 2025 High

   Showed SEC24D marks a biophysically distinct, cholesterol-rich ERES subpopulation that sorts raft-preferring cargo, revealing membrane-domain-based isoform specialization.

   Evidence RUSH synchronized trafficking, fluorescent cholesterol analog labeling, and isoform/TMED2/10 knockdowns distinguishing raft-preferring vs. raft-excluded cargo

   PMID:41309618

   Open questions at the time

   • Mechanism by which SEC24D establishes or senses raft character unknown
   • Relationship between raft-ERES sorting and collagen export not connected
10. 2025 Medium

    Connected SEC24D loss to an ER-stress-driven transcriptional defect in osteogenesis, defining a downstream signaling axis.

    Evidence siRNA and OI mutant (R313H) expression in MSCs with osteogenic assays, transcriptomics, and ATF6 epistasis for TGF-β/Runx2

    PMID:40374976

    Open questions at the time

    • Whether ATF6/TGF-β effects are secondary to ER stress vs. specific cargo loss unclear
    • Single-lab study without in vivo confirmation
11. 2026 Medium

    Identified a translational regulatory mechanism of disease, where a 5'UTR uAUG variant lowers SEC24D protein and is correctable by ASO.

    Evidence Patient fibroblast Western blot/qPCR showing protein-mRNA dissociation and ASO functional rescue

    PMID:41495099

    Open questions at the time

    • Generalizability to other patients not established
    • Mechanism of uAUG-mediated translational repression not biochemically detailed

• How SEC24D mechanistically distinguishes and captures large secreted ECM cargo versus transmembrane cargo, and how its raft-ERES specialization, O-GlcNAc regulation, and autophagy role are integrated, remains unresolved.
• No structure of SEC24D bound to a secreted ECM cargo
• Integration of PTM regulation, membrane-domain sorting, and autophagosome closure into one model is incomplete