SEC24A

SEC24A is a cargo-selective subunit of the COPII coat complex that mediates ER-to-Golgi transport of specific membrane and soluble cargoes bearing LxxLE-class, DxE, and di-leucine export signals, with its IxM-binding groove structurally occluded relative to the SEC24C/D paralogs (PMID:18843296, PMID:17255961). SEC24A is specifically required for efficient ER exit of PCSK9, and SEC24A-deficient mice exhibit reduced plasma cholesterol due to upregulated hepatic LDL receptor levels (PMID:23580231). Beyond canonical vesicular trafficking, SEC24A maintains ER–mitochondria contact sites and inter-organelle Ca²⁺ flux; its loss increases autophagic flux and confers resistance to thapsigargin-induced apoptosis, functions not compensated by other SEC24 paralogs (PMID:33622772, PMID:30588337). SEC24A is also specifically exploited by hepatitis B virus for envelope protein export via a di-arginine motif–dependent interaction with its N-terminal domain (PMID:32017353).

1. 2007 High

   The question of whether individual human SEC24 paralogs select distinct ER export signals was answered by showing that SEC24A specifically mediates transport of di-leucine-bearing cargoes such as ERGIC-53, establishing isoform-specific cargo sorting within the COPII coat.

   Evidence siRNA knockdown of individual SEC24 isoforms combined with in vitro binding assays in mammalian cells

   PMID:17255961

   Open questions at the time

   • Structural basis for di-leucine signal recognition by SEC24A was not resolved
   • Full repertoire of SEC24A-selective cargoes was unknown
2. 2008 High

   Crystallographic comparison of all four human SEC24 isoforms revealed that the IxM-binding groove is occluded in SEC24A/B while a distinct site selectively binds LxxLE and DxE signals, providing the structural explanation for paralog-specific cargo discrimination.

   Evidence X-ray crystallography of SEC24A/B/C/D trunk domains with biochemical COPII vesicle packaging assays

   PMID:18843296

   Open questions at the time

   • How SEC24A recognizes di-leucine signals structurally was not addressed
   • Whether additional binding sites exist for non-canonical signals remained open
3. 2013 High

   A physiological role for SEC24A cargo selectivity was demonstrated in vivo: SEC24A-deficient mice showed reduced plasma cholesterol through impaired ER exit of PCSK9 and consequent upregulation of hepatic LDL receptors, linking COPII cargo selection to systemic lipid metabolism.

   Evidence Germline SEC24A knockout mice; epistasis genetics with Apoe and Ldlr mutations; plasma cholesterol and hepatic LDLR quantification

   PMID:23580231

   Open questions at the time

   • The ER export signal on PCSK9 recognized by SEC24A was not mapped
   • Extent of functional redundancy between SEC24A and SEC24B for other cargoes was only partially explored
4. 2015 Medium

   SEC24A was shown to directly bind and mediate ER export of the voltage-gated potassium channel Kv1.3, extending its cargo repertoire to ion channels and implicating C-terminal acidic residues as the recognition determinant.

   Evidence siRNA knockdown trafficking assays; in vitro binding with recombinant proteins; site-directed mutagenesis

   PMID:26156069

   Open questions at the time

   • Only demonstrated in a single lab; independent confirmation is lacking
   • Whether the acidic residues constitute a general SEC24A recognition motif was not tested
5. 2018 Medium

   A genome-wide CRISPR screen revealed that SEC24A is specifically required for thapsigargin-induced ER stress cell death but not for other ER stressors, indicating a stressor-selective function upstream of the UPR beyond its canonical trafficking role.

   Evidence Genome-wide CRISPR/Cas9 loss-of-function screen in HAP1 cells; pharmacological ER stress assays

   PMID:30588337

   Open questions at the time

   • Molecular mechanism connecting SEC24A to thapsigargin-specific death was not defined
   • Whether this reflects ER–mitochondria contact or cargo trafficking defects was unresolved
6. 2020 Medium

   HBV was shown to co-opt SEC24A specifically for envelope protein ER export via a di-arginine motif in the S domain, demonstrating that pathogen cargo can exploit paralog-specific COPII sorting.

   Evidence Yeast two-hybrid, co-immunoprecipitation, siRNA knockdown of SEC24 paralogs, and mutagenesis of the HBV S domain di-arginine motif

   PMID:32017353

   Open questions at the time

   • Structural details of the SEC24A–HBV S domain interface are unresolved
   • Whether other viral envelopes use the same SEC24A-dependent pathway is untested
   • Findings from a single lab; awaits independent replication
7. 2021 Medium

   SEC24A was found to maintain ER–mitochondria contacts and Ca²⁺ transfer; its loss reduced organelle colocalization, impaired inter-organelle Ca²⁺ flux, increased autophagic flux, and conferred apoptosis resistance — a function unique to SEC24A among all paralogs. This retroactively connected the 2018 thapsigargin sensitivity finding to defective ER–mitochondria Ca²⁺ signaling.

   Evidence CRISPR/Cas9 knockout in HAP1 cells; organelle Ca²⁺ indicators; colocalization imaging; autophagic flux and apoptosis assays; paralog rescue experiments

   PMID:33622772

   Open questions at the time

   • Whether SEC24A directly tethers ER–mitochondria membranes or acts indirectly through cargo trafficking of tethering factors is unknown
   • Molecular identity of the SEC24A-dependent cargo(es) maintaining these contacts is unresolved
   • Findings from a single lab; awaits independent replication

• The molecular mechanism by which SEC24A uniquely supports ER–mitochondria contacts and Ca²⁺ transfer — whether through direct membrane tethering, trafficking of tethering machinery, or another mechanism — remains the key unresolved question.
• No tethering or adaptor cargo has been identified as the SEC24A-dependent factor at ER–mitochondria contacts
• No structural model exists for SEC24A recognition of di-leucine or di-arginine export signals
• In vivo validation of the ER–mitochondria contact function has not been performed