Chylomicron assembly and ER export in enterocytes; MTTP, APOB (apoB-48) and SAR1B

Dietary fat is exported from absorptive enterocytes as chylomicrons, large triglyceride-rich lipoproteins built on a single apolipoprotein B-48 molecule. In the endoplasmic reticulum, microsomal triglyceride transfer protein (MTTP, with protein disulfide isomerase as its small subunit) transfers neutral lipid onto apoB-48 as it is translated, producing a primordial lipoprotein that is then expanded into a pre-chylomicron. ApoB-48 is the non-exchangeable structural scaffold of the particle. The pre-chylomicron leaves the ER in a COPII-dependent carrier whose formation is started by the small GTPase SAR1B: GTP-bound SAR1B inserts into the ER membrane at exit sites and recruits the SEC23-SEC24 and SEC13-SEC31 coats. After Golgi maturation, the chylomicron is secreted basolaterally into lymph. The three steps correspond to three recessive fat-malabsorption disorders. Abetalipoproteinemia (MTTP) and homozygous familial hypobetalipoproteinemia 1 (APOB) block particle formation. Chylomicron retention disease (SAR1B) leaves particle formation intact but blocks ER exit.

MODULE:chylomicron_assembly_secretionDRAFTBiological Processmodules/chylomicron_assembly_secretion.yaml
chylomicron assemblyGO:0034378
GO:0034378
chylomicron assembly
The module covers chylomicron assembly (lipidation of apoB-48 in the ER) and the COPII-dependent ER export of the nascent particle.
file:human/SAR1B/SAR1B-ai-review.yaml
SAR1B gene review (human)
The ER-export step (UniProtKB:Q9Y6B6, GO:0003924 GTPase activity, COPII vesicle coat assembly, lipoprotein transport) matches the core functions of the human SAR1B review.
file:human/APOB/APOB-ai-review.yaml
APOB gene review (human)
The scaffold step (UniProtKB:P04114, GO:0005319 lipid carrier activity, ER lumen) matches the first core function of the human APOB review.
PMID:12692552
Mutations in a Sar1 GTPase of COPII vesicles are associated with lipid absorption disorders.
SAR1B (SARA2) mutations cause chylomicron retention disorders. The authors conclude that chylomicrons are selectively recruited by the COPII machinery for ER export despite exceeding typical COPII vesicle size.
PMID:28982670
Understanding Chylomicron Retention Disease Through Sar1b Gtpase Gene Disruption: Insight From Cell Culture.
SAR1B deletion in Caco-2/15 enterocyte-like cells reduces apoB-48 and chylomicron secretion; a SAR1A/SAR1B double knockout almost abolishes it.
UniProtKB:P55157
Microsomal triglyceride transfer protein large subunit
UniProt records MTTP lipid-transfer activity (triglyceride, cholesteryl ester and phospholipid transfer) and its requirement for apoB-containing lipoprotein secretion (PMID:8876250; PMID:15897609). MTTP has no ai-gene-review gene review yet.

Scope is the enterocyte biogenesis and ER-export arm. Hepatic VLDL assembly uses the same MTTP and SAR1B steps with full-length apoB-100, and SURF4 acts as the SAR1B-paired cargo receptor for hepatic lipoprotein export (PMID:33186557); these are context rather than separate parts here. Golgi maturation and basolateral secretion are downstream of this module. SAR1A partially overlaps SAR1B in enterocyte chylomicron export (PMID:28982670), but upregulated SAR1A does not compensate in patients (PMID:21235735). PANTHER family ids are omitted because the participants are grounded as concrete human gene products.

4Nodes
3Parts
0Variant Sets
0Variants
3Annotons
2Connections

Derived QC

Recommended-field compliance

100.0% recommended fields populated

All recommended fields populated.

Module deep research

✗ none found

No MODULE:chylomicron_assembly_secretion deep-research report alongside the module YAML.

Leaf nodes lacking representative members

✓ every leaf node grounds to a representative protein.

Template conformance

✓ every declared conforms_to bundle matches its template motif.

Reaction chaining (advisory)

✓ every PRECEDES step chains, or its break is acknowledged via chaining_status.

  • apob48_scaffold → sar1b_er_export [NOT_CHECKED]

Function consistency and core coverage

2 gene-function check(s) reflected in core functions · 0 conflict(s) · 1 evidence or coverage gap(s) (3 comparison(s))

Retained noncore functions are reported separately; they do not require promotion to core functions. Support from representative genes does not establish conservation across the whole family. Missing reviews or identifiers leave a claim unverified.

AnnotonParticipantFunctionAssessment
mttp_lipid_transfer MTTP
UniProtKB:P55157
Gene 		triglyceride transfer activity
GO:0140344 		Review Missing
No ai-review exists for this protein; its function and core-function coverage cannot yet be checked.
apob_scaffold APOB
UniProtKB:P04114
Gene 		lipid carrier activity
GO:0005319 		Core Supported
The GO molecular function is reflected in core_functions (exact term or a reviewed subclass).
sar1b_copii_gtpase SAR1B
UniProtKB:Q9Y6B6
Gene 		GTPase activity
GO:0003924 		Core Supported
The GO molecular function is reflected in core_functions (exact term or a reviewed subclass).

Gene-review completeness (2/3 grounded genes reviewed)

1 complete review(s) · 1 with deep research · 1 missing review · 1 reviewed but lacking deep research

Gene Review Complete Deep research
APOB P04114 ✓ 126/158 ✗
MTTP P55157 ✗ — —
SAR1B Q9Y6B6 ✓ ✓ ✓

Details

Context
enterocyteCL:0000584
Chylomicron assembly and ER exportBiological Processchylomicron_assembly_secretion
chylomicron assemblyGO:0034378
Context
enterocyteCL:0000584

Connections

mttp_lipidation -> apob48_scaffold Provides Input For
MTTP-mediated lipid transfer converts nascent apoB-48 into a lipidated particle.
The assembled pre-chylomicron is the cargo exported from the ER in a SAR1B-initiated COPII carrier.
Part 1: apoB-48 lipidation
MTTP transfers neutral lipid onto nascent apoB-48Reactionmttp_lipidation

Annotons

MTTP: triglyceride transfer
mttp_lipid_transfer
Participant: Gene Product: MTTP
Gene Product:
MTTPUniProtKB:P55157

Function

triglyceride transfer activityGO:0140344
Targets: apolipoprotein B-48

Processes

chylomicron assemblyGO:0034378

Locations

endoplasmic reticulum lumenGO:0005788

Loads triglyceride, cholesteryl ester and phospholipid onto apoB as it is translocated, preventing apoB degradation and producing the primordial lipoprotein. Loss causes abetalipoproteinemia.

Part 2: structural scaffold
ApoB-48 forms the non-exchangeable scaffold of the particleMolecular Functionapob48_scaffold

Annotons

APOB (apoB-48): lipid carrier scaffold
apob_scaffold
Participant: Gene Product: APOB
Gene Product:
APOBUniProtKB:P04114

Function

lipid carrier activityGO:0005319

Processes

chylomicron assemblyGO:0034378

Locations

endoplasmic reticulum lumenGO:0005788

The intestinal apoB-48 isoform, produced by APOBEC1 editing of APOB mRNA, is the single structural protein of each chylomicron. Biallelic truncating APOB variants abolish chylomicron formation.

Part 3: COPII-dependent ER export
SAR1B starts COPII-dependent ER export of the pre-chylomicronTransport Stepsar1b_er_export

Annotons

SAR1B: COPII GTPase switch
sar1b_copii_gtpase
Participant: Gene Product: SAR1B
Gene Product:
SAR1BUniProtKB:Q9Y6B6

Function

GTPase activityGO:0003924

Processes

COPII vesicle coat assemblyGO:0048208 endoplasmic reticulum to Golgi vesicle-mediated transportGO:0006888 lipoprotein transportGO:0042953

Locations

endoplasmic reticulum exit siteGO:0070971

SEC12-loaded SAR1B-GTP inserts into the ER membrane and recruits the COPII coat that forms the pre-chylomicron transport carrier. SAR1B, rather than SAR1A, is required for this export. Loss causes chylomicron retention disease, in which assembled particles accumulate in the enterocyte ER.