LRATD2

Gap: The direct molecular activity by which LRATD2 selects or advances EGFR cargo is unknown, including whether it binds EGFR, a cargo receptor, a coat component, or a regulatory GTPase directly.

OPEN BIOLOGY MF_DARK

What is known: LRATD2 depletion selectively delays EGFR ER-to-Golgi transport and rescue restores it, while ShhN and IGF2 transport are unaffected in the same human-cell system.

Significance: Resolving the direct activity is necessary to distinguish a cargo adaptor from a coat regulator or another class of trafficking factor.

What would resolve it: Purified-component binding and reconstitution assays with LRATD2, EGFR cytosolic regions, cargo receptors, AP-1, COPII components, and candidate small GTPases.

Provenance (the field's own admissions):

• "Knockdown of FAM84B/LRATD2 caused a significant delay of EGFR transport from the ER to the Golgi in the RUSH transport system (Fig. 3C and SI Appendix, Fig. S3 A and B). The defects were rescued by expressing a siRNA-resistant construct of FAM84B-HA (Fig. 3 D and E). In contrast, knockdown of FAM84B/LRATD2 did not cause defects in the ER-to-Golgi transport of SBP-EGFP-ShhN and SBP-EGFP-IGF2-HA (Fig. 3 F–I)." — PMID:34433667

Gap: The endogenous composition, stoichiometry, and lifetime of LRATD2-containing trafficking assemblies are unknown.

OPEN BIOLOGY MF_DARK

What is known: Tagged LRATD2 coimmunoprecipitates with AP1gamma1 and SEC23A/B only under the reported cross-linking conditions; this supports proximity but not a stable, stoichiometric complex.

Significance: Endogenous interaction architecture would locate LRATD2 within AP-1-, COPII-, or other transport machinery without overinterpreting tagged cross-linking assays.

What would resolve it: Endogenous tagging followed by quantitative affinity purification, proximity labeling, cross-linking mass spectrometry, and perturbation of individual partners.

Provenance (the field's own admissions):

• "FAM84B-HA coimmunoprecipitated with AP1γ1 and Sec23A/B, but not Sar1A in the presence of a cross-linker (Fig. 3B)." — PMID:34433667

Gap: The endogenous occupancy and trafficking consequence of LRATD2 N-myristoylation are unknown.

OPEN BIOLOGY MF_DARK

What is known: Chemical labeling, NMT inhibition, and Gly2 mutation establish overexpressed LRATD2 as an NMT1/2 substrate in human cells, but do not define how the modification affects membrane recruitment or EGFR transport.

Significance: N-myristoylation could provide the missing physical basis for regulated vesicle association.

What would resolve it: Quantify endogenous LRATD2 myristoylation and compare wild-type and Gly2-mutant proteins for membrane partitioning, coat proximity, and EGFR transport rescue.

Provenance (the field's own admissions):

• "Alk12 labeling signals decreased with the treatment of NMT inhibitor in LRATD1, LRATD2, and ERICH5 and were completely removed in G2A mutants, confirming LRATD1, LRATD2, and ERICH5 as substrates of NMT1/2 (Figure 4C)." — PMID:34956690

Gap: The normal tissue-level physiological role of LRATD2 and the relationship between its trafficking function and tumor-associated phenotypes remain unresolved.

OPEN BIOLOGY BP_DARK

What is known: LRATD2 amplification or perturbation changes proliferation, migration, invasion, and signaling in several human cancer-cell models, whereas direct trafficking evidence is currently centered on a reconstituted HEK293T cargo assay.

Significance: This distinction is needed to separate an endogenous trafficking function from context-dependent consequences of cancer amplification and overexpression.

What would resolve it: Analyze endogenous LRATD2 loss in nontransformed, tissue-relevant human models and connect cargo-trafficking defects to cellular and organismal phenotypes.

Provenance (the field's own admissions):

• "Knockdown of FAM84B in ESCC cell lines significantly reduced in vitro cell growth, migration and invasion." — PMID:26759717

Gap: The structural function of the catalytically divergent LRAT-like region is unknown.

OPEN BIOLOGY MF_DARK

What is known: The region is required for LRATD2-driven invasion and anchorage-independent growth in DU145 cells, but its conserved catalytic cysteine is absent and enzymatic activity has not been demonstrated.

Significance: A structural explanation could reveal whether the domain mediates membrane, cargo, or protein contacts rather than catalysis.

What would resolve it: Determine the LRATD2 structure and map separation-of-function mutations across the LRAT-like region in trafficking and partner-binding assays.

Provenance (the field's own admissions):

• "Since the catalytic residue Cys is not conserved (Figure 2), FAM84B is unlikely to have either enzymatic activities, which implies that FAM84B facilitates Ras signaling." — PMID:31205500