YIF1A

YIF1A is an integral multi-pass transmembrane protein of the ER–Golgi system that functions in COPII-dependent ER-to-Golgi membrane transport, characterized largely through its yeast ortholog Yif1p (PMID:10970842, PMID:12657649). Yif1p forms a tight, stable complex with the YIP1-family protein Yip1p, acting downstream of Yip1p; loss of Yif1p function blocks ER-to-Golgi protein transport and causes accumulation of ER membranes and 40–50 nm vesicles (PMID:10970842). This complex, together with the additional subunit Yos1p, is efficiently packaged into COPII vesicles and is required for the fusion competence of ER-derived vesicles with the Golgi, an activity it confers in part through binding the ER-to-Golgi SNAREs Bos1p and Sec22p (PMID:11157978, PMID:12657649, PMID:15659647). The cytosolic N-terminal domain interacts with transport Rab/Ypt GTPases in a prenylation-dependent manner, coupling the complex to vesicle-trafficking GTPases (PMID:10970842, PMID:11943201). The human protein localizes to the Golgi and ER and belongs to a five-pass transmembrane (FinGER/YIPF) family whose members interact with one another and whose overexpression fragments the Golgi (PMID:14680843); human YIF1A binds human Yip1A, which specifies its Golgi localization (PMID:15990086). In neurons, YIF1A localizes to the ER–Golgi intermediate compartment, recycles between ER and Golgi, and interacts via its transmembrane regions with VAPB to support membrane trafficking into dendrites; the ALS8-linked VAPB-P56S mutant sequesters YIF1A into ER-like inclusions, displacing it from the ERGIC (PMID:23736259). YIF1A function is separable from Yip1A's role in regulating ER structure, since YIF1A knockdown does not reproduce the ER whorl phenotype of Yip1A depletion (PMID:23342155).

1. 2000 High

   Established that Yif1p partners with Yip1p and is functionally required for ER-to-Golgi transport, defining the gene's core trafficking role and its epistatic position downstream of Yip1p.

   Evidence Yeast two-hybrid, affinity studies, conditional-lethal mutants, and electron microscopy of the yeast ortholog

   PMID:10970842

   Open questions at the time

   • Mechanism by which the complex promotes transport not defined
   • Direct GTPase interactions shown only in two-hybrid, not biochemically validated here
2. 2001 Medium

   Showed Yif1p is selectively packaged into COPII vesicles, placing it physically in the ER-derived transport carriers rather than only at static membranes.

   Evidence Purification of COPII vesicles with MALDI mass spectrometry and antibody localization

   PMID:11157978

   Open questions at the time

   • Single study
   • Does not establish a functional role within the vesicle
3. 2002 Medium

   Demonstrated that Yif1p binds Rab/Ypt GTPases in a prenylation-dependent manner and associates with other YIP1-family members, linking it mechanistically to GTPase-controlled trafficking.

   Evidence In vitro biochemical binding assays and sequence analysis

   PMID:11943201

   Open questions at the time

   • In vitro only
   • Functional consequence of GTPase binding unresolved
4. 2003 High

   Resolved the step at which the complex acts: the Yip1p–Yif1p complex confers fusion competence on ER-derived vesicles and binds the SNAREs Bos1p and Sec22p, distinguishing vesicle fusion from vesicle budding/packaging.

   Evidence Reconstituted in vitro ER-to-Golgi transport assay with timed antibody inhibition and co-IP of SNARE partners

   PMID:12657649

   Open questions at the time

   • Molecular mechanism by which the complex activates SNARE-mediated fusion not defined
   • No structural model of the complex–SNARE interaction
5. 2003 Medium

   Identified Btn2p-dependent retrieval as the route maintaining Yif1p Golgi localization, addressing how the protein is recycled rather than degraded.

   Evidence Yeast two-hybrid, BTN2 deletion, and fluorescence localization

   PMID:12615067

   Open questions at the time

   • Single lab
   • Mechanism of retrieval not detailed beyond Btn2p requirement
6. 2004 Medium

   Mapped the cytoplasmic N-terminal domain as an interaction hub for PX-domain proteins, with the protein-binding surface distinct from the lipid-binding pocket.

   Evidence Genome-wide yeast two-hybrid screen and in vitro binding with domain dissection

   PMID:15263065

   Open questions at the time

   • Functional significance of PX-domain interactions in vivo unresolved
   • Single study
7. 2005 High

   Expanded the complex to a third essential subunit, Yos1p, refining the composition of the trafficking module required for ER-to-Golgi transport.

   Evidence Co-IP, subcellular fractionation, COPII packaging assay, and genetic depletion

   PMID:15659647

   Open questions at the time

   • Stoichiometry of the Yip1p–Yif1p–Yos1p complex not determined
   • Specific contribution of Yos1p to fusion not isolated
8. 2005 Medium

   Translated the yeast paradigm to humans, showing YIF1A localizes to the Golgi, binds human Yip1A, and depends on Yip1A for its Golgi localization.

   Evidence Immunofluorescence, yeast two-hybrid, co-IP, and dominant-negative overexpression

   PMID:15990086

   Open questions at the time

   • Functional trafficking role of human YIF1A not directly tested here
   • Single lab
9. 2006 High

   Detailed the endosome-to-Golgi retrieval pathway for Yif1p, implicating retromer/sorting-nexin machinery (Ypt6, Snx4, Vps26) alongside Btn2p.

   Evidence Co-IP, GST binding, GFP fluorescence, and genetic epistasis across multiple deletions

   PMID:17101785

   Open questions at the time

   • Relevance of this retrieval route to mammalian YIF1A unknown
10. 2013 High

    Defined a neuronal role: YIF1A occupies the ERGIC, recycles between ER and Golgi, and interacts with VAPB via transmembrane regions to support dendritic membrane trafficking, with the ALS8 VAPB-P56S mutant sequestering YIF1A into ER inclusions.

    Evidence Reciprocal co-IP, neuronal immunofluorescence, live imaging, siRNA knockdown, and dominant-negative overexpression

    PMID:23736259

    Open questions at the time

    • Whether YIF1A mislocalization is causal in ALS8 pathology not established
    • Cargo selectivity of dendritic trafficking not mapped
11. 2013 Medium

    Showed that YIF1A function is dissociable from Yip1A's ER-shaping activity, since YIF1A knockdown does not produce the ER whorls caused by Yip1A depletion.

    Evidence siRNA knockdown with ER-morphology fluorescence readout

    PMID:23342155

    Open questions at the time

    • Negative finding; single lab
    • Does not define YIF1A's positive contribution to ER structure
12. 2018 High

    Provided in vivo developmental evidence that the Yif1–Yip1 complex maintains Golgi integrity and ER-to-Golgi transport required for dendrite pruning, via downregulation of the adhesion molecule Neuroglian.

    Evidence Drosophila clonal genetic screen, co-IP in S2 cells, live imaging, and epistasis with Rab1 and Sar1

    PMID:29769219

    Open questions at the time

    • Direct molecular link from transport defect to Neuroglian endocytosis not fully resolved
    • Mammalian YIF1A counterpart of this developmental role untested

• How human YIF1A mechanistically confers vesicle fusion competence and selects cargo for dendritic delivery, and whether its displacement by VAPB-P56S is causal in disease, remains unresolved.
• No structural model of the human YIF1A–Yip1A complex
• Direct cargo repertoire of human YIF1A undefined
• Causality of YIF1A in ALS8 not demonstrated