TMEM74

TMEM74 is a multi-pass transmembrane protein that functions at the intersection of autophagy, apoptosis regulation, and neuronal excitability. It localizes to lysosomes and autophagosomes and promotes autophagy through direct physical interactions with ATG16L1 and ATG9A via a BECN1/PI3KC3- and ULK1-independent pathway, with TMEM74 itself undergoing autophagic degradation in a self-regulatory loop (PMID:18294959, PMID:29048433). Independently of its autophagy function, TMEM74 inhibits BIK-mediated apoptosis through a transmembrane domain–BH3 domain interaction, and it promotes surface trafficking of HCN1 channels in neurons, enhancing Ih current and restraining neuronal excitability in amygdala and prefrontal cortical circuits that govern anxiety- and autism-like behaviors (PMID:28412412, PMID:30886335, PMID:36690791).

1. 2008 High

   The first functional characterization established that TMEM74 is a lysosome/autophagosome-resident protein whose overexpression is sufficient and whose knockdown is necessary for starvation-induced autophagy, placing it as a positive regulator of this pathway.

   Evidence Subcellular fractionation, GFP-LC3 dot formation, MDC staining, LC3-II immunoblotting, siRNA knockdown, and wortmannin inhibition in HeLa cells

   PMID:18294959

   Open questions at the time

   • No direct binding partners identified
   • Mechanism of action within the autophagy cascade unknown
   • Partial wortmannin sensitivity left the PI3K-dependence ambiguous
2. 2009 Medium

   An independent high-throughput screen confirmed TMEM74 as one of the strongest autophagy inducers among >1,000 human cDNAs, providing unbiased validation of its autophagy-promoting activity.

   Evidence Automated GFP-LC3 dot quantification screen, TEM, and LC3-II immunoblotting

   PMID:19029833

   Open questions at the time

   • Screen-level data without detailed mechanistic follow-up
   • Binding partners and pathway position still undefined
3. 2017 High

   The mechanistic basis of TMEM74-driven autophagy was resolved: TMEM74 directly binds ATG16L1 and ATG9A to promote autophagosome formation independently of BECN1/PI3KC3 and ULK1, and is itself degraded by autophagy, forming a negative-feedback loop that confers pro-survival advantage under metabolic stress.

   Evidence Reciprocal co-immunoprecipitation, siRNA epistasis for ATG16L1 and ATG9A, GFP-LC3 microscopy, autophagy flux assays, and cell viability assays under stress

   PMID:29048433

   Open questions at the time

   • Structural details of the TMEM74–ATG16L1 and TMEM74–ATG9A interfaces unknown
   • Whether TMEM74 functions as a scaffold or an activator of these partners is unresolved
   • In vivo relevance of the BECN1/ULK1-independent pathway not tested
4. 2017 High

   A second, autophagy-independent function was uncovered: TMEM74 physically interacts with the pro-apoptotic BH3-only protein BIK through its transmembrane domains to suppress BIK-induced apoptosis, establishing TMEM74 as a dual regulator of autophagy and apoptosis.

   Evidence Co-immunoprecipitation, co-localization imaging, TM domain-deletion mutagenesis, and caspase/cell death apoptosis assays

   PMID:28412412

   Open questions at the time

   • Whether TMEM74 sequesters BIK or alters its conformation is unknown
   • Physiological context (cell type, stress condition) for the anti-apoptotic function not defined
   • Relationship between autophagy and apoptosis arms at endogenous expression levels unclear
5. 2019 High

   A neuronal role was established: TMEM74 binds HCN1 channels and promotes their surface trafficking, enhancing Ih current in basolateral amygdala pyramidal neurons; loss of TMEM74 increases excitability and produces anxiety-like behavior, revealing a non-autophagic in vivo function.

   Evidence Co-immunoprecipitation, surface biotinylation, TM1 domain deletion, whole-cell patch-clamp, Tmem74 knockout mice, BLA-specific AAV-shRNA knockdown, and behavioral rescue

   PMID:30886335

   Open questions at the time

   • Whether TMEM74 chaperones HCN1 through the secretory pathway or stabilizes it at the plasma membrane is unresolved
   • Interaction interface on HCN1 not mapped
   • Whether this trafficking function extends to other HCN family members unknown
6. 2023 High

   Circuit-level dissection showed that TMEM74 loss in prelimbic cortex pyramidal neurons increases excitability and produces dissociable behavioral phenotypes—autism-like behaviors via projections to dorsal striatum fast-spiking interneurons and anxiety-like behaviors via projections to BLA—establishing TMEM74 as a cell-autonomous regulator of neuronal excitability with circuit-specific behavioral outputs.

   Evidence Conditional Tmem74 deletion in PL (AAV-Cre), optogenetic and chemogenetic circuit manipulation, whole-cell patch-clamp, and multi-domain behavioral testing with rescue

   PMID:36690791

   Open questions at the time

   • Whether HCN1 trafficking is the sole mechanism underlying excitability changes in PL neurons is untested
   • Molecular partners mediating TMEM74 function in cortical (vs. amygdala) neurons not identified
   • Human genetic association with anxiety or autism spectrum disorder not established

• Key unresolved questions include the structural basis of TMEM74's interactions with ATG16L1, ATG9A, BIK, and HCN1; whether its autophagy and ion channel trafficking functions are mechanistically linked or fully independent; and whether TMEM74 variants contribute to human neuropsychiatric disease.
• No structural data for any TMEM74 complex
• No human genetic evidence linking TMEM74 to disease
• Relationship between autophagy/apoptosis function and neuronal trafficking function unexplored