📄 molecular biology

FAM136A is an essential chaperone for mitochondrial membrane protein biogenesis

This study identifies FAM136A as a conserved, essential chaperone residing in the mitochondrial intermembrane space that solubilizes and facilitates the insertion of voltage-dependent anion channels (VDACs) and specific electron transport chain subunits into the outer mitochondrial membrane.

Original authors: Ernst, M., Zhang, J., Xu, H., Ma, A., Boegeholz, L. A. K., Szabo, M., Wang, T.-Y., Chou, T.-F., Guna, A., Voorhees, R. M.

Published 2026-08-04

📖 4 min read☕ Coffee break read

CC BY 4.0

Original authors: Ernst, M., Zhang, J., Xu, H., Ma, A., Boegeholz, L. A. K., Szabo, M., Wang, T.-Y., Chou, T.-F., Guna, A., Voorhees, R. M.

Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). ⚕️ This is an AI-generated explanation of a preprint that has not been peer-reviewed. It is not medical advice. Do not make health decisions based on this content. Read full disclaimer

Imagine the inside of a cell as a bustling, high-tech city. At the heart of this city are the mitochondria, the power plants that keep everything running. But these power plants have a tricky problem: they need to import thousands of different parts from the outside world to build and repair themselves. Some of these parts are like sturdy, barrel-shaped pipes (called $\beta$ -barrels) that need to be inserted into the outer wall of the power plant.

Here's the catch: these pipes are made of oily, water-fearing materials. Once they leave the safety of the factory (the cell's main body) and enter the watery hallway inside the power plant (called the intermembrane space), they want to stick to each other and clump up, like oil droplets in a glass of water. If they clump, they can't get installed, and the power plant breaks down. To prevent this, the cell uses "chaperones"—think of them as specialized bodyguards or guides. These guides hold the oily pipes, keep them separated, and walk them safely to the installation crew. For a long time, scientists knew about a few of these guides, mostly in simple organisms like yeast. But human cells are much more complex, with more parts and more complicated jobs. The big question was: do humans have extra, specialized guides for their more complex power plants, and if so, who are they?

This paper introduces us to a new, essential bodyguard named FAM136A. The researchers discovered that this protein is a critical chaperone specifically for the human power plant's outer wall. They found that without FAM136A, the "barrel pipes" (specifically a family of proteins called VDACs) get lost, clump together in the watery hallway, and fail to get installed. The team showed that FAM136A acts like a soluble life raft: it grabs the oily, unfinished pipes as soon as they enter the hallway, keeps them from sticking to each other, and guides them to the installation site.

The scientists didn't just guess this; they proved it with a mix of clever experiments. First, they used a genome-wide "search party" (a CRISPRi screen) to see which genes were needed to build these pipes. When they turned off FAM136A, the pipes stopped working. They then went into the lab to watch the action in real-time. They showed that FAM136A physically grabs the pipes and, crucially, can do this all by itself without needing other helpers. They even built a model of FAM136A and found it has a special "groove" lined with negative electric charges. This groove acts like a magnet that attracts the pipes (which have positive charges) while repelling other proteins that don't belong there.

Interestingly, the paper suggests that FAM136A is a relatively new invention in the evolutionary story. It is found in humans, other animals, and plants, but it is completely missing in fungi (like yeast). This suggests that as life evolved to become more complex, the simple guides used by yeast weren't enough, and FAM136A evolved to handle the extra complexity of human mitochondria. The researchers also found that FAM136A isn't just for the barrel pipes; it helps a few other specific inner-wall proteins too.

Why does this matter? Because FAM136A is essential for life. If it's broken, the power plant fails. The paper connects this to a real-world disease: a mutation in FAM136A causes a condition called Ménière's disease, which leads to hearing loss. The authors suggest this happens because the cells in our inner ear are super-sensitive to energy and need these pipes to work perfectly. If FAM136A is missing, the pipes clump, the energy supply drops, and the hearing cells die.

In short, this paper identifies FAM136A as a vital, specialized bodyguard for human mitochondria. It solves the mystery of how our complex cells keep their essential outer-wall parts from clumping up in the watery hallway. It suggests that this protein is a key piece of the puzzle for understanding how human cells generate energy and why certain genetic mutations lead to specific diseases like hearing loss. The authors are confident that FAM136A is a direct, necessary chaperone, but they also suggest that it works alongside other systems, hinting that the full story of mitochondrial maintenance is a complex team effort.

Technical Summary: FAM136A is an essential chaperone for mitochondrial membrane protein biogenesis

Problem Statement
Mitochondrial biogenesis faces a unique challenge in metazoans compared to fungi: the mammalian mitochondrial proteome is significantly expanded, particularly in the number of voltage-dependent anion channel (VDAC) paralogs and other membrane proteins. While the biogenesis of $\beta$ -barrel proteins in the outer mitochondrial membrane (OM) relies on conserved machinery inherited from bacteria (TOM complex for translocation, SAM complex for insertion), the soluble chaperone network in the intermembrane space (IMS) required to prevent the aggregation of nascent, hydrophobic $\beta$ -strands in the aqueous IMS is less understood in mammals. Previous studies in yeast identified small TIM chaperones (Tim9-10, Tim8-13) as key factors, but their sufficiency for the expanded mammalian substrate diversity, particularly the three human VDAC paralogs (VDAC1, 2, and 3), was unclear. Furthermore, the molecular function of FAM136A, a conserved but essential gene linked to Ménière's disease, remained unknown despite its localization to the IMS and presence of twin-CX3C motifs similar to TIM proteins.

Methodology
The authors employed a multi-faceted approach combining genome-wide screening, cell biology, and in vitro reconstitution:

Reporter Systems: A split-GFP system was utilized where GFP1-10 was targeted to the IMS, and GFP11 was fused to the N-terminus of mitochondrial substrates (VDAC1-3, TIMs, etc.). Fluorescence upon complementation served as a ratiometric readout (normalized to RFP) for successful mitochondrial integration and biogenesis.
CRISPRi Screens: A genome-wide CRISPR interference (CRISPRi) screen using the VDAC3 reporter identified FAM136A as a critical factor for VDAC biogenesis. A dual-guide CRISPRi screen was subsequently used to identify genetic modifiers and interactions within the IMS chaperone network.
Biochemical Characterization:
- Immunoprecipitation (IP): Native IPs from mitochondria and isolated IMS fractions were performed to identify endogenous interactors of FAM136A.
- In Vitro Translation: Rabbit reticulocyte lysate (RRL) and the PURE system (chaperone-free) were used to translate nascent VDACs and test binding to recombinant FAM136A purified from E. coli.
- Solubility Assays: Sucrose gradient fractionation was used to determine if FAM136A could prevent the aggregation of nascent VDACs in an aqueous environment.
- Crosslinking: Site-specific photo-crosslinking (using Bpa) and chemical crosslinking (DSS) were employed to confirm direct physical interactions between FAM136A and VDAC substrates.
- Insertion Assays: An in vitro reconstitution assay using purified mitochondria from wild-type and FAM136A knockout cells, followed by hypotonic swelling and proteinase K digestion, distinguished between proteins translocated into the IMS versus those successfully integrated into the OM.
Structural and Mutational Analysis: AlphaFold3 models were used to identify a hydrophobic groove in FAM136A. Site-directed mutagenesis of FAM136A (hydrophobic and charge-altering residues) and VDAC3 (hydrophobicity and charge modifications) was performed to map the binding interface and substrate specificity determinants. Chimeric constructs between human VDAC3 and yeast Porin1 were used to dissect sequence requirements.

Key Results

Identification of FAM136A: The CRISPRi screen revealed that FAM136A is essential for the biogenesis of all three VDAC paralogs, with the most pronounced effect on VDAC3. Depletion of FAM136A did not affect general mitochondrial morphology or the biogenesis of control proteins in the matrix or inner membrane, indicating a specific role in OM $\beta$ -barrel biogenesis.
Direct Chaperone Function: FAM136A physically associates with nascent, unfolded VDACs in the IMS. In vitro experiments demonstrated that recombinant FAM136A alone is sufficient to bind and solubilize nascent VDAC3, preventing its aggregation in the absence of mitochondria or other chaperones.
Mechanism of Action: FAM136A facilitates the insertion of VDACs into the OM. In FAM136A knockout mitochondria, VDAC3 is translocated across the OM but fails to integrate into the lipid bilayer, remaining in a protease-sensitive state.
Structural Basis for Specificity: FAM136A contains a hydrophobic groove (~1000 Å²) lined with conserved negatively charged residues.
- Hydrophobic Interaction: The groove binds the exposed hydrophobic $\beta$ -strands of VDACs.
- Electrostatic Selectivity: The negative charges lining the groove confer substrate specificity. VDACs are enriched in positively charged residues, facilitating binding, whereas the yeast homolog Porin1 is enriched in negative charges, leading to electrostatic repulsion and lack of binding. This explains why FAM136A is required in metazoans (with expanded VDACs) but lost in fungi.
Substrate Scope: Beyond VDACs, FAM136A also chaperones a subset of small, single-spanning $\alpha$ -helical inner membrane proteins (e.g., specific ETC subunits), which are distinct from the canonical substrates of the small TIM chaperones.
Genetic Context: FAM136A functions in a parallel pathway to the disaggregase CLPB and its chaperone HAX1. While TIM8B/TIM13 can partially compensate for VDAC3 biogenesis in the absence of FAM136A, FAM136A does not bind canonical TIM substrates (AAC1, TIM23), indicating a non-overlapping, specialized role.

Significance and Claims
The paper concludes that FAM136A is a specialized, IMS-resident chaperone essential for the biogenesis of mammalian $\beta$ -barrel proteins and a subset of $\alpha$ -helical inner membrane proteins. Its discovery fills a gap in understanding how mammalian mitochondria manage their expanded proteome compared to fungi. The authors propose that FAM136A maintains nascent membrane proteins in a folding-competent, soluble state via a hydrophobic groove and electrostatic selectivity, facilitating their hand-off to the SAM complex for OM insertion.

The study provides a biochemical explanation for the essentiality of FAM136A and the pathophysiology of FAM136A mutations causing Ménière's disease. Specifically, the loss of FAM136A function likely disrupts VDAC3 biogenesis, impairing electron transport chain regulation and ROS management in high-energy-demand cells like inner ear hair cells, which cannot regenerate. The work suggests that the IMS chaperone network in mammals has evolved to mimic the complexity of the bacterial periplasm, utilizing specialized factors like FAM136A to ensure targeting specificity and prevent aggregation in a crowded, diverse proteome.

More like this