Proteolytic remodeling by Yme1 enables mitochondrial-derived compartment formation

This study identifies the conserved i-AAA protease Yme1 as a critical regulator that enables mitochondrial-derived compartment (MDC) formation by proteolytically remodeling lipid transfer and MICOS complex proteins to relieve constraints on membrane biogenesis during stress.

The Gist

The Big Picture: The Cell's "Emergency Room"

Imagine your cell is a bustling city, and the mitochondria are the power plants keeping the lights on. Sometimes, the city gets stressed (maybe due to a lack of food or a toxic spill). When this happens, the power plant needs to do some emergency renovations.

One specific renovation project is called the MDC (Mitochondrial-Derived Compartment). Think of an MDC as a special "trash chute" or a "quarantine zone" built into the outer wall of the power plant. Its job is to grab specific, broken, or dangerous parts of the wall (hydrophobic proteins) and seal them off so they don't cause chaos inside the main building.

For a long time, scientists knew that these trash chutes formed, but they didn't know how the construction crew knew when to start building them. This paper discovers the foreman who gives the order: a protein named Yme1.

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The Foreman: Yme1

Yme1 is like a strict, highly efficient foreman working inside the power plant. His main job is to use a pair of "molecular scissors" (proteolytic activity) to cut up old or unnecessary parts of the machinery.

The researchers found that if you fire the foreman (delete the YME1 gene), the construction crew stops working. Even when the city is stressed and desperately needs the trash chute, no MDCs are built. The power plant gets clogged with junk, and the cell gets sick.

Key Discovery: It's not just that the foreman is missing; it's that his scissors are missing. If you give the cell a foreman who is present but has broken scissors (a mutant version of Yme1), the trash chute still doesn't get built. The cutting action is the magic ingredient.

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The Obstacles: The "Traffic Jams"

Why does the foreman need to cut things to build the trash chute? The paper reveals that there are two main "traffic jams" or "roadblocks" preventing the construction. Yme1's job is to clear these roads.

1. The Lipid Delivery Trucks (The Ups Family)

Imagine a fleet of delivery trucks (called Ups proteins) that constantly move building materials (lipids) from the outer wall to the inner wall of the power plant.

• The Problem: When the cell is stressed, these trucks keep working overtime, trying to fix the inner wall. But this constant traffic jams the outer wall, making it impossible to build the new trash chute (MDC).
• The Fix: Yme1 uses his scissors to cut down the number of these trucks. By slowing down the delivery traffic, he clears the outer wall so the trash chute can be built.
• The Experiment: When the researchers removed the trucks (deleted UPS2) in cells that also lacked the foreman, the trash chute started building again! This proved that the trucks were the main obstacle.

2. The Structural Scaffolding (The MICOS Complex)

Imagine a massive scaffolding structure (MICOS complex) that holds the inner and outer walls of the power plant together. It's very sturdy and keeps everything rigid.

• The Problem: This scaffolding is so strong that it physically prevents the outer wall from bending and forming a new compartment. It's like trying to build a new room in a house where the walls are made of steel beams.
• The Fix: Yme1 also cuts up parts of this scaffolding. By weakening the rigid structure, he allows the outer wall to become flexible enough to fold inward and create the trash chute.
• The Experiment: When the researchers removed the scaffolding (deleted MICOS) in cells without the foreman, the trash chute formed again!

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The "Double Trouble" Solution

Here is the coolest part of the story:

• Removing just the trucks (Ups) helped a little bit.
• Removing just the scaffolding (MICOS) helped a little bit.
• But if you removed BOTH the trucks and the scaffolding, the trash chute formed perfectly, even without the foreman (Yme1) being there!

This tells us that Yme1's main job is to clear both of these obstacles simultaneously. When the cell is stressed, Yme1 gets to work, cuts down the trucks, and weakens the scaffolding, finally giving the green light for the MDC to form.

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The "Green Light" Metaphor

You might ask, "Why doesn't Yme1 just cut everything all the time?"

The paper shows that Yme1 is like a security guard with a master key.

• Normally: The guard is there, doing minor maintenance (cutting a few old parts), but the "Green Light" for the big renovation isn't on.
• During Stress: When the cell is stressed (like when you add a drug called Rapamycin), the "Green Light" turns on. Now, Yme1 goes into overdrive. He cuts the trucks and the scaffolding aggressively, allowing the MDC to form.
• The Catch: Even if you force the guard to work overtime (overexpress Yme1), he can't build the trash chute if the city is in "lockdown mode" (lack of amino acids). The cell needs the right metabolic conditions to give the final permission.

Summary in One Sentence

The protein Yme1 acts as a molecular foreman who, during times of stress, uses its scissors to cut down specific "traffic" (lipid trucks) and "scaffolding" (structural proteins) that are blocking the cell from building a vital emergency trash chute (MDC) to keep the mitochondria healthy.

Technical Summary

1. Problem Statement

Mitochondrial-derived compartments (MDCs) are specialized, multilamellar structures that bud from the outer mitochondrial membrane (OMM) in response to metabolic and proteotoxic stress. They function to selectively sequester hydrophobic OMM proteins (such as the import receptor Tom70) to maintain cellular homeostasis. While previous research established that MDC biogenesis relies on specific lipid compositions (particularly reductions in phosphatidylethanolamine, PE) and occurs at membrane contact sites, the molecular mechanisms translating stress signals into the physical remodeling of the OMM remained poorly defined. Specifically, it was unclear how the cell regulates the constraints that normally prevent spontaneous MDC formation.

2. Methodology

The authors employed a multi-faceted approach combining genetics, quantitative proteomics, lipidomics, and live-cell imaging in Saccharomyces cerevisiae:

• Genetic Manipulation: The study utilized deletion mutants (e.g., yme1Δ, ups2Δ, MICOSΔ), catalytically inactive mutants (yme1E541Q), and overexpression strains. They also used suppressor mutants (e.g., rpt3-P215L) to decouple mitochondrial morphology defects from MDC formation.
• MDC Assays: MDC formation was quantified using fluorescence microscopy. Cells were stained with Tom70-GFP (OMM marker) and Tim50-mCherry (IMM marker). MDCs were identified as Tom70-positive, Tim50-negative puncta. Induction was achieved using Rapamycin (Rap), Concanamycin A (ConcA), or Cycloheximide (CHX).
• Quantitative Proteomics: Mitochondria were isolated from wild-type and yme1Δ cells under stress (Rap treatment). TMT-based mass spectrometry was used to identify differentially expressed proteins, focusing on substrates of the i-AAA protease Yme1.
• Lipidomics: Whole-cell lipid extraction followed by LC-MS (QTOF) was performed to measure phospholipid levels, specifically Phosphatidylethanolamine (PE), across different genotypes and treatments.
• Genetic Epistasis: Double and triple mutants (e.g., yme1Δ ups2Δ, mic60Δ ups2Δ yme1Δ) were constructed to test if disrupting specific pathways could bypass the requirement for Yme1.

3. Key Contributions and Results

A. Yme1 Proteolytic Activity is Essential for MDC Formation

• Loss of Function: Deletion of YME1 completely blocked MDC formation in response to Rap, ConcA, and CHX.
• Specificity: This defect was not a secondary consequence of the aberrant mitochondrial morphology or loss of mitochondrial DNA (mtDNA) typically seen in yme1Δ cells. Restoring morphology via rpt3-P215L or using rho⁰ cells did not rescue MDC formation.
• Catalytic Requirement: Expression of wild-type Yme1 rescued MDC formation in yme1Δ cells, but a catalytically inactive mutant (yme1E541Q) failed to do so. Conversely, overexpression of wild-type Yme1 (but not the inactive mutant) induced MDCs even in the absence of stress, confirming that proteolytic activity is the critical driver.

B. Identification of Yme1 Substrates via Proteomics

Quantitative proteomics revealed that in the absence of Yme1, specific protein groups accumulate under MDC-inducing conditions:

1. Ups Lipid Transfer Proteins: Ups1 and Ups2 (involved in phospholipid transport between membranes) were significantly elevated.
2. MICOS Complex: Components of the Mitochondrial Contact Site and Cristae Organizing System (Mic12, Mic19, Mic26, Mic27) were elevated.
3. Protein Import Machinery: Small TIM chaperones and Pam17 were also elevated.

C. The Ups Pathway Constrains MDC Formation

• Regulation: Yme1 normally degrades Ups2. In yme1Δ cells, Ups2 accumulates, and the stress-induced decline in cellular PE levels is blunted.
• Genetic Interaction: Deleting UPS2 in a yme1Δ background (yme1Δ ups2Δ) partially restored MDC formation (~20% of cells formed MDCs vs. 0% in yme1Δ alone). This suggests that Ups2 accumulation acts as a constraint on MDC biogenesis, likely by maintaining PE levels that inhibit membrane curvature.

D. The MICOS Complex Constrains MDC Formation

• Regulation: MICOS components accumulate in yme1Δ cells.
• Genetic Interaction: Complete deletion of the MICOS complex (MICOSΔ) caused constitutive MDC formation (~35% of untreated cells).
• Partial Rescue: Deleting MICOS in a yme1Δ background (yme1Δ MICOSΔ) partially restored MDC formation (~10% of cells).
• Synthetic Lethality: A mic60Δ yme1Δ double mutant was synthetically lethal, indicating a strong functional interdependence between Yme1 and MICOS.

E. Combined Disruption Bypasses Yme1

• Triple Mutant Analysis: While a full triple mutant was inviable, the authors tested mic60Δ ups2Δ yme1Δ. This strain showed substantial restoration of MDC formation (~45% of cells upon Rap treatment), effectively bypassing the requirement for Yme1.
• Conclusion: This demonstrates that Yme1 promotes MDC formation by coordinately relieving constraints imposed by both lipid transfer (Ups pathway) and membrane architecture (MICOS).

F. Metabolic Gating

• Yme1 overexpression drives MDC formation only in nutrient-rich media. In minimal media lacking amino acids, Yme1 overexpression fails to induce MDCs. This indicates that while Yme1 removes structural constraints, metabolic signals are required to gate the final activation of the pathway.

4. Significance and Model

The paper proposes a unified model for MDC biogenesis:

1. Basal State: Yme1 constitutively turns over Ups proteins and MICOS components to maintain mitochondrial homeostasis, keeping MDC formation repressed.
2. Stress Response: Metabolic or proteotoxic stress amplifies Yme1 activity (or substrate susceptibility).
3. Relief of Constraints: Enhanced Yme1 proteolysis reduces Ups2 levels (altering lipid composition, specifically lowering PE) and disrupts MICOS organization.
4. Biogenesis: The removal of these two distinct constraints (lipid composition and membrane architecture) permits the OMM to undergo the extensive invagination and elongation required to form MDCs.

Significance:

• Mechanistic Insight: This study identifies the first specific protease (Yme1) responsible for initiating MDC biogenesis, linking proteostasis directly to membrane remodeling.
• Dual Regulation: It reveals that MDC formation is not controlled by a single factor but requires the coordinated dismantling of both lipid transport systems and structural scaffolding complexes.
• Conservation: Given the conservation of Yme1 (YME1L in mammals) and MICOS, these findings likely extend to mammalian mitochondrial quality control mechanisms, offering new targets for understanding mitochondrial diseases associated with membrane remodeling defects.