CDK/mTOR-dependent phosphorylation of UBE2H restrains its charging with ubiquitin and regulates CTLH-dependent degradation

This study reveals that CDK- and mTOR-dependent phosphorylation of the E2 enzyme UBE2H at residues S3/S5 restricts its ubiquitin charging to dynamically regulate CTLH complex activity in response to cell cycle progression and nutrient status, thereby controlling the degradation of specific substrates like NEK9 and AAMP.

The Gist

Imagine your cell is a bustling, high-tech factory. Its job is to keep everything running smoothly, recycling old parts and building new ones. To do this, it has a specialized "trash crew" called the CTLH complex. This crew's job is to tag unwanted proteins with a little "trash can" sticker (called ubiquitin) so they can be destroyed.

But here's the problem: The trash crew needs a specific tool to pick up those stickers. That tool is a protein called UBE2H. Think of UBE2H as the delivery truck that carries the trash stickers from the warehouse to the trash crew.

This new research discovers a fascinating "off-switch" that controls how many delivery trucks are actually loaded and ready to work. Here is the story in simple terms:

1. The Traffic Light System (The Off-Switch)

The factory has two main managers:

• The Nutrient Manager (mTOR): Checks if there is enough food (nutrients) in the factory.
• The Clock Manager (CDK): Checks what time of day it is (specifically, when the cell is dividing).

When things are busy or when the cell is dividing (mitosis), these managers want to slow down the trash crew. Why? Because if the trash crew works too fast during a delicate operation like cell division, they might accidentally throw away important machinery, causing the factory to crash.

To stop the trash crew, the managers send a signal to the delivery trucks (UBE2H). They slap a "Do Not Load" sticker (a phosphate group) on the front of the trucks.

• Result: The trucks can't pick up the trash stickers. They sit empty and idle. The trash crew (CTLH) has nothing to do, so it stops working.

2. The "Hyper-Truck" Experiment

The scientists decided to test what happens if they build a "super-truck" that cannot get the "Do Not Load" sticker. They created a mutant version of UBE2H (called the AA mutant) that ignores the managers' signals.

What happened when they used the Hyper-Truck?

• The Factory Went Haywire: The trash crew never stopped. It worked 24/7, even when it shouldn't have.
• The Consequences:
  • Bad Timing: During cell division, the trash crew started throwing away essential parts (like the protein NEK9, which helps organize the cell's skeleton).
  • The Crash: The cells tried to divide but failed. They ended up with broken chromosomes and extra, tiny nuclei (micronuclei), which is like a factory trying to split in half but ending up with two halves that are missing crucial blueprints.
  • Starvation: The trash crew also ate up a protein called HMGCS1, which is needed to make fuel for the cell. Without this fuel, the cell couldn't build the necessary parts to divide properly.

3. The "DR" Tag (How the Trash Crew Knows What to Throw Away)

While studying the Hyper-Truck, the scientists also discovered how the trash crew recognizes what to throw away. They found a specific "ID tag" on the proteins.

• Imagine the trash crew has a scanner. They are looking for a very specific code at the very end of a protein: "DR" (or similar codes like "ER").
• They found that a protein called MKLN1 acts as the scanner. It grabs any protein with this "DR" tail and hands it to the trash crew.
• The scientists also found that another protein, FAM72A, has a similar tail. It can sneak in and block the scanner, acting like a "fake ID" that tricks the system into ignoring the real trash.

The Big Picture

This paper teaches us that cells don't just turn the trash crew on or off; they carefully control the delivery trucks (UBE2H) that supply the trash crew.

• When the cell is resting or eating: The managers (mTOR) put the "Do Not Load" sticker on the trucks to slow things down.
• When the cell is dividing: The clock manager (CDK) puts the sticker on to ensure the trash crew doesn't accidentally delete vital parts during the split.

If you remove this safety switch (like the Hyper-Truck), the cell becomes chaotic, makes mistakes during division, and eventually fails. It's a perfect example of how a cell uses simple "on/off" switches to keep its complex machinery running safely.

Technical Summary

1. Problem Statement

The ubiquitin-proteasome system relies on an enzymatic cascade (E1-E2-E3) to regulate protein homeostasis. While the regulation of E3 ligases and substrates is well-characterized, the in vivo regulation of E2 enzyme catalytic activity remains poorly understood. Specifically, the CTLH complex (C-terminal to LisH), a multi-subunit E3 ligase, is known to respond to cell cycle and nutrient cues, but the mechanism governing its overall catalytic capacity is unclear. The authors sought to determine how the CTLH complex's activity is dynamically tuned by physiological signals and whether its cognate E2 enzyme, UBE2H, serves as a regulatory node.

2. Methodology

The study employed a multidisciplinary approach combining quantitative proteomics, cell biology, and structural modeling:

• Quantitative Proteomics (TMT-MS): Used Xenopus egg extracts (interphase vs. mitosis) and mammalian cell lines (RPE1, HeLa, HEK293T) to profile ubiquitin-charged enzymes. An in vitro screening assay involved adding HA-tagged ubiquitin to extracts, immunodepleting with anti-HA beads, and analyzing the supernatant via Tandem Mass Tag (TMT) mass spectrometry to identify proteins depleted by ubiquitin conjugation.
• Biochemical Assays:
  • Thioester Detection: Used non-reducing SDS-PAGE and radiolabeled $^{35}$S-methionine to visualize E2~Ub thioester bonds.
  • Kinase Inhibition/Activation: Treated cells with CDK inhibitors (RO3306, Palbociclib), mTOR inhibitors (Torin1, Rapamycin), and phosphatase inhibitors (Okadaic acid). Used TSC2 knockout cells to model constitutive mTOR activation.
  • Mutagenesis: Generated UBE2H mutants: S3A/S5A (AA) (non-phosphorylatable, hyperactive), S3D/S5D (DD) (phosphomimetic, inactive), and various truncation/mutation constructs for substrates NEK9 and AAMP.
• Cell Biology:
  • Live-cell Imaging: Tracked mitotic duration, chromosome segregation, and micronucleus formation in cells expressing WT or mutant UBE2H.
  • Metabolic Rescue: Supplemented cells with geranylgeraniol (GGOH) and farnesol (FOH) to test if mitotic defects were due to impaired prenylation.
• Proteomics & Interaction Mapping: Performed global proteomic profiling of cells expressing WT vs. AA UBE2H. Used Immunoprecipitation-Mass Spectrometry (IP-MS) and AlphaFold 3 to map interactions between the CTLH substrate receptor MKLN1 and its substrates.

3. Key Contributions

1. Discovery of E2 Phosphoregulation: Identified that the E2 enzyme UBE2H is directly phosphorylated by CDK and mTOR at N-terminal serine residues (S3 and S5).
2. Mechanism of Inhibition: Demonstrated that this phosphorylation acts as a "brake," preventing UBE2H from being charged with ubiquitin by E1, thereby reducing the pool of active E2 available to the CTLH complex.
3. Context-Dependent Regulation: Established a "handoff" model where mTOR restrains UBE2H activity during interphase (nutrient-rich conditions), while CDK takes over to inactivate UBE2H during mitosis.
4. New Substrate Identification: Utilized a hyperactive UBE2H mutant to uncover novel CTLH substrates: the mitotic kinase NEK9 and AAMP (Angio-associated migratory cell protein).
5. Degron Definition: Defined a novel DR-like C-degron motif recognized by the CTLH substrate receptor MKLN1.

4. Key Results

A. UBE2H Charging is Cell-Cycle Dependent

• Mitotic Discharge: UBE2H is heavily charged with ubiquitin during interphase but becomes discharged (inactive) during mitosis in both Xenopus extracts and human cells.
• Phosphorylation Mechanism: The loss of charging in mitosis is driven by phosphorylation at S3 and S5.
  • The AA mutant (S3A/S5A) remains charged and active during mitosis.
  • The DD mutant (S3D/S5D) remains uncharged even in interphase.
  • CDK2/Cyclin A can phosphorylate UBE2H in vitro.

B. mTOR and CDK Converge on UBE2H

• Nutrient Signaling: In interphase, mTOR signaling maintains UBE2H phosphorylation, limiting its activity. Inhibition of mTOR (via Torin1 or starvation) increases UBE2H charging and enhances CTLH-dependent degradation of substrates like HMGCS1.
• Signaling Hierarchy: The AA mutant phenocopies mTOR inhibition; cells expressing AA are insensitive to further mTOR inhibition, placing UBE2H phosphorylation downstream of mTOR.
• The "Handoff": mTOR restrains UBE2H in interphase, while CDK inactivates it during mitosis, ensuring CTLH activity is suppressed when cell division is critical.

C. Phenotypic Consequences of Hyperactive UBE2H

• Mitotic Defects: Cells expressing the AA mutant exhibit:
  • Accelerated mitotic exit.
  • Increased micronucleus formation.
  • Reduced proliferation and colony formation.
• Metabolic Link: The defects are linked to the accelerated degradation of HMGCS1 (a key enzyme in the mevalonate pathway). This reduces the production of geranylgeraniol (GGOH), required for protein prenylation. Supplementation with GGOH, but not farnesol, rescued the mitotic defects.

D. Novel Substrates and Degron Recognition

• NEK9: Degraded specifically during mitosis in the presence of hyperactive UBE2H. Degradation requires the CTLH component MKLN1 and a C-terminal degron (residues 961–979).
• AAMP: Degraded in both interphase and mitosis. Requires MKLN1 and a C-terminal DR-like motif (specifically "VQRPDR").
  • Mutations in the "DR" tail (e.g., D433N, R434H) stabilize AAMP.
  • MKLN1 acts as the substrate receptor, binding the DR tail via its Kelch-repeat domain.
  • FAM72A, an adaptor for UNG2, competes with AAMP for MKLN1 binding via its own C-terminal "IR" tail, suggesting a competitive gating mechanism for substrate selection.

5. Significance

• Paradigm Shift in E2 Regulation: This study challenges the view that E2 enzymes are constitutively active "passive" carriers. It demonstrates that E2 charging can be dynamically regulated by phosphorylation to act as a primary control point for E3 ligase output.
• Integration of Signals: It reveals how the CTLH complex integrates cell cycle (CDK) and metabolic (mTOR) signals through a single E2 enzyme to modulate proteostasis.
• Biological Implications: The findings explain why mTOR inhibitors enhance CTLH-dependent turnover and highlight the importance of inactivating UBE2H during mitosis to prevent the degradation of essential proteins (like NEK9 and HMGCS1) required for accurate chromosome segregation and cell division.
• Therapeutic Potential: The identification of the DR-like C-degron and the MKLN1 receptor provides a new framework for understanding substrate specificity in the CTLH complex, which could be targeted in diseases involving metabolic dysregulation or cancer.

In summary, the paper establishes UBE2H phosphorylation as a critical regulatory switch that gates CTLH activity, ensuring that ubiquitin-mediated degradation is tightly coupled to the cell's metabolic state and cell cycle phase.