Deep quantitative phosphoproteomics identifies non-canonical pH-sensitive yeast phosphorylation networks

This study employs deep quantitative phosphoproteomics to reveal a distinct, non-canonical acid-responsive phosphorylation network in yeast that operates independently of the traditional TORC2 and PKC pathways, identifying the membrane-tethered kinase Yck1 as a key mediator of acid stress signaling.

The Gist

Imagine a yeast cell as a tiny, bustling city. This city has a perimeter wall (the cell wall) and a security fence (the plasma membrane). Usually, when the city gets hit by a storm of acid (like vinegar), the first thing that happens is the fence and wall get damaged. The city's emergency response team, known as the TORC2 and PKC pathways, rushes to the scene to patch the holes and send out repair crews. This is the "canonical" (standard) way the city reacts to acid.

But this new study discovered something fascinating: The city has a secret, underground command center that reacts to the acid before the fence even breaks.

Here is the story of how the scientists found it, explained simply:

1. The Experiment: The "Sorbitol Shield"

The scientists wanted to see if there were reactions happening independent of the fence damage. They knew that if they gave the yeast cells a "shield" made of a sugar called sorbitol, it would stiffen the fence and stop the standard emergency team (TORC2/PKC) from reacting to the acid.

• Scenario A (No Shield): Acid hits $\rightarrow$ Fence breaks $\rightarrow$ Standard team reacts.
• Scenario B (With Shield): Acid hits $\rightarrow$ Fence stays strong (thanks to sorbitol) $\rightarrow$ Standard team stays calm.

By comparing these two scenarios, the scientists could filter out the "standard" reactions and look for the "secret" ones that happened even when the fence was safe.

2. The Discovery: A Hidden Network

They found over 1,000 specific "switches" (phosphorylation sites) on proteins that flipped on when the acid hit, regardless of whether the fence was broken or not.

Think of these switches like light switches in a house.

• The Standard Team: Only flips switches near the front door (membrane repair) when the door is kicked in.
• The Secret Team: Flips switches in the kitchen, the basement, and the garden immediately when the acid smell hits the air, even if the front door is perfectly fine.

3. Who is the Secret Boss? (Meet Yck1)

The scientists asked: "Who is flipping these secret switches?"
They found the culprit is a protein called Yck1.

• The Analogy: Imagine the standard team uses "heavy" tools (basophilic kinases) that need a lot of basic building blocks to work. Yck1, however, is a "lightweight" specialist (an acid-loving kinase) that thrives in the acidic environment itself.
• The Location: Yck1 is tethered to the city's security fence. It acts like a sentinel that senses the acid directly and starts a different kind of operation: organizing the city's internal delivery trucks (endocytosis) and managing the city's shape (cell polarity).

4. The "Code" of the Secret Switches

The scientists noticed that the secret switches (the ones Yck1 flips) have a very specific "password" or code.

• The Standard Code: Often involves "Proline" (a specific amino acid).
• The Secret Code: In the secret network, the code is rich in Acidic amino acids (like Aspartic acid) and Histidine.

The Histidine Mystery:
The study found that if a switch has a Histidine amino acid nearby, it gets flipped extra hard when the acid hits.

• Analogy: Think of Histidine as a magnet for protons (the acid particles). When the acid level rises, these magnets get "charged up," which instantly tells the Yck1 boss, "Hey, turn on the lights!" It's like a direct radio signal from the acid itself to the machinery.

5. What Does This Secret Network Do?

While the standard team is busy fixing the fence, this secret network (driven by Yck1) is busy:

• Reorganizing the delivery trucks: It tells the cell to grab things from the outside and bring them inside (endocytosis).
• Shaping the city: It helps the yeast cell decide where to grow its "bud" (how it reproduces).
• Managing the G-proteins: These are like the cell's internal messengers.

The Big Picture

For years, scientists thought yeast only reacted to acid by fixing its damaged skin. This paper shows that acid is also a direct messenger.

It's like a city that doesn't just react to a storm by fixing broken windows; it also has a smart home system that instantly turns on the heaters, locks the doors, and calls the fire department the moment the temperature drops, even if no windows are broken yet.

In short: The yeast cell has a "Plan B" signaling network that bypasses the usual damage-control team. It uses a specialized boss (Yck1) and a unique chemical code to reorganize the cell's internal traffic and shape the moment it senses acid, ensuring the cell survives and adapts instantly.

Technical Summary

1. Problem Statement

Cellular pH homeostasis is critical for proteome function, and intracellular acidification acts as a potent signal that rewires phosphorylation networks. In the yeast Saccharomyces cerevisiae, the canonical response to acid stress is understood to occur primarily through two pathways triggered by plasma membrane (PM) and cell wall (CW) stress:

1. TORC2-Ypk1 Pathway: Activated by membrane deformation.
2. Cell Wall Integrity (CWI) Pathway: Mediated by PKC1 and the MAPK cascade (Slt2).

However, previous observations indicated that a significant subset of proteins undergoes acid-dependent phosphorylation independently of these canonical pathways. The specific mechanisms, kinase drivers, and substrate motifs governing these non-canonical, TORC2/PKC-independent acid stress responses remained uncharacterized. The study aimed to systematically dissect the yeast phosphoproteome to distinguish between phosphorylation events driven by membrane/cell wall stress versus those driven directly by intracellular acidification.

2. Methodology

The authors employed a deep quantitative phosphoproteomics approach using SILAC (Stable Isotope Labeling by Amino acids in Cell culture) to decouple the effects of acid stress from membrane stress.

• Experimental Design (Triple SILAC):
  • Light (L): Untreated control cells.
  • Medium (M): Cells treated with 45 mM acetic acid for 15 minutes (induces acid stress + membrane/cell wall stress).
  • Heavy (H): Cells pre-equilibrated with 1M sorbitol (an osmotic protectant) before acid treatment. Sorbitol stabilizes the cell wall and plasma membrane, effectively blocking the activation of the TORC2 and CWI pathways while allowing intracellular acidification to occur.
• Data Acquisition:
  • Cells were lysed, digested with trypsin, and phosphopeptides were enriched.
  • Analysis was performed using two mass spectrometers: Q-Exactive Plus (Orbitrap) and timsTOF HT (PASEF mode) to maximize depth, identifying over 19,000 unique phosphosites.
  • Data was normalized against whole-proteome abundance changes to isolate phosphorylation-specific regulation.
• Classification Strategy:
  • Phosphosites were categorized based on their response ratios ($Log_2[M/L]$ for acid effect; $Log_2[M/H]$ for sorbitol effect).
  • Key Class of Interest: AiSu (Acid-induced / Sorbitol-unaffected). These sites increase upon acid treatment but remain unchanged by sorbitol, representing the non-canonical, membrane-stress-independent response.
  • Control Class: AiSr (Acid-induced / Sorbitol-repressed). These sites represent the canonical TORC2/PKC-dependent response.
• Bioinformatic Analysis:
  • Gene Ontology (GO) enrichment (g:Profiler, STRING).
  • Sequence motif analysis (pLogo, sliding window enrichment, pairwise amino acid enrichment).
  • Structural analysis using AlphaFold 3 pLDDT scores to assess disorder vs. order.
  • Kinase-substrate mapping using BioGRID and Position Weight Matrices (PWMs) to predict kinase drivers.

3. Key Contributions

1. Discovery of a Distinct Signaling Network: The study provides the first proteome-wide evidence of a large-scale phosphorylation network (approx. 1,767 sites) that responds to acid stress independently of the canonical TORC2 and CWI pathways.
2. Identification of Yck1 as a Primary Driver: Through motif scoring and network overlap analysis, the authors identify the membrane-tethered Casein Kinase 1 (Yck1) as the major kinase driving this non-canonical response.
3. Definition of Acidophilic vs. Basophilic Motifs: The paper establishes a clear dichotomy in substrate specificity:
   • Non-canonical (AiSu): Enriched in acidophilic motifs (aspartic acid/glutamic acid upstream).
   • Canonical (AiSr): Enriched in basophilic motifs (arginine/lysine upstream).
4. Novel Sequence Constraints:
   • Proline Repression: Phosphorylation of proline-rich sequences is quantitatively repressed by acid stress.
   • Histidine Sensitivity: Phosphosites with histidine at positions -4 or -3 show hyper-phosphorylation in the canonical (basophilic) response, suggesting protons act as second messengers to activate basophilic kinases.

4. Key Results

• Phosphoproteome Landscape:

  • Of 19,079 unique sites, 1,767 were classified as AiSu (non-canonical).
  • AiSu sites are significantly enriched in proteins associated with the plasma membrane, GTPase-mediated signaling, endocytosis, and cell polarity (specifically at the bud neck).
  • AiSr sites (canonical) are enriched in TOR complex processes, cell wall remodeling, and basophilic kinase substrates.

• Sequence and Structural Motifs:

  • AiSu Motif: Characterized by acidic residues (D/E) at positions -3 and -2 relative to the phosphosite. This matches the substrate preference of Yck1.
  • AiSr Motif: Characterized by basic residues (R/K) at -3 and -2, consistent with AGC kinases like Ypk1.
  • Structural Differences: AiSu sites are enriched in ordered structures (high AlphaFold pLDDT scores >70), whereas AiSr sites are predominantly in disordered regions.
  • Proline Effect: There is a strong anti-correlation between acid stress and phosphorylation of proline-containing sequences (anti-correlation with +1 Proline).

• Kinase Identification:

  • Yck1 was the only kinase significantly enriched for the Cat1 motif (defined by -3/-2 D and +1 N/L/I/V) found in the AiSu class.
  • Validation: Known Yck1 substrates (e.g., Pma1, Rim8) were confirmed to be acid-induced and sorbitol-unaffected in the dataset.
  • Cat2 Motif: Enriched in -3/-2 R, associated with basophilic kinases (Ypk1, Pkh1/2) and the canonical AiSr response.

• Functional Implications:

  • The non-canonical network (AiSu/Cat1) is heavily linked to ubiquitin-mediated endocytosis (specifically $\alpha$-arrestins), cytokinesis, and cell polarization.
  • The study suggests that Yck1 activation under acid stress is distinct from its role in glucose sensing, likely triggered directly by intracellular acidification rather than membrane stress.

5. Significance

• Paradigm Shift: This work challenges the prevailing view that acid stress signaling in yeast is solely a consequence of membrane/cell wall deformation. It reveals a direct, intracellular pH-sensing mechanism mediated by Yck1.
• Mechanistic Insight: It elucidates how protons can function as second messengers, potentially by protonating histidine residues to activate basophilic kinases (canonical) or by directly activating acidophilic kinases like Yck1 (non-canonical).
• Biological Relevance: The identified network connects pH stress to critical cellular processes like endocytosis, cell budding, and polarity, suggesting that pH regulation is a central coordinator of morphogenesis and membrane trafficking.
• Methodological Advance: The use of sorbitol pre-equilibration combined with deep SILAC phosphoproteomics provides a robust framework for dissecting stress responses where multiple signaling layers (membrane vs. cytosolic) overlap.

In conclusion, the paper uncovers a Yck1-mediated, non-canonical acid stress signaling pathway that operates independently of the TORC2/CWI axis, characterized by acidophilic substrates, ordered structural contexts, and a specific role in regulating endocytosis and cell polarity.