A Global Ligandability Map of Tryptoline Butynamide Stereoprobes Identifies Covalent Inhibitors of the Actin Maturation Protease ACTMAP

This study demonstrates that stereodefined butynamide probes, despite lower intrinsic reactivity than acrylamides, uniquely target the actin-maturing protease ACTMAP in human cancer cells, thereby validating butynamides as a differentiated electrophile for expanding covalent ligand discovery.

The Gist

Imagine your body is a bustling city made of billions of tiny workers called proteins. These workers keep everything running, from moving your muscles to copying your DNA. Sometimes, to fix a broken machine, you need a tool that can permanently lock a specific part in place. In the world of medicine, these tools are called drugs, and the "lock" mechanism is often a chemical bond that sticks to a specific protein.

For a long time, scientists have had a favorite "key" to open these locks: a chemical shape called an acrylamide. It's like a standard, reliable screwdriver that fits into many different screws (proteins) in the body. It's been so successful that it's the go-to tool for discovering new medicines.

The Big Idea: Trying a New Tool
In this paper, a team of scientists from Scripps Research and Vividion Therapeutics asked a simple question: "What if we tried a different kind of screwdriver?"

They created a new set of tools based on a chemical shape called a butynamide. Think of the acrylamide as a standard, slightly sharp screwdriver, and the butynamide as a slightly duller, more specialized one. Because it's "duller" (less reactive), it doesn't stick to everything it touches. Scientists thought this might be a bad thing, but they suspected it might actually be a good thing for finding very specific targets that the standard screwdriver misses.

The Experiment: A City-Wide Search
To test this, the researchers used a high-tech method called Activity-Based Protein Profiling (ABPP). Imagine sending out two different teams of detectives into a crowded city (human cancer cells):

1. Team A carries the standard "acrylamide" keys.
2. Team B carries the new "butynamide" keys.

Both teams are looking for proteins they can lock onto. They tag the proteins they find with a glowing light so they can be seen under a microscope.

The Surprise Findings
The results were fascinating:

• Team A (Acrylamide) found many proteins. They were like a generalist, sticking to hundreds of different workers.
• Team B (Butynamide) found fewer proteins overall, which makes sense because their keys were "duller." However, they found a special group of proteins that Team A completely ignored!

It turned out that the "duller" key was actually better at fitting into very specific, tricky pockets that the "sharper" key was too clumsy to enter. This proves that having different types of chemical keys allows us to find new targets we didn't know existed.

The Star Discovery: The "Actin Maturation" Fixer
Among the proteins that only the new "butynamide" key could lock onto was a protein called ACTMAP.

• What does ACTMAP do? Imagine a factory assembly line making "Actin," which is the muscle fiber that helps your cells move and hold their shape. When Actin is first made, it has a little "tag" on the end that needs to be cut off to make it work properly. ACTMAP is the scissor that cuts off this tag.
• What happened? When the scientists used their new butynamide key to lock onto ACTMAP, they effectively jammed the scissors.
• The Result: The "tags" on the Actin fibers weren't cut off. The cells were left with "unfinished" muscle fibers. This is a big deal because it stops the cells from moving and growing properly.

Why This Matters
This study is like discovering a new type of lockpick that opens doors the old ones couldn't.

1. New Targets: It shows that by changing just one small part of a drug molecule (swapping the "acrylamide" for "butynamide"), we can find entirely new proteins to target. This expands the map of "druggable" targets in the human body.
2. New Medicine: The discovery that we can jam the ACTMAP scissors offers a new way to potentially stop cancer cells, which rely heavily on moving and dividing.
3. Better Tools: The scientists created a "stereoprobe"—a tool that comes in left-handed and right-handed versions. Only one version worked, proving they can be incredibly precise. This helps them study exactly how these proteins work without accidentally breaking other things in the cell.

In a Nutshell
The scientists took a known chemical tool, tweaked its shape to make it slightly less aggressive, and discovered that this "gentler" tool could find and lock onto unique proteins that the original tool missed. One of these new locks was a protein essential for muscle cell health, opening the door to new ways of treating diseases like cancer. It's a reminder that sometimes, being a little less aggressive is the key to finding the most important things.

Technical Summary

1. Problem Statement

• Limitations of Current Covalent Probes: While Activity-Based Protein Profiling (ABPP) has successfully mapped covalent ligandability across the human proteome, these efforts have heavily relied on acrylamides as the primary cysteine-directed electrophile.
• The "Acrylamide Bias": Acrylamides are favored for their tempered reactivity and small size, but this reliance may obscure the ligandability of proteins that prefer alternative electrophiles or possess microenvironments that favor different chemical reactivities.
• Knowledge Gap: The global reactivity and ligandability profile of butynamides (an alternative cysteine-directed electrophile found in some drugs) remain largely unexplored. It is unknown whether butynamides engage a distinct set of proteins compared to acrylamides or if they offer advantages in targeting specific "undruggable" or cryptic pockets.

2. Methodology

The study employed a comparative chemical proteomics approach using stereoprobes (stereochemically defined electrophilic compounds) to map ligandability.

• Chemical Design:
  • A common tryptoline core was used to generate two sets of stereoprobes: acrylamides and butynamides.
  • Four stereoisomers were synthesized for each reactive group (e.g., 1S,3S; 1R,3S; 1R,3R; 1S,3R), creating "parent" (non-alkyne) competitors and "alkyne-modified" probes for detection.
• Proteomic Workflow:
  • Cell Models: Experiments were conducted in a pooled set of five human cancer cell lines (Ramos, 22Rv1, UACC-257, U-2 OS, Hep G2) to broaden proteomic coverage.
  • Gel-ABPP: Initial screening to visualize global proteomic reactivity and stereoselectivity.
  • Protein-Directed ABPP (Mass Spectrometry):
    • Cells were pre-treated with parent stereoprobes (competitors) followed by alkyne-modified probes.
    • Proteins were enriched via CuAAC (copper-catalyzed azide-alkyne cycloaddition) to biotin-azide, digested, and analyzed via TMT (Tandem Mass Tag) multiplexed MS.
  • Cross-Competition Analysis: A novel workflow compared the blockade of alkyne-probe enrichment by parent acrylamides vs. parent butynamides to categorize targets as:
    1. Butynamide-preferring
    2. Acrylamide-preferring
    3. Shared
  • Validation: Hits were validated using recombinant protein expression in HEK293T cells, site-directed mutagenesis (Cysteine to Alanine/Serine), gel-ABPP, Western blotting, and Cryo-EM/AlphaFold2 structural modeling.

3. Key Contributions

• Global Ligandability Map: The first comprehensive comparison of acrylamide vs. butynamide reactivity across the human proteome, revealing that reactive group identity significantly dictates protein engagement.
• Discovery of ACTMAP Inhibitors: Identification of C19orf54/ACTMAP as a highly specific target for (1S, 3R)-tryptoline butynamides, which are inert toward the corresponding acrylamides.
• Mechanistic Insight into Degradation: Demonstration that covalent binding of butynamides to ACTMAP triggers proteasome-dependent degradation of the protease, leading to the accumulation of immature actin.
• Expansion of Chemical Space: Validation that butynamides, despite lower intrinsic electrophilicity, can access unique protein targets (including transcriptional regulators and scaffolding proteins) that acrylamides miss or engage poorly.

4. Key Results

A. Comparative Reactivity and Ligandability

• Attenuated Reactivity: Tryptoline butynamides exhibited lower intrinsic reactivity (measured by glutathione consumption) and lower overall proteomic reactivity compared to acrylamides.
• Distinct Target Profiles: Despite lower reactivity, butynamides preferentially engaged 20 unique proteins that were not significantly liganded by acrylamides. Conversely, 52 proteins were acrylamide-preferring.
• Avoidance of Toxicity: Unlike acrylamides, which stereoselectively engaged the spliceosome factor SF3B1 (causing anti-proliferative effects), the corresponding butynamides did not engage SF3B1, suggesting a potential safety advantage.

B. Characterization of Butynamide-Preferring Proteins

• AGPS (Alkyldihydroxyacetonephosphate Synthase):
  • Preferentially liganded by (1S, 3S)-butynamide.
  • Covalent modification occurs at C190, located near the FAD-binding site but distal to known inhibitor sites, suggesting potential for allosteric modulation.
• HELLS (Lymphoid-Specific Helicase):
  • Preferentially liganded by (1R, 3R) and (1R, 3S)-butynamides.
  • Modification occurs at C364, a residue located near the DNA-protein interaction surface, identified via Lys-C digestion (as it was missed by trypsin due to peptide size).
• ACTMAP (C19orf54):
  • Highly Selective: Showed robust stereoselective reactivity with (1R, 3S)-butynamide (WX-02-570/WX-02-623) but negligible reactivity with the matched acrylamide.
  • Target Site: Covalent modification occurs at the catalytic nucleophile C132.
  • Functional Consequence: Inhibition of ACTMAP prevents the N-terminal processing of $\beta$- and $\gamma$-actin. This leads to the accumulation of immature, N-acetylated actin in cancer cells.
  • Proteasomal Degradation: Covalent binding of the butynamide to ACTMAP induces a conformational change that promotes binding to chaperones (HSP90, CCT2) and subsequent proteasome-dependent degradation of ACTMAP. This degradation is stereoselective and blocked by the proteasome inhibitor MG132.

C. Structural Insights

• Docking: AlphaFold2-based docking confirmed that the active enantiomer (WX-02-623) positions its electrophilic carbon within 5.16 Å of the C132 sulfur atom, while the inactive enantiomer fails to achieve this geometry.
• Cysteine Accessibility: The study highlighted that cysteine-directed ABPP can miss specific sites if the resulting tryptic peptides are too small or large, necessitating alternative proteases (like Lys-C) for full coverage.

5. Significance

• Chemical Probe Discovery: This work establishes butynamides as a differentiated electrophile class for chemical probe discovery. They offer a strategy to target proteins that are inaccessible to acrylamides, expanding the "ligandable proteome."
• ACTMAP as a Therapeutic Target: The identification of potent, stereoselective covalent inhibitors of ACTMAP provides a new chemical tool to study actin maturation. Since ACTMAP knockout mice exhibit muscle weakness due to immature actin accumulation, these inhibitors could be used to model this phenotype or explore the role of actin dynamics in cancer cell migration and proliferation.
• Mechanism of Action: The finding that covalent modification can trigger target degradation (via chaperone recruitment and the proteasome) rather than just inhibition adds a new layer of complexity to the design of covalent drugs.
• Methodological Advancement: The use of pooled cell lines for protein-directed ABPP successfully maintained sensitivity for detecting low-abundance or cell-line-restricted proteins, validating this approach for broad ligandability mapping.

In summary, the paper demonstrates that diversifying the reactive group in covalent probes (from acrylamide to butynamide) reveals a distinct landscape of protein interactions, leading to the discovery of specific inhibitors for ACTMAP and providing a robust framework for exploring the ligandability of the human proteome beyond traditional chemical scaffolds.