⚗️ biochemistry

Identification and targeting of a novel drug pocket in the tumour angiogenesis regulator RhoJ

This study identifies a unique N-terminal allosteric pocket in the tumour angiogenesis regulator RhoJ, demonstrates that the antifungal drug terconazole binds to this site to disrupt its interaction with PAK3, and validates this pocket as a promising target for developing selective anti-angiogenic therapies.

Original authors: Chen, W., Shoemark, D. K., Corey, R. A., Andrienko, T. N., Mellor, H.

Published 2026-10-20

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CC BY 4.0

Original authors: Chen, W., Shoemark, D. K., Corey, R. A., Andrienko, T. N., Mellor, H.

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 ⚕️ This is an AI-generated explanation of a preprint that has not been peer-reviewed.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

Inside the human body, a vast network of blood vessels constantly delivers oxygen and nutrients to every cell. When this system works correctly, it maintains health, but when it goes wrong, it can fuel the growth of cancer. Tumors are greedy; as they grow, their centers run out of oxygen, triggering a desperate signal to build new blood vessels. This process, known as angiogenesis, allows the tumor to steal nutrients and spread to other parts of the body. For decades, doctors have tried to stop this by blocking the signals that tell blood vessels to grow, but these treatments often have limits. They can be difficult for the body to absorb, and tumors eventually learn to bypass the specific blockades. Scientists are now looking for new ways to cut off a tumor's supply line by targeting the tiny molecular machines inside cells that control this growth. One such machine is a protein called RhoJ, which acts as a master switch for building new blood vessels in the lining of blood vessels. The challenge has been that RhoJ looks almost identical to another protein, Cdc42, which performs essential, life-sustaining jobs in every cell of the body. If a drug blocks RhoJ but also accidentally blocks Cdc42, it would be toxic to the patient. Finding a way to stop the cancer-fueling protein without harming the healthy one has been a major hurdle in drug design.

Researchers at the University of Bristol have found a potential solution by looking at a small, unique feature of the RhoJ protein that its look-alike partner lacks. While RhoJ and Cdc42 share a nearly identical core structure, RhoJ has a small extension at its beginning, like a tiny tail made of a specific sequence of building blocks. The team used powerful computer simulations to watch how these proteins behave when they interact with their partners. They discovered that when RhoJ connects to a specific signaling partner called PAK3, this unique tail folds into a distinct pocket or hollow space. This pocket does not exist in Cdc42, making it a perfect target for a drug that wants to be specific. The simulations showed that this pocket is stable and plays a significant role in holding the two proteins together. By focusing on this unique nook, the researchers hoped to find a molecule that could slip inside and jam the mechanism, stopping the protein from doing its job without touching the similar, essential proteins in the rest of the body.

To find a drug that fits this pocket, the team performed a virtual search through a library of over 1,500 medicines already approved by regulators for other uses. They tested how well each of these existing drugs might fit into the newly discovered pocket on the RhoJ protein. One compound stood out: terconazole, an antifungal medication commonly used to treat yeast infections. The computer models suggested that terconazole could nestle comfortably into the RhoJ pocket, effectively blocking the spot where the protein normally grabs its partner. To prove this was not just a computer prediction, the scientists built a test system inside living human cells. They created a glowing sensor that would light up when RhoJ and its partner PAK3 connected. When they added terconazole to these cells, the glow dimmed significantly, showing that the drug had successfully disrupted the connection between the two proteins. The drug worked in a dose-dependent way, meaning higher amounts of the drug caused a stronger disruption. Crucially, when they tested the same drug against the look-alike protein Cdc42, it had no effect. The drug ignored the essential protein and only targeted the cancer-related one.

The study confirms that this unique tail on the RhoJ protein creates a specific opening that can be targeted by small molecules. While the drug they found, terconazole, is not yet a perfect medicine—it requires a relatively high concentration to work and is primarily held in place by the way it fits into the oily parts of the pocket rather than forming tight chemical bonds—it serves as a vital proof of concept. It demonstrates that it is possible to design a drug that distinguishes between two nearly identical proteins by targeting a small, unique feature. The researchers believe this approach opens the door for developing a new class of anti-cancer drugs that can stop tumor blood vessel growth with high precision, avoiding the side effects that come from blocking essential cellular functions. By focusing on these small, unique structural differences, scientists may finally be able to outsmart the body's own defenses against cancer in a way that was previously thought too difficult to achieve.

Technical Summary: Identification and Targeting of a Novel Drug Pocket in the Tumour Angiogenesis Regulator RhoJ

Problem Statement
The Rho GTPase RhoJ is a critical regulator of tumour angiogenesis, making it an attractive target for anti-cancer therapeutics. However, developing specific small-molecule inhibitors for RhoJ is challenging due to its high sequence conservation (63–65% identity) with the ubiquitously expressed and essential GTPase Cdc42. Global inhibition of Cdc42 would likely cause prohibitive toxicity. While previous efforts have identified pockets on Cdc42, dual-targeting compounds remain a risk. The authors posit that RhoJ possesses unique structural features absent in Cdc42—specifically an N-terminal extension (residues Asn2–Lys19)—that could serve as a basis for selective drug discovery.

Methodology
The study employed a multi-disciplinary approach combining computational modeling, virtual screening, and cell-based biochemical assays:

Molecular Dynamics (MD) Simulations: The authors generated homology models of the activated RhoJ (Mg2+-GppCp) in complex with the p21-binding domain (PBD) of PAK3, the predominant RhoJ effector in endothelial cells. All-atom MD simulations (500 ns) were performed using GROMACS to analyze the conformational behavior of the RhoJ N-terminal extension. Membrane-associated systems were also modeled to account for lipidation (palmitoylation and farnesylation).
Computational Analysis: Computational alanine scanning (CAS) using the Bristol University Docking Engine (BUDE) was used to identify energetic hotspots at the RhoJ:PAK3 interface. Solvation free-energy gains ( $\Delta_iG$ ) were calculated using PDBePISA.
Structure-Based Virtual Screening (SBVS): A library of 1,537 FDA-approved drugs was docked into the identified RhoJ N-terminal pocket using an ensemble of five representative frames from the stable MD trajectory. Binding energies were averaged to rank candidates.
Cell-Based Biosensor Development: An enhanced bioluminescence resonance energy transfer (eBRET2) biosensor was engineered in HEK293T cells to monitor the RhoJ:PAK3 interaction in live cells. This involved fusing RhoJ to RLuc8 and the PBD of PAK3 to GFP2.
Experimental Validation: Top virtual screening hits were tested in the eBRET2 assay to assess their ability to disrupt the RhoJ:PAK3 interaction. Specificity was validated against Cdc42 using a G-LISA binding assay.

Key Contributions and Results

Discovery of a Novel Allosteric Pocket: MD simulations revealed that the unique N-terminal extension of RhoJ (residues 2–19) is not disordered but folds into a defined conformation upon binding to PAK3. This folding creates a novel allosteric pocket at the RhoJ:PAK3 interface that is structurally absent in Cdc42.
Structural Specificity: The pocket formation was highly reproducible and stable specifically in the RhoJ:PAK3 complex. Simulations with other effectors (PAK1, N-WASP, BORG3) showed that this stable pocket formation did not occur or was less stable, suggesting the N-terminus contributes to effector selectivity by stabilizing the RhoJ:PAK3 complex geometry rather than merely increasing global affinity.
Energetic Contribution: Computational alanine scanning indicated that the RhoJ N-terminus contributes approximately 19.2% of the total binding energy to the RhoJ:PAK3 interaction. In membrane-embedded models, this region accounted for ~14.0% of the total interface area.
Identification of Terconazole: Virtual screening of FDA-approved drugs identified the antifungal agent terconazole as a top candidate. Docking studies showed terconazole occupies the N-terminal pocket, sharing critical contacts with the RhoJ:PAK3 interface.
Experimental Validation:
- The optimized eBRET2 biosensor successfully detected the RhoJ:PAK3 interaction.
- Terconazole disrupted the RhoJ:PAK3 interaction in a dose-dependent manner with an apparent $IC_{50}$ of 101 $\mu$ M.
- Crucially, terconazole showed no effect on Cdc42 binding to PAK1 at concentrations up to 100 $\mu$ M, demonstrating selectivity for RhoJ over Cdc42.

Significance and Claims
The paper claims to provide a "proof of concept" for the specific targeting of RhoJ in the development of novel anti-angiogenic drugs. The authors argue that by targeting the RhoJ-specific N-terminal extension, it is possible to disrupt tumour angiogenesis without cross-reacting with the conserved, housekeeping pathways of Cdc42.

While the authors acknowledge that terconazole is a relatively weak binder ( $IC_{50}$ 101 $\mu$ M) and primarily relies on hydrophobic contacts rather than specific hydrogen bonds, they emphasize its value as a lead scaffold. The study demonstrates that lineage-specific extensions in closely related small GTPases create exploitable surfaces for selective drug discovery. The identification of this pocket offers a rational path toward a new class of inhibitors that act downstream of multiple pro-angiogenic signals, potentially overcoming the limitations of current antibody-based therapies that target upstream factors like VEGF.

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