Small Molecules Flip the Switch on a Staphylococcus aureus Protease, Opening a New Antibiotic Avenue

Antibiotic resistance keeps tightening its grip on hospitals worldwide, and few pathogens embody the crisis as starkly as Staphylococcus aureus. Methicillin-resistant strains of the bacterium emerge at high frequency, and the pipeline of genuinely new antibiotics has been thin for decades. Traditional targets such as protein synthesis and DNA replication have been mined so thoroughly that researchers are increasingly looking elsewhere for vulnerabilities. One of the most promising is not an enzyme that bacteria need to build something, but a machine they use to destroy things: the ClpC/ClpP protease, the bacterial equivalent of the proteasome that degrades damaged or unwanted proteins in our own cells.

A team led by Timo Jenne and Axel Mogk at Heidelberg University has now taken a significant step toward drugging this machine in S. aureus. Writing in the open-access journal iScience, the researchers report the first examples of small-molecule activators of the S. aureus ClpC unfoldase, identified through a high-throughput screen of roughly 110,000 chemically diverse compounds. The work demonstrates that the protease, normally held in a tightly regulated resting state, can be chemically forced into persistent, uncontrolled activity — a strategy that in principle turns the bacterium’s own protein-quality-control machinery against it.

The logic of the approach rests on how ClpC works. The protein is an AAA+ ATPase, a ring-forming molecular motor that, in its active hexameric state, unfolds substrate proteins and threads them through a central channel into the barrel-shaped serine peptidase ClpP, where they are chopped into fragments. In its basal state, however, ClpC is inactive: it forms an inactive resting state composed of two half-spirals, stabilized by head-to-head interactions of its coiled-coil middle domains and by contacts between its N-terminal domain (NTD) and the ATPase domains. Activation normally requires adapter proteins such as MecA or McsB, or the binding of substrates tagged with phosphoarginine, a degron recognized by dedicated pockets in the NTD. Crucially, the adapters themselves are degraded by the protease, creating a negative feedback loop that restricts activity to times when substrates are present.

Because deregulating this machinery is toxic — as shown by acyldepsipeptide antibiotics that hijack ClpP directly, and by natural cyclic peptides such as cyclomarin A that deregulate the mycobacterial ClpC1 homolog — the Heidelberg team reasoned that small molecules capable of locking S. aureus ClpC into its active state could similarly disrupt protein homeostasis. They screened their library at a single concentration of 40 micromolar, monitoring degradation of the fluorescent model substrate FITC-casein by ClpC and ClpP. Around 900 preliminary hits emerged, which the team prioritized by chemical clustering, keeping only compound families with multiple active analogs. Fifty-seven representative compounds were repurchased for rigorous biochemical validation.

About half of those failed to reproduce their initial activity and were discarded as false positives, but eight bona fide activators survived. The most potent, compound 8, boosted ClpC/ClpP proteolysis of FITC-casein nearly eightfold, approaching the activation achieved by the natural adapter MecA. Dose-response curves placed the compounds’ potencies in the low-to-moderate micromolar range, with EC50 values as low as 4.4 micromolar for compound 10. Importantly, the compounds activated degradation of a native substrate as well: the bacterial cell division protein FtsZ was broken down 1.7- to 4.2-fold faster in their presence, with compound 30 driving essentially complete FtsZ degradation within two hours. All eight compounds also stimulated ClpC’s ATPase activity, and the degree of ATPase stimulation correlated with proteolytic activation, indicating that the molecules work by overriding the protease’s built-in activity control rather than by an assay artifact.

Structural and biophysical experiments then revealed what the compounds do to the machine. Using an ATPase-deficient ClpC variant to trap complexes, analytical size-exclusion chromatography showed that compounds 8, 10, and 41 promote the formation of ClpC/ClpP assemblies. Negative-stain electron microscopy revealed that these assemblies are strikingly large and heterogeneous: instead of the canonical single-hexamer-on-ClpP complex seen with MecA, the compounds drive the formation of ring dimers, tetrahedral arrangements of four ClpC hexamers, and even larger networks of ClpC rings interconnected by ClpP, which offers two docking sites for unfoldase hexamers. These architectures mirror those previously observed with the toxic cyclic peptide cyclomarin A and with phosphoarginine-tagged substrates, suggesting that different deregulating ligands all funnel ClpC through a common activation pathway linked to higher-order assembly.

Where do the compounds bind? Every piece of evidence pointed to the N-terminal domain, the protease’s central allosteric hub. A ClpC variant lacking the NTD was completely unresponsive to all eight compounds, and adding an excess of isolated NTD sequestered the molecules and abolished their stimulatory effect. Computational docking with the AI-based tool DiffDock-L, followed by physics-based HADDOCK simulations, mapped the compounds onto two chemically ligandable regulatory sites: the hydrophobic substrate-binding groove, which is the target of all known ClpC1-directed cyclic peptides in mycobacteria, and the pArg1 pocket, which normally recognizes phosphoarginine degrons. Mutational analysis supported this split: compounds 10 and 41 lost their activity against a mutant disrupting the pArg1 site, while several groove-targeting compounds showed reduced stimulation of a mutant at Met92, a residue lining the groove.

The team went further, obtaining co-crystal structures of the NTD bound to compound 8 at 1.90 angstrom resolution. The structure revealed that only part of the ligand, a pyrazoline-containing fragment the authors call Cpd-8a, makes direct protein contacts, mostly hydrophobic, with just a single hydrogen bond to Gly4. Structure-activity studies with commercial derivatives confirmed the importance of this fragment, and superposition with the cryo-EM structure of the resting state suggested a mechanism: ligand binding reorients the protein’s N-terminal tail, which would sterically clash with the AAA1 domain and destabilize the inactive conformation. For compound 10, docking and derivative testing converged on the pArg1 pocket as the binding site, with predicted clashes between the ligand and the tip of the regulatory middle domain — the same region whose disruption by mutation unleashes autonomous proteolysis. Notably, this is the first demonstration that the pArg1 pocket can be engaged by classical small molecules, extending the site’s known repertoire beyond phosphoarginine-bearing ligands and BacPROTAC degraders.

There is, however, an important caveat. When the compounds were tested against live S. aureus, all eight impaired growth to some degree, and compound 10 fully prevented growth at 200 micromolar with a 90-fold drop in viability and a minimum inhibitory concentration of 64 micrograms per milliliter. Yet experiments with clpC and clpP knockout strains showed that this toxicity does not depend on ClpC/ClpP at all — the knockouts were inhibited just as strongly, or more so. The authors suggest that compound 10’s coumarin-derived aesculetin moiety, related to aminocoumarin antibiotics that inhibit DNA gyrase, may drive off-target effects, and they caution that low affinity, poor permeability, or unknown secondary binding sites could all contribute to the disconnect between potent in vitro activation and ClpC-independent cellular toxicity.

Even so, the study marks a conceptual advance. It establishes that S. aureus ClpC, like its mycobacterial counterpart, is chemically addressable, and it maps two distinct ligandable sites within the NTD whose engagement produces a defined functional outcome: sustained, adaptor-independent proteolysis. Unlike natural adapters, which are consumed in the process, the small molecules are not degraded, enabling longer-lasting protease activity — a fundamental difference between biological regulation and chemical deregulation with clear implications for manipulating bacterial proteostasis. The researchers also note that mycobacteria deploy decoy proteins, ClpC2 and ClpC3, that sequester cyclic peptides and protect ClpC1; no such shield is expected for compounds targeting S. aureus ClpC. With structural blueprints for compound 8 and compound 10 in hand, the path forward is structure-guided optimization: raising affinity, confirming direct binding, and engineering the cellular engagement needed to convert biochemical deregulation into genuine, ClpC-dependent antibacterial lethality.

Subject of Research: Small-molecule activation of the Staphylococcus aureus ClpC/ClpP AAA+ protease as a potential antibiotic strategy

Article Title: Small-molecule activators of the Staphylococcus aureus ClpC/ClpP AAA+ protease

Article References: Jenne, T., Viliuga, V., Uhrig, U., Jehle, B., Schwan, M., Kopp, J., Flemming, D., Seebach, E., Sinning, I., Bukau, B., & Mogk, A. (2026). Small-molecule activators of the Staphylococcus aureus ClpC/ClpP AAA+ protease. iScience, 29(10), Article 117738. https://doi.org/10.1016/j.isci.2026.117738

Image Credits: AI Generated

DOI: 10.1016/j.isci.2026.117738

Keywords: Staphylococcus aureus, ClpC/ClpP protease, AAA+ ATPase, antibiotic resistance, high-throughput screening, allosteric regulation, protein homeostasis, crystallography, molecular docking, acyldepsipeptides, MRSA, antibacterial drug discovery