In the relentless arms race between hosts and intracellular pathogens, the human immune system constantly evolves intricate defense mechanisms to thwart microbial invasion and proliferation. Among these defenses, cell-autonomous immunity serves as a potent frontline barrier, directly targeting pathogens that invade and reside within host cells. While the ubiquitination of intracellular bacteria and their subsequent degradation via the proteasome have long been recognized as critical antimicrobial strategies, the precise molecular mechanisms by which host cells eradicate these ubiquitinated bacteria have remained enigmatic. Novel research published in Nature Microbiology now sheds light on this mystery by unveiling the pivotal role of the host AAA-ATPase enzyme VCP/p97 in dismantling intracellular bacterial pathogens.
The study reveals that VCP/p97, a multifunctional ATPase known for its role in protein homeostasis and degradation, associates specifically with diverse cytosol-exposed ubiquitinated bacteria, including Streptococcus pneumoniae, Salmonella enterica serovar Typhimurium, and Streptococcus pyogenes. This interaction is not merely a passive binding; rather, the ATPase activity of the VCP/p97’s D2 domain actively drives the reduction of bacterial loads within infected cells. This discovery places VCP/p97 as a central player in the host’s intracellular antimicrobial arsenal, unlocking a new dimension of cellular defense previously unappreciated in the microbial immunology field.
Delving deeper, the researchers employed a multifaceted experimental approach integrating cutting-edge optical trap techniques, molecular dynamics simulations, in vitro reconstitution assays, and immunogold transmission electron microscopy (TEM). By leveraging optical trap technology, the team was able to measure the minute mechanical forces generated by p97 during its interaction with ubiquitinated bacterial substrates. These observations suggested that p97 applies physical pulling forces capable of disrupting bacterial surface components, a hypothesis further supported by detailed molecular dynamics simulations that provided a structural basis for this mechanical action at the atomic level.
The in vitro reconstitution studies revealed that p97 directly extracts ubiquitinated surface proteins, specifically BgaA and PspA, from the membranes of S. pneumoniae. These two proteins are integral to the bacterial cell membrane’s function and structural integrity. By forcibly removing these surface proteins, p97 initiates a catastrophic cascade of membrane destabilization, leading to extensive membrane lysis. Electron microscopy offered visual confirmation, revealing membrane breaches and cytosolic content leakage in bacteria exposed to active p97 complexes. This destruction culminates in the effective killing of the pathogen, thereby halting its intracellular proliferation.
Strikingly, the study underscores that the ATPase activity localized within the D2 domain of p97 is critical for this antibacterial function. Mutations or inhibitors targeting this domain abrogated the enzyme’s ability to reduce bacterial numbers, emphasizing the enzyme’s mechanical force generation as essential for bactericidal activity. This mechanistic insight distinguishes p97’s role from conventional proteasomal degradation, which primarily unfolds proteins for recycling, suggesting a unique function of p97 in lysing entire bacterial cells via membrane disruption.
Crucially, these findings extend beyond cellular models into whole organism physiology. Experimental infection models using mice illustrated that p97 activity significantly curtails S. pneumoniae proliferation in vivo. Animals with compromised p97 function exhibited heightened bacterial burdens and increased susceptibility to fatal sepsis, a severe systemic inflammatory response to bacterial invasion. These in vivo results position p97 not only as an essential molecular machine within cells but also as a critical determinant of host survival during bacterial infections.
By demonstrating the broad spectrum of bacterial targets affected—spanning Gram-positive S. pneumoniae and S. pyogenes as well as Gram-negative S. enterica—the researchers illustrate the conserved and versatile nature of p97’s antimicrobial role. This generalist activity indicates that p97 likely recognizes a ubiquitous molecular pattern, potentially the ubiquitin modifications decorating invading bacteria, thereby targeting multiple species without reliance on species-specific immune receptors.
These insights fundamentally shift our understanding of cell-autonomous immunity. Traditionally, ubiquitination marked bacterial components for proteasome recognition and degradation, yet how entire bacterial cells succumbed to this tag was a lingering unknown. This study elucidates that p97 acts as a mechanochemical agent that exploits ubiquitin signals to physically dismantle bacterial surface structures, leading to lethal membrane damage. This mechanism complements and reinforces classical proteasomal pathways, reflecting the multifaceted nature of intracellular bacterial clearance.
The authors also highlight intriguing implications for therapeutic development. Since p97 function hinges on targeted ATPase activity—and given the enzyme’s evolutionary conservation—pharmacological modulation of p97 could serve as a novel host-directed therapy to enhance antibacterial defense without directly targeting bacterial components, thereby reducing selective pressures for antibiotic resistance. Conversely, understanding how pathogens might evade or inhibit p97-mediated clearance could reveal new bacterial virulence strategies and inform countermeasures.
Furthermore, the work opens avenues to investigate whether variations in p97 activity or expression influence susceptibility to bacterial infections in human populations. Genetic polymorphisms or acquired dysfunctions of p97, implicated previously in neurodegenerative diseases and cancer, may also impact innate immunity, suggesting broader pathological connections. Future research might explore p97’s role across diverse cell types, tissues, and infectious contexts, as well as its interplay with autophagy, inflammation, and adaptive immunity.
The methodological synergy achieved—combining biophysical force measurements, high-resolution electron microscopy, computational simulations, and animal models—exemplifies an integrated systems biology approach that reveals complex molecular actions with physiological outcomes. This study underscores the power of cross-disciplinary research to unravel the hidden mechanics of host-pathogen interactions and uncovers a striking example of nature’s molecular ingenuity.
In conclusion, the identification of VCP/p97 as an innate immune effector that physically ruptures intracellular bacterial membranes via extraction of ubiquitinated surface proteins heralds a paradigm shift in antimicrobial biology. This discovery not only enriches the conceptual framework of cell-autonomous immunity but also spotlights a potential molecular target for innovative anti-infective strategies. As bacterial pathogens continue to evolve resistance to traditional antibiotics, harnessing or enhancing intrinsic host defenses like those mediated by p97 may prove indispensable in securing human health against persistent microbial threats.
This seminal research, led by Ghosh, Roy, Baid, and colleagues, pushes the frontier of microbiology and immunology, revealing the mechanical prowess by which host cells convert a post-translational ubiquitin signal into lethal force against invading bacteria. With a robust foundation of experimental evidence, this work sets the stage for translational efforts aiming to manipulate p97 activity in clinical settings, promising a new arsenal in the fight against lethal bacterial infections.
Subject of Research:
The study investigates the role of the host AAA-ATPase VCP/p97 in recognizing, mechanically disrupting, and killing ubiquitinated intracellular bacteria as an innate immune defense mechanism.
Article Title:
Host AAA-ATPase VCP/p97 lyses ubiquitinated intracellular bacteria as an innate antimicrobial defence
Article References:
Ghosh, S., Roy, S., Baid, N. et al. Host AAA-ATPase VCP/p97 lyses ubiquitinated intracellular bacteria as an innate antimicrobial defence. Nat Microbiol (2025). https://doi.org/10.1038/s41564-025-01984-y
Image Credits:
AI Generated
