Tuberculosis protein PE5 hijacks host ubiquitin machinery

Tuberculosis remains a leading infectious killer worldwide and infects about one quarter of the global population. The bacterium Mycobacterium tuberculosis (Mtb) survives inside people by secreting effector proteins that interfere with the immune system. One very large family of these effectors, called the PE/PPE proteins, takes up roughly 10% of the Mtb genome and includes some 169 to 200 family members depending on the species. Because many PE/PPE proteins have repetitive sequences and very high GC content, studying their roles inside host cells has been difficult, and most family members remain poorly understood. One of these proteins, named PE5, is suspected to be important for Mtb disease but its actions inside host cells were not known. In new work led by Samantha L. Bell, researchers set out to define what PE5 does inside host cells. They asked whether PE5 interacts with human cellular machinery and how that interaction might help Mtb evade immune defenses. By focusing on direct molecular partners of PE5, Bell and colleagues aimed to open a window on how a single, previously uncharacterized mycobacterial effector could influence infection and pathogenesis.

To find proteins that bind PE5, the team used affinity purification coupled mass spectrometry (AP-MS), a method that isolates a protein of interest together with whatever binds to it and then identifies the partners by mass spectrometry. AP-MS revealed that PE5 associates with a host E3 ubiquitin ligase complex known as CRL2. The interaction is mediated through PE5's C-terminal Gly-Gly motif, which is recognized by the CRL2 substrate receptor KLHDC2. Surprisingly, PE5 itself does not become ubiquitinated after binding; instead, it is degraded upon being bound by KLHDC2. Binding of PE5 also increases autoubiquitination of KLHDC2, a self-targeting modification of the receptor, yet this change does not stop KLHDC2 from degrading its known substrates. Nor does it block CRL2 from degrading substrates that are handled by other substrate receptors. In short, AP-MS showed a specific PE5 — CRL2-KLHDC2 interaction: PE5 binds, is degraded, and triggers increased autoubiquitination of KLHDC2 without shutting down overall CRL2 activity.

These findings reveal a surprising connection between a mycobacterial effector and a host protein-degradation pathway. By showing that PE5 binds CRL2 via KLHDC2 and affects KLHDC2 autoubiquitination, the study raises new questions about how Mtb manipulates cell biology to its advantage. Because PE5 does not block CRL2 activity broadly, its role may be to subtly redirect or fine-tune host protein turnover in ways that favor bacterial survival, rather than to shut down the ubiquitin system outright. The results also implicate CRL2 complexes in cell-intrinsic responses to Mtb infection, suggesting a novel role for this ubiquitin ligase complex in host defense or in pathogen manipulation of host processes. More broadly, the work expands our view of how PE/PPE proteins could target innate immune responses and contributes to understanding how many uncharacterized PE/PPE family members might promote Mtb virulence through specific interactions with host machinery.