Nuclear Condensate Assembly by Drosophila Keap1 in Oxidative Stress

Study Background and Research Question

The Keap1-Nrf2 pathway is a central regulator of cellular defense mechanisms, orchestrating transcriptional responses against oxidative and xenobiotic stress. In canonical models, cytoplasmic Keap1 (Kelch-like ECH-associated protein 1) represses Nrf2 (NF-E2–related factor 2) by targeting it for proteasomal degradation, while oxidative cues disrupt this interaction, enabling Nrf2 to translocate to the nucleus and activate a battery of antioxidant genes. However, emerging research suggests broader roles for Keap1 family proteins, including nuclear localization and direct modulation of gene expression. The molecular details of these nuclear functions and their interplay with canonical cytoplasmic signaling remain incompletely understood. The reference study (Antioxidants 2026, 15, 134) seeks to clarify how the Drosophila Keap1 ortholog (dKeap1) behaves in response to oxidative stress and whether it employs phase-separation mechanisms to organize nuclear functions.

Key Innovation from the Reference Study

The study's principal innovation is the demonstration that dKeap1 assembles into stable nuclear condensates following oxidative challenge. Unlike previous work that largely focused on cytoplasmic dynamics, this research shows that dKeap1 can accumulate in the nucleus and organize into discrete, nonmembranous foci. Through domain mapping and live-cell imaging, the authors reveal that both the N-terminal and C-terminal domains of dKeap1 are required for this process, with intrinsically disordered regions (IDRs) in the C-terminal domain playing a critical role in phase separation. This mechanistic insight advances the understanding of how Keap1-family proteins might directly regulate nuclear processes, including chromatin interaction and developmental gene expression, through condensate formation.

Methods and Experimental Design Insights

The experimental framework combines genetic, imaging, and biochemical approaches to dissect dKeap1 behavior. Key methods include:

• Genetic models: Drosophila stocks expressing various tagged or mutated forms of dKeap1 enabled precise dissection of functional domains.
• Fluorescence live imaging: Live-cell confocal microscopy was used to visualize dKeap1 subcellular localization and foci formation under basal and oxidative conditions.
• FRAP (Fluorescence Recovery After Photobleaching): Assessed the mobility of dKeap1 within nuclear condensates, revealing reduced diffusion and suggesting stable assembly.
• In vitro phase separation assays: Recombinant fusion proteins harboring the C-terminal domain (CTD-YFP) were tested for their capacity to form condensates, supporting the role of IDRs in phase separation.
• Domain deletion analysis: Systematic truncation and domain swaps (including deletion of the Kelch domain) clarified the structural contributions to condensate formation and subcellular localization.

This comprehensive approach allowed the authors to link specific structural features with functional outcomes relevant to nuclear organization and stress response.

Core Findings and Why They Matter

Central findings from the study include:

• Nuclear accumulation and condensate formation: Upon oxidative stress, dKeap1 transitions from diffuse nuclear localization to the assembly of stable nuclear foci. FRAP analysis confirms that dKeap1 within condensates exhibits reduced mobility, characteristic of biomolecular condensates formed by liquid–liquid phase separation.
• Structural requirements: Both the N-terminal and C-terminal domains of dKeap1 are necessary for condensate assembly. Notably, the C-terminal domain harbors IDRs that are sufficient to drive in vitro condensate formation, highlighting the role of intrinsic disorder in phase separation.
• Kelch domain as a regulatory element: Deletion of the Kelch domain results in robust cytoplasmic condensate formation even in the absence of stress, and in vitro data show that the Kelch domain suppresses condensate formation. This suggests a regulatory mechanism whereby the Kelch domain restrains inappropriate phase separation under basal conditions.
• Functional implications for transcriptional regulation: These findings support a model wherein dKeap1 nuclear condensates may scaffold the assembly of transcriptional complexes or chromatin-modifying machinery. Given prior evidence that dKeap1 binds chromatin and activates developmental genes, the ability to form condensates could facilitate localized control of gene expression during oxidative stress and development (reference study).

The implications extend to understanding how stress response signaling is integrated with chromatin organization and nuclear architecture. The findings also suggest that phase separation is a conserved regulatory mechanism among nuclear proteins involved in transcriptional control.

Comparison with Existing Internal Articles

The intersection of protease technology, condensate biology, and oxidative stress signaling is explored in several recent expert articles. For example, the article "Precision Proteolysis in Translational Research" discusses how advanced proteases like PreScission Protease (PSP) enable precise fusion protein tag cleavage, supporting workflows in condensate research and mechanistic studies of stress signaling. This complements the reference study by illustrating how accurate tag removal is vital for functional assays of proteins like dKeap1, which are sensitive to structural modifications. Similarly, "From Protein Purification to Nuclear Condensates" highlights the utility of HRV 3C protease-based tools for producing native protein constructs used in phase separation assays. These resources collectively reinforce the importance of optimized protein purification enzymes and low-temperature protease activity for research at the interface of protein structure, function, and nuclear organization.

Limitations and Transferability

While the evidence for dKeap1 nuclear condensate formation in Drosophila is compelling, several limitations should be considered. First, the study relies predominantly on Drosophila models and in vitro systems; the direct extrapolation to mammalian Keap1, or to human disease contexts, will require further validation. Additionally, the functional consequences of condensate formation—beyond association with chromatin—are not fully delineated. It remains to be established whether condensate assembly is required for specific gene activation or silencing events, and how these structures are dynamically regulated in vivo. The observed mechanisms may, however, provide a conceptual framework for exploring Keap1-Nrf2 signaling in other organisms and in pathological states such as cancer or neurodegeneration.

Protocol Parameters

• Oxidative stress induction: Apply defined oxidative agents (e.g., paraquat) to Drosophila cells or tissues to trigger nuclear translocation and condensate assembly of dKeap1.
• Live imaging: Use fluorescence-tagged dKeap1 constructs and confocal microscopy for real-time assessment of nuclear foci formation before and after oxidative challenge.
• FRAP analysis: Perform photobleaching and recovery measurements on nuclear dKeap1 condensates to quantify mobility and assess phase separation properties.
• Protein purification for in vitro assays: Express and purify dKeap1 domains using affinity tags; protease cleavage (e.g., HRV 3C-based enzymes) is recommended for removal of fusion tags to ensure protein integrity in condensate formation assays.

Research Support Resources

For experimental workflows investigating protein phase separation, nuclear condensate assembly, or fusion protein tag cleavage, researchers may benefit from robust protease tools. PreScission Protease (PSP) (SKU K1101) offers sequence-specific cleavage at the Gln-Gly bond and maintains activity at low temperatures, facilitating the recovery of native proteins for sensitive downstream assays. APExBIO’s PSP is particularly suited for preparing high-purity recombinant proteins for condensate research, as described in recent methodological articles. For further reading on technical applications and mechanistic rationale, see "PreScission Protease: Precision Tag Cleavage for Purification".