Drosophila Keap1 Nuclear Condensates in Oxidative Stress Response

Study Background and Research Question

The Keap1-Nrf2 signaling axis is a central orchestrator of cellular defense against oxidative and xenobiotic stress, with broad relevance to cancer, neurodegeneration, and metabolic disease. Under homeostatic conditions, Keap1 (Kelch-like ECH-associated protein 1) sequesters Nrf2 in the cytoplasm, targeting it for ubiquitin-mediated proteasomal degradation. Oxidative insults disrupt this interaction, allowing Nrf2 to accumulate in the nucleus and activate genes encoding antioxidant and detoxification enzymes. While the cytoplasmic regulatory function of Keap1 is well established, emerging evidence suggests that Keap1 family proteins, including the Drosophila ortholog dKeap1, also exert direct nuclear functions in gene regulation and chromatin organization. However, the molecular underpinnings and physiological significance of these nuclear activities have remained incompletely understood.

Key Innovation from the Reference Study

The referenced study (Antioxidants 2026, 15, 134) provides new mechanistic insight into the noncanonical nuclear roles of Keap1 proteins. Specifically, it demonstrates that dKeap1 translocates to the nucleus and assembles into stable nuclear condensates in Drosophila cells upon oxidative challenge. These biomolecular condensates—membraneless subnuclear compartments arising from liquid–liquid phase separation—are increasingly recognized as key regulators of gene expression, chromatin architecture, and cellular stress adaptation. The study further dissects the domain requirements for dKeap1 condensate formation, revealing the essential roles of both N- and C-terminal domains and the presence of intrinsically disordered regions (IDRs) that facilitate phase separation. This work establishes a direct mechanistic link between oxidative stress signaling and the assembly of nuclear condensates by Keap1 family proteins, extending the functional repertoire of this pathway far beyond cytoplasmic Nrf2 regulation.

Methods and Experimental Design Insights

The authors employed a combination of live-cell fluorescence microscopy, domain-deletion mutagenesis, and in vitro condensate reconstitution assays to interrogate dKeap1’s behavior and requirements for condensate formation. Drosophila stocks expressing fluorescently tagged dKeap1 variants enabled real-time visualization of subcellular localization dynamics in response to controlled oxidative treatment. Fluorescence recovery after photobleaching (FRAP) was used to assess the mobility and stability of dKeap1 within nuclear foci, providing evidence for condensate maturation. The study also leveraged domain truncations and fusion constructs—such as CTD-YFP fusions—to delineate the specific sequence elements mediating phase separation. Notably, deletion of the Kelch domain promoted robust cytoplasmic condensate formation, highlighting its negative regulatory role in condensate assembly. In vitro, purified dKeap1 CTD fusion proteins recapitulated condensate formation, supporting the sufficiency of IDRs in driving this process.

Protocol Parameters

• Oxidative Stress Induction: Drosophila cells were exposed to defined oxidative agents (e.g., paraquat) to trigger dKeap1 nuclear accumulation and condensate formation.
• Fluorescent Protein Tagging: dKeap1 constructs fused to YFP or GFP enabled direct observation of subcellular localization and foci assembly in live cells.
• Domain Dissection: Deletion or fusion of N-terminal, C-terminal, and Kelch domains allowed mapping of regions required for condensate formation and subcellular targeting.
• FRAP Analysis: Photobleaching of dKeap1 foci was used to quantify molecular exchange rates, supporting the interpretation of condensate stability and maturation.
• In Vitro Condensate Reconstitution: Recombinant CTD-YFP proteins were used to assess intrinsic phase separation capacity and IDR involvement.

Core Findings and Why They Matter

The study’s principal finding is that dKeap1 rapidly accumulates in the nucleus and forms discrete, stable nuclear condensates upon oxidative stress exposure (reference study). FRAP experiments revealed reduced mobility for dKeap1 within these condensates, consistent with phase-separated, dynamic assemblies. Both terminal domains of dKeap1 were required for condensate formation, and specifically, two IDRs within the C-terminal domain proved sufficient to drive condensate assembly in vitro. Deletion of the Kelch domain, however, led to aberrant cytoplasmic condensate formation, suggesting this domain normally restricts phase separation to appropriate nuclear contexts. These findings extend the paradigm of oxidative response regulation to include the spatial organization of nuclear signaling hubs by Keap1-family proteins, with direct implications for chromatin remodeling and transcriptional control during stress adaptation.

More broadly, this work situates dKeap1 among a growing class of nuclear regulators—including Mediator, RNA polymerase II, HP1α, and Polycomb group proteins—that utilize biomolecular condensates for the regulation of gene expression and chromatin state. The demonstration that IDRs in dKeap1 mediate phase separation aligns with established principles of condensate biology, reinforcing the concept that proteins with modular domain architectures and intrinsic disorder are uniquely poised to orchestrate dynamic nuclear compartments in response to environmental cues.

Comparison with Existing Internal Articles

Several internal resources complement and contextualize these findings. For example, "Drosophila Keap1 Assembles Nuclear Condensates in Oxidative Stress" and "Drosophila Keap1 Forms Nuclear Condensates Under Oxidative Stress" both emphasize the mechanistic requirement for dKeap1’s terminal domains and IDRs in condensate assembly, reinforcing the reference study's conclusions regarding the structural basis for phase separation. These resources further discuss the functional implications of condensate formation for chromatin regulation and stress-responsive transcription.

On the methodological front, internal articles such as "PreScission Protease: Next-Generation Precision for Fusion Protein Tag Cleavage" and "PreScission Protease: Precision Tag Cleavage for Advanced Workflows" provide scenario-driven guidance for the removal of fusion tags from recombinant proteins used in nuclear condensate studies. These workflows highlight the importance of using highly specific protein purification enzymes, such as HRV 3C protease (PreScission Protease), to preserve protein integrity and function during experimental reconstitution of phase-separating domains.

Limitations and Transferability

While the study establishes a clear mechanistic link between dKeap1 structure and nuclear condensate formation in Drosophila, several limitations should be considered. The work focuses on acute oxidative stress models in cultured Drosophila cells, and the physiological relevance of dKeap1 condensates in intact tissues or whole-animal models remains to be fully elucidated. Furthermore, the generalizability of these findings to mammalian Keap1 proteins, and the extent to which similar condensate dynamics regulate human oxidative stress responses, require direct comparative studies. The in vitro reconstitution assays, though informative, may not fully recapitulate the complex nuclear environment in vivo. Nonetheless, the identification of IDR-driven condensate assembly in dKeap1 provides a robust framework for exploring analogous mechanisms in other chromatin regulators and across species.

Why this cross-domain matters, maturity, and limitations

The link between oxidative stress signaling and nuclear condensate formation by Keap1 proteins bridges the traditionally distinct fields of stress response biology and biomolecular phase separation. This cross-domain insight expands our understanding of how cells spatially and temporally coordinate gene expression programs in response to environmental challenges. However, while the Drosophila model offers a tractable system for dissecting condensate biology, further work is needed to translate these findings to mammalian and human systems, including disease contexts.

Research Support Resources

For researchers aiming to replicate or extend these studies, precise manipulation of recombinant protein domains is crucial. The use of highly specific fusion protein tag cleavage enzymes, such as PreScission Protease (PSP) (SKU K1101), is recommended for workflows involving sensitive nuclear proteins and intrinsically disordered regions. PreScission Protease, a recombinant HRV 3C protease-GST fusion, enables efficient and site-specific removal of affinity tags at low temperatures, preserving native protein structure—an important consideration in phase separation experiments. Additional workflow advice on maximizing specificity and minimizing experimental artifacts can be found in internal scenario-driven articles, such as "Scenario-Driven Solutions with PreScission Protease (PSP)...". These resources provide protocols and troubleshooting guidance to ensure reproducible outcomes in advanced nuclear condensate and chromatin research.