Drosophila Keap1 Proteins Assemble Nuclear Condensates in Response to Oxidative Stress

Study Background and Research Question

The Keap1-Nrf2 signaling pathway is a central regulator of cellular responses to oxidative and xenobiotic stress, with wide-reaching implications in disease and development. Under basal conditions, Keap1 (Kelch-like ECH-associated protein 1) binds Nrf2 (NF-E2–related factor 2) in the cytoplasm and targets it for proteasomal degradation, thereby limiting antioxidant gene expression. Upon exposure to oxidative stress, this interaction is disrupted, allowing Nrf2 to accumulate in the nucleus and activate detoxifying gene programs (source: paper). Recent studies indicate Keap1 family proteins—including Drosophila’s ortholog dKeap1—also localize to the nucleus, where they may regulate transcription directly and participate in developmental processes. However, the mechanisms governing nuclear functions of Keap1, particularly how it might exert gene regulatory roles or interact with chromatin, are not fully elucidated.

Key Innovation from the Reference Study

This work by Ji et al. establishes that Drosophila Keap1 (dKeap1) assembles into stable nuclear condensates in response to oxidative stress. The study identifies the structural requirements for condensate formation, including the necessity of both N- and C-terminal domains and the presence of intrinsically disordered regions (IDRs) within the C-terminal domain. Furthermore, the Kelch domain is demonstrated to suppress condensate formation, delineating a nuanced regulatory mechanism for dKeap1 localization and function (source: paper).

Methods and Experimental Design Insights

The authors employed a combination of live-cell fluorescence imaging, in vitro phase separation assays, and domain-mapping mutagenesis. After subjecting Drosophila cells to oxidative conditions, dKeap1 localization was monitored using fluorescently tagged constructs. Fluorescence recovery after photobleaching (FRAP) was used to assess the mobility and assembly state of dKeap1 within nuclear foci. To dissect structural requirements, the team generated deletion and domain-swap mutants, focusing on the N-terminal domain (NTD), C-terminal domain (CTD), and the Kelch and IDR regions. Recombinant fusion proteins were used in vitro to probe propensity for condensate formation and to validate the sufficiency of specific domains in driving phase separation (source: paper).

Protocol Parameters

• assay | live-cell fluorescence imaging | 37°C | applicable to Drosophila S2 cells | enables visualization of dynamic dKeap1 localization in situ | paper
• assay | in vitro condensate assembly | 4–25°C | applicable to recombinant dKeap1 CTD-YFP fusion proteins | recapitulates phase separation behavior and domain dependence | paper
• assay | FRAP (Fluorescence Recovery After Photobleaching) | NA | determination of protein mobility within nuclear condensates | quantifies stability and exchange rates of dKeap1 assemblies | paper
• assay | HRV 3C protease-based tag cleavage | 4°C | applicable to protein purification workflows for condensate biology | minimizes protease-induced denaturation, preserves native structure | workflow_recommendation

Core Findings and Why They Matter

The central discovery is that dKeap1 forms stable nuclear condensates upon oxidative stress, a process that is both domain- and IDR-dependent. FRAP analysis revealed that these foci exhibit reduced protein mobility, suggesting a transition from a diffusive state to stable assemblies. Deletion of either the NTD or CTD abrogated nuclear condensate formation, highlighting a cooperative requirement. Within the CTD, two IDRs were pinpointed as critical drivers of in vitro condensate formation. Notably, CTD-YFP fusion proteins alone were sufficient to form condensates, whereas deletion of the Kelch domain led to aberrant cytoplasmic foci even under non-stress conditions—indicating the Kelch domain acts as a negative regulator of condensate assembly (source: paper).

These findings expand our understanding of Keap1-Nrf2 pathway regulation beyond cytoplasmic sequestration and degradation of Nrf2. They provide mechanistic insight into how Keap1 family proteins may directly modulate gene expression in response to cellular redox state by assembling nuclear condensates that likely scaffold chromatin remodeling or transcriptional machinery. This is particularly relevant to development and disease, where precise gene regulatory programs are sensitive to oxidative cues.

Comparison with Existing Internal Articles

This study’s mechanistic clarity on dKeap1 nuclear condensation aligns closely with the internal article "Nuclear Condensate Assembly by Drosophila Keap1 in Oxidative Stress", which similarly emphasizes the dual-domain and IDR requirements for nuclear condensate formation. Both converge on the conclusion that condensate assembly is integral to dKeap1’s nuclear function and stress adaptation. Further, "Redefining Precision in Protein Purification: Mechanistic..." discusses how advances in condensate biology interface with the need for precise protein purification tools—such as HRV 3C protease-based tag cleavage—in workflows studying dynamic assemblies like nuclear condensates. The integration highlighted in these resources underscores the translational value of understanding condensate biology for both basic and applied molecular research.

Limitations and Transferability

While the domain requirements and phase separation properties of dKeap1 in Drosophila cells are rigorously established, there are limitations in direct transferability to mammalian systems. The biochemical context, presence of homologous domains, and post-translational modifications may differ, and functional consequences of condensate assembly could be context-dependent. The study also relies primarily on overexpressed, tagged constructs; thus, endogenous behaviors may diverge under physiological conditions. Nonetheless, the strong parallels with mammalian Keap1 localization and function suggest relevance to broader eukaryotic stress response pathways (source: paper).

Research Support Resources

For researchers investigating condensate biology or chromatin-associated complexes, robust protein purification and tag cleavage strategies are critical—especially when preserving native protein structure and minimizing proteolytic artifacts. PreScission Protease (PSP, SKU K1101) from APExBIO, which combines HRV 3C protease specificity with GST fusion for efficient tag removal at low temperatures, can be utilized to recover native proteins during studies of phase-separating domains (workflow_recommendation). This enables researchers to generate untagged constructs suitable for in vitro assays on condensate formation or interaction mapping. For further insights into precision tag cleavage and its integration with condensate research, see "PreScission Protease: Precision Tag Cleavage for Protein Purification".