PreScission Protease for Condensate-Ready Proteins

Fusion-tag removal is often treated as the final housekeeping step in recombinant protein purification. For proteins that assemble into biomolecular condensates, however, cleavage can be an experimental control: an affinity tag may change solubility, effective valency, steric accessibility, or the apparent threshold for assembly. This distinction is especially important for Drosophila Keap1, whose stress-responsive nuclear foci and intrinsically disordered regions suggest that small changes in construct architecture can influence measurable behavior.

This article develops a workflow-centered perspective on PreScission Protease (PSP; K1101). Rather than presenting it only as a general precision protease, the discussion shows how HRV 3C protease cleavage can help separate purification effects from genuine sequence- and domain-dependent condensate phenotypes. It therefore extends beyond the broad tag-removal emphasis of the existing HRV 3C precision article, which focuses on product capabilities and routine workflow optimization, by focusing on assay interpretation and construct comparability.

Why tag removal matters in condensate assays

Affinity tags improve expression, capture, and solubility, but they are not biologically invisible. A fused domain can contribute additional interaction surfaces, alter the local charge distribution, restrict access to a disordered segment, or change the concentration at which a protein begins to form visible assemblies. Fluorescent reporters add a further structural element that may affect the behavior being measured. Consequently, a condensate observed with a tagged construct should not automatically be interpreted as a property of the unmodified protein.

The distinction is relevant to the 2026 study by Ji and colleagues, which found that Drosophila Keap1, or dKeap1, accumulates in nuclear foci after oxidative treatment and becomes less mobile within those foci by fluorescence recovery after photobleaching. Their experiments also identified intrinsically disordered regions in the C-terminal domain and showed that C-terminal-domain fluorescent fusion proteins formed condensates in vitro. These observations establish a strong connection between sequence architecture and assembly, but they also create a practical question: does the fusion partner merely report assembly, or does it help drive it?

A matched cleavage strategy can address that question. Researchers can compare the same recombinant construct before and after fusion protein tag cleavage, while keeping expression history and most purification variables constant. The comparison is not a substitute for genetic controls, but it is a useful way to test whether a fluorescent or affinity module is influencing the phenotype.

Mechanism and design logic of PreScission Protease

PSP is a recombinant fusion protease composed of human rhinovirus type 14 3C protease fused to glutathione S-transferase and produced in Escherichia coli. The catalytic component is commonly called HRV 3C protease. It recognizes the octapeptide sequence Leu-Glu-Val-Leu-Phe-Gln-Gly-Pro, written LEVLFQGP, and cleaves between glutamine and glycine. This defined protease cleavage at the Gln-Gly bond is the central reason the enzyme is useful for controlled removal of recombinant tags.

For construct planning, the cleavage sequence must be encoded at the junction between the tag and the target protein. The recovered N terminus is determined by the position of that site: because cleavage occurs after Gln, the downstream product begins with the Gly-Pro portion of the recognition sequence unless the construct has been designed to accommodate or eliminate those residues. Thus, PSP can support recovery of a native or near-native target, but the prescission protease cleavage site should be evaluated against the intended biochemical and structural definition of the protein.

The GST fusion architecture is also operationally relevant. It provides a soluble recombinant fusion protease format and may permit separation of the enzyme from a cleaved target through a compatible glutathione-affinity step, although that separation should be confirmed for each protein and buffer system. The enzyme is therefore more than a generic protein purification enzyme: its sequence selectivity and tag-compatible format can be integrated into a construct-specific purification plan.

Low temperature is particularly valuable when the target contains intrinsically disordered regions or is prone to aggregation. The product information specifies optimal operation at 4°C in specially formulated cleavage buffers, supporting low temperature protease activity during a step in which fragile proteins remain at elevated concentration. This condition can help preserve a soluble population, but it should not be interpreted as proof that a protein will retain its native condensate behavior after purification.

Reference insight: converting a condensate finding into assay controls

The most meaningful innovation in the dKeap1 study was not simply the observation of nuclear foci. It was the combination of cell imaging, mobility analysis, domain deletion, and in vitro reconstitution. According to the reference study by Ji et al. in Antioxidants, both the N-terminal and C-terminal domains were required for stress-associated foci formation, two intrinsically disordered regions were present in the C-terminal domain, and C-terminal-domain fusion proteins formed condensates in vitro. In contrast, deleting the Kelch domain produced robust cytoplasmic foci even under basal conditions, while in vitro experiments indicated that the Kelch domain suppresses condensate formation.

This design reveals an important assay principle: a domain can promote or restrain assembly depending on the molecular context. A construct containing only a disordered C-terminal region may be highly assembly-prone, whereas the intact protein may require stress-dependent regulation or relief of intramolecular restraint. The study therefore argues against using a single positive condensate signal as evidence for a universal phase-separation mechanism.

For practical assay decisions, the paper supports three controls. First, compare full-length and domain-restricted proteins rather than treating a truncated construct as a direct surrogate for the native factor. Second, evaluate mobility, not only punctum abundance; FRAP can distinguish a relatively dynamic assembly from a less mobile focus. Third, include tag-matched and tag-cleaved versions when feasible. PSP was not established as a reagent used in the reference study, so its application here is a workflow extension, not a claim about the original experimental procedure. Its value is that it can create a more rigorous comparison between a reporter-containing construct and a construct with the reporter or affinity tag removed.

A workflow for tag-cleaved dKeap1 and related proteins

A robust workflow begins during cloning rather than at the cleavage step. Place the LEVLFQGP recognition sequence between the affinity tag and the target, confirm that the junction is accessible, and decide whether the post-cleavage Gly-Pro residues are compatible with the intended protein. For a fluorescent construct, it may be useful to prepare parallel versions: one retaining the reporter for imaging and one designed for removal before a biochemical assembly assay.

1. Capture and initial quality check: Purify the fusion protein under conditions that preserve solubility, then assess enrichment and degradation before cleavage. An intact, soluble substrate is generally easier to interpret than a partially degraded one.
2. Cleavage as a controlled transition: Apply PSP in the recommended cleavage buffer at low temperature. Keep the substrate concentration, buffer composition, and handling time comparable across tag-retained and tag-cleaved samples whenever possible.
3. Protease and tag removal: Separate the released target from the GST-containing protease and liberated tag using a validated downstream purification step. Confirm the target identity and homogeneity rather than assuming that disappearance of the fusion band proves complete processing.
4. Functional comparison: Examine microscopy, solubility, and FRAP behavior using matched protein preparations. If cleavage changes assembly, test whether the effect is reproducible across full-length and domain-specific constructs.

Protocol Parameters

• Recognition sequence: Verify the engineered LEVLFQGP prescission protease cleavage site and its position relative to the target N terminus before expression.
• Cleavage temperature: Use 4°C in the specially formulated cleavage buffer described in the K1101 product information; treat this as the recommended starting condition rather than a replacement for substrate-specific optimization.
• Storage: Store PSP at −80°C and prepare aliquots to limit repeated freeze–thaw exposure. The product information reports that aliquots may be stored at −20°C for up to six months.
• Analytical confirmation: Verify cleavage by an orthogonal method such as SDS-PAGE, immunodetection, intact-mass analysis, or chromatography, then assess whether the recovered protein remains suitable for assembly assays.

How PSP compares with alternative tag-removal strategies

The principal advantage of PSP is sequence-defined processing at a compact, engineered site. In comparison, thrombin or factor Xa workflows may be less attractive when residual recognition sequences or nonspecific susceptibility could complicate a sensitive protein preparation. TEV protease is another highly selective option and may be preferable when its recognition sequence and buffer requirements fit the construct. The correct choice depends on site accessibility, target stability, downstream separation, and the acceptable N-terminal residue pattern.

PSP is not universal. It cannot be expected to process an unrelated recognition sequence, and a poorly exposed junction may remain incompletely cleaved even when the enzyme is active. Chemical cleavage approaches can also be useful in specialized cases, but harsher reaction environments may be difficult to reconcile with disordered, aggregation-prone, or multivalent proteins. For condensate work, the most informative comparison is usually not a race between enzymes; it is whether the selected method produces matched, structurally credible proteins with minimal unintended modification.

Applying the strategy to nuclear condensate biology

The dKeap1 findings provide a logical test system for this workflow. A purified C-terminal construct can be analyzed with and without its fusion partner to determine whether its assembly tendency persists after tag removal. A full-length construct can then be examined separately to test whether the C-terminal behavior is retained when the Kelch domain and N-terminal region are present. Finally, a Kelch-deleted construct can be used as a context-specific comparison because the reference study associated that deletion with constitutive cytoplasmic foci.

These experiments should preserve the distinction between three biological levels. Nuclear foci in living cells reflect localization, stress state, chromatin context, and cellular regulation. In vitro condensates reveal the assembly capacity of a purified protein under defined conditions. FRAP reports molecular mobility within an assembly. A tag-cleaved protein may improve biochemical relevance, but it does not convert an in vitro droplet into a nuclear condensate or establish that the same regulatory mechanism operates in both settings.

The linked article on nuclear condensate formation by Drosophila Keap1 emphasizes the biological discovery that oxidative stress promotes dKeap1 foci and that C-terminal disordered regions contribute to assembly. The present article builds on that finding from a different angle: it treats protease-mediated construct cleanup as a way to improve causal inference. In other words, the biological article explains what dKeap1 does, whereas a PSP-centered workflow helps test how much of the observed behavior belongs to dKeap1 itself rather than to its fusion architecture.

Why this cross-domain matters, maturity, and limitations

The bridge from protein purification to condensate biology is experimentally useful but still requires careful qualification. Fusion-tag cleavage is a mature biochemical operation; using it as a causal control for biomolecular condensation is an application-level extension. The reference study supports domain-dependent assembly, altered mobility, and a suppressive role for the Kelch domain, but it does not validate PSP as the cause of those observations.

Several limitations remain. Cleavage can alter solubility by exposing a new terminus, and the residual Gly-Pro sequence may itself matter in a narrowly engineered construct. Buffer changes during protease treatment can influence assembly independently of tag removal. Finally, removing a fluorescent reporter may prevent direct visualization, requiring a separate reporter-containing sample. These variables should be recorded and tested rather than hidden within a nominally identical purification protocol.

Conclusion

PreScission Protease is most valuable in condensate-oriented workflows when it is used as an experimental design tool rather than merely as a final purification reagent. Its HRV 3C protease activity, defined Gln-Gly cleavage, GST fusion format, and low-temperature operating profile can support controlled production of tag-cleaved proteins while helping researchers interrogate construct-dependent assembly.

For dKeap1, the key opportunity is to connect the reference study's domain and mobility findings with matched biochemical preparations. Comparing tag-retained and tag-cleaved proteins, while preserving appropriate full-length, truncated, and deletion controls, can sharpen conclusions about intrinsically disordered regions and domain-mediated restraint. The result is a more defensible path from recombinant expression to mechanistic interpretation of nuclear condensates.