PreScission Protease for Condensate Biology

Protein tags are valuable during expression and capture, but they can become experimental variables when the purified protein is used for phase separation, chromatin binding, oligomerization, or fluorescence-based assays. PreScission Protease (PSP) offers a controlled way to remove a fusion tag before those downstream measurements. The recombinant fusion protease is an HRV 3C protease linked to GST and is designed to recognize the eight-residue sequence Leu-Glu-Val-Leu-Phe-Gln-Gly-Pro, cleaving between Gln and Gly.

That specificity makes PSP useful in workflows built around structurally sensitive proteins such as Drosophila Keap1. APExBIO supplies PSP as a sterile liquid for research use, with storage and handling practices intended to preserve activity. The central practical question is not simply whether a tag can be removed, but whether cleavage can be performed without compromising protein solubility, oligomeric state, or the biological readout.

Setup and principle: designing a clean cleavage experiment

In a typical construct, the affinity or solubility tag is positioned upstream of the target protein, followed by the prescission protease cleavage site. After expression and initial purification, PSP is added in a compatible cleavage buffer. Because the enzyme recognizes a defined sequence rather than cutting broadly across exposed peptide bonds, it supports targeted fusion protein tag cleavage while reducing the risk of nonspecific degradation.

The cleavage-site architecture should be checked before expression. The enzyme cuts between the glutamine and glycine in LEVLFQGP, so the residues remaining on the target depend on how the site is placed relative to the protein’s intended terminus. For proteins used in condensate assays, even a short residual peptide or an uncleaved tag may influence multivalent interactions. Confirm the expected post-cleavage sequence in silico and verify the product experimentally by SDS-PAGE, intact-mass analysis, or N-terminal sequencing when the terminus is mechanistically important.

PSP’s low-temperature operating profile is particularly relevant for unstable or aggregation-prone proteins. The PreScission Protease (PSP) product information identifies 4 °C as the optimal operating temperature and recommends storage at −80 °C; aliquots may be stored at −20 °C for up to six months. Keep both enzyme and substrate cold, but remember that lower temperature can slow reaction kinetics. A time course is therefore more informative than assuming that a short incubation will provide complete cleavage.

Workflow: from tagged expression product to assay-ready protein

1. Audit the construct. Confirm the orientation of tag, cleavage site, and target. Check for internal sequences that could complicate interpretation and calculate the predicted molecular masses of the uncleaved and cleaved species.
2. Purify before cleavage. Use the tag to enrich the fusion protein and remove as much host-cell protein, nucleic acid, and insoluble material as practical. A cleaner substrate usually makes cleavage monitoring easier and reduces nonspecific background.
3. Exchange into a compatible buffer. Desalt or dialyze the fusion protein into the recommended cleavage buffer before adding PSP. Avoid making untested buffer changes immediately before the reaction; salt, detergent, reducing conditions, and cofactors can affect both protease activity and the target protein’s state.
4. Run a small pilot matrix. Rather than committing an entire preparation, test several PSP-to-substrate ratios and incubation times at 4 °C. Include a no-enzyme control to distinguish proteolysis or precipitation caused by the substrate from cleavage caused by PSP.
5. Track the reaction analytically. Withdraw small samples at defined time points, quench or cool them consistently, and compare the disappearance of the fusion band with the appearance of the expected target band. If the target is fluorescent, remember that removing a fluorescent tag also removes the signal; use an independently labeled preparation or analyze an uncleaved matched control.
6. Separate products after cleavage. Remove the released tag and PSP using a compatible affinity step, size-exclusion chromatography, or another validated polishing method. PSP itself is GST-fused, so enzyme carryover should be considered when the downstream assay is sensitive to contaminating proteins or when glutathione-based purification is used.
7. Qualify the final protein. Measure concentration, inspect purity, and compare soluble recovery before and after cleavage. For condensate studies, evaluate turbidity, microscopy, and concentration-dependent assembly using the same buffer history across all conditions.

Protocol Parameters

• Temperature: Perform the cleavage reaction at 4 °C, with buffer, tubes, and chromatography fractions pre-chilled to 4 °C.
• Pilot concentration: For a 100 µL reaction, begin with 0.5 mg/mL fusion protein and test PSP at 0.005, 0.01, and 0.02 mg/mL, corresponding to approximate enzyme-to-substrate mass ratios of 1:100, 1:50, and 1:25.
• Time course: Collect analytical aliquots at 0, 2, 4, 8, and 16 h while maintaining the reaction at 4 °C.
• Gel monitoring: Load 2–5 µL of each pilot aliquot on SDS-PAGE, adjusting the volume so that uncleaved and cleaved samples receive comparable total protein.
• Enzyme storage: Maintain the working PSP aliquot at −80 °C for long-term preservation; when short-term convenience is required, use −20 °C storage for no longer than 6 months and avoid repeated freeze-thaw cycles.

The concentration and time points above are an executable starting screen, not a universal specification. Optimize them for the fusion protein’s mass, accessibility of the cleavage site, and stability. The product’s low-temperature protease activity is best treated as a workflow advantage that still requires empirical confirmation for every construct.

Key Innovation from the Reference Study

The reference study on Drosophila Keap1 nuclear condensates reported that dKeap1 accumulates in stable nuclear foci after oxidative treatment and showed, using FRAP, that dKeap1 becomes less mobile within those structures. Domain analysis identified a requirement for both the N-terminal and C-terminal regions, while two intrinsically disordered regions in the C-terminal domain promoted condensate formation in vitro. In contrast, deleting the Kelch domain produced robust cytoplasmic foci and supported the conclusion that the Kelch region restrains condensate assembly.

These findings translate into clear assay choices. First, purify a construct in which a removable tag does not obscure the dKeap1 C-terminal region or its disordered segments. Second, compare tagged and cleaved preparations when testing concentration-dependent assembly, because the tag may change effective valency, solubility, or steric accessibility. Third, use domain-deletion controls to separate genuine C-terminal assembly from nonspecific aggregation. PSP was not identified as a reagent in the reference study; its value here is as a practical protein purification enzyme for constructing cleaner, better-controlled follow-up experiments.

Why this cross-domain matters, maturity, and limitations

Moving from a protein purification workflow to nuclear-condensate biology is a method-development bridge, not evidence that PSP itself drives dKeap1 assembly. The reference study supports the biological importance of dKeap1 domains, IDRs, nuclear foci, and altered mobility. PSP can help test those findings with tag-reduced or tag-free preparations, but it cannot establish that a cleavage product is correctly folded, functional, or equivalent to endogenous dKeap1. Those questions require orthogonal controls, including matched uncleaved protein, domain mutants, concentration series, and independent measurements of protein integrity.

Advanced applications and comparative advantages

Condensate reconstitution: Cleave tags before microscopy or turbidity measurements when the experimental goal is to evaluate the target protein’s intrinsic assembly behavior. Retain an uncleaved fluorescent fusion as a localization control, but interpret differences cautiously because fluorophores and affinity tags can alter assembly thresholds. If visualization requires a fluorophore after cleavage, use a separate labeling strategy rather than assuming that the tag-removal reaction preserves fluorescence.

Structure-function comparisons: A common PSP site allows a series of dKeap1 constructs to be purified under comparable conditions. Cleavage can then be applied to full-length protein, C-terminal fragments, and Kelch-domain variants, helping distinguish tag effects from domain-dependent behavior. This is especially useful when the study’s conclusion depends on whether the C-terminal region promotes assembly or whether the Kelch region suppresses it.

Native-protein recovery: Compared with broad-spectrum proteases, HRV 3C protease offers a narrow recognition sequence and cleavage at a defined Gln-Gly bond. That precision can reduce unwanted damage to sensitive targets, although it does not guarantee complete reaction or eliminate the need for downstream polishing. The GST fusion format also supports practical enzyme handling, while requiring investigators to plan for PSP removal when the final preparation must be highly defined.

For additional context, Precision Cleavage, Translational Reach complements this workflow by connecting tag removal with native-protein quality and nuclear-architecture assays. The article Drosophila Keap1 Nuclear Condensates Under Oxidative Stress provides the biological context for the dKeap1 application, whereas this article focuses on the purification decisions needed to test that biology reproducibly.

Troubleshooting and optimization tips

Little or no cleavage

Check that the complete LEVLFQGP recognition sequence is present and correctly oriented. A buried site may be inaccessible in a folded fusion protein, and a mutation near the Gln-Gly junction can reduce recognition. Confirm that the PSP aliquot has not experienced repeated freeze-thaw cycles. Extend the reaction from 4 h to 8 or 16 h at 4 °C before increasing enzyme concentration, because excessive enzyme can complicate downstream purification.

Partial cleavage or heterogeneous bands

Run a ratio and time matrix rather than judging a single condition. Partial cleavage may reflect limited site accessibility, aggregation, or a mixture of folded states. Include a no-enzyme control and compare soluble and insoluble fractions. If cleavage improves after dilution, lower substrate concentration or mild buffer exchange may be exposing the site; validate that the dilution itself does not alter the target’s oligomeric state.

Target precipitation during the reaction

Separate protease performance from target stability. Examine the substrate at 4 °C without PSP, reduce handling time, and test a small concentration series before scaling. For dKeap1 or other IDR-containing proteins, precipitation and regulated condensation can look similar by eye. Use microscopy, centrifugation, and a soluble-protein measurement rather than relying only on turbidity.

Unexpected background after purification

Residual tag, PSP, or host proteins can interfere with condensate measurements and binding assays. Use a second polishing step and inspect the final sample by SDS-PAGE. Because PSP contains GST, plan an orthogonal cleanup strategy if glutathione-binding components are undesirable. Also confirm the final protein concentration with a method that is not distorted by the tag-removal chemistry.

Future outlook

The dKeap1 findings establish a testable model in which domain architecture and intrinsically disordered regions influence stress-responsive nuclear condensates, while the Kelch domain can restrain assembly. A carefully optimized PreScission Protease workflow can strengthen the next generation of experiments by separating tag-dependent behavior from target-dependent behavior. The most informative path forward is comparative: analyze cleaved and uncleaved matched proteins, retain domain-specific controls, and pair condensate formation with mobility and protein-integrity measurements. In that role, PSP is not the biological mechanism; it is a precision control point that helps make the mechanism easier to measure.