PreScission Protease for Condensate-Ready Proteins

Recombinant proteins used in phase-separation, chromatin, and stress-response studies often need an affinity tag for expression and purification but benefit from tag removal before functional analysis. PreScission Protease (PSP) from APExBIO addresses this transition with a recombinant HRV 3C protease fused to GST. The enzyme recognizes the octapeptide Leu-Glu-Val-Leu-Phe-Gln-Gly-Pro, commonly written LEVLFQGP, and cuts between glutamine and glycine.

That specificity makes PSP a useful protein purification enzyme for workflows in which an intact, near-native target is more informative than a permanently tagged construct. The approach is especially relevant to the study of Drosophila Keap1, where domain architecture, intrinsically disordered regions, and protein concentration can influence nuclear condensate formation. PSP does not replace biological validation, but it can help researchers create cleaner matched samples for comparing tagged, cleaved, and domain-deletion proteins.

Setup and Principle Overview

PSP is a recombinant fusion protease composed of HRV14 3C protease and GST and is produced in Escherichia coli. Its recognition sequence must be engineered into the fusion construct, usually between the affinity tag and the target protein. Because cleavage occurs at the defined Gln-Gly bond, the released target begins with the intended downstream sequence rather than a long residual protease recognition tail.

The key design checkpoint is therefore not simply whether a protein carries GST or another affinity handle. The construct must contain an accessible prescission protease cleavage site with the correct amino-acid order. A target lacking LEVLFQGP, or carrying substitutions within the recognition region, should not be expected to undergo efficient cleavage. This is an important distinction between GST fusion protein cleavage and nonspecific proteolysis.

PSP is formulated for activity at low temperature, with 4 °C identified as the operating optimum in the product information. The same source recommends storage at −80 °C to preserve activity, while aliquots may be stored at −20 °C for up to 6 months. Small aliquots reduce repeated freeze-thaw exposure, a practical safeguard for any protease used across multiple purification batches.

Key Innovation from the Reference Study

The reference study, Drosophila Keap1 Proteins Assemble Nuclear Condensates in Response to Oxidative Stress, identified a previously underappreciated nuclear behavior of dKeap1. Following oxidative treatment, dKeap1 accumulated in stable nuclear foci with reduced mobility by fluorescence recovery after photobleaching. The investigators further found that both the N-terminal and C-terminal domains were required for foci formation, while two intrinsically disordered regions within the C-terminal domain supported condensate formation in vitro.

The study also reported a useful domain-level contrast: CTD-YFP fusion proteins readily formed condensates in vitro, whereas deletion of the Kelch domain promoted robust cytoplasmic foci under basal conditions. The Kelch domain therefore acted as a suppressive structural element in the tested system. These findings suggest that assay design must preserve domain boundaries and distinguish intrinsic assembly from effects caused by an affinity or fluorescent tag.

PSP translates that insight into a practical construct strategy. A researcher can express a tagged dKeap1 domain or CTD fusion, purify it, and then compare the same preparation before and after removal of the tag when a cognate PSP site has been built into the linker. Cleaved material can be evaluated by microscopy, turbidity or sedimentation assays, and FRAP-style mobility measurements. The most informative design is a matched panel containing uncleaved protein, PSP-treated protein, and a cleavage control lacking the recognition site. This separates genuine domain-driven behavior from changes caused by the tag, protease exposure, or buffer exchange.

Step-by-Step Workflow for Fusion Protein Tag Cleavage

1. Design the fusion construct. Place LEVLFQGP between the affinity tag and the target, and inspect the junction for unintended residues, secondary structure, or steric obstruction. For condensate studies, document whether the tag is attached to the N or C terminus because terminal placement can alter apparent assembly.
2. Express and capture the fusion protein. Use the affinity method appropriate for the chosen tag, then remove cell debris and insoluble material before cleavage. A clean starting sample reduces the chance that contaminants will bind, precipitate, or complicate interpretation after tag removal.
3. Exchange into a compatible cleavage environment. Desalt or dialyze the captured protein into the recommended cleavage buffer or a buffer confirmed to support PSP stability. Avoid changing salt, reducing conditions, or detergent at the same time as cleavage unless the effect is being deliberately tested.
4. Run a small pilot before scaling. Use a short time course and several PSP inputs rather than committing an entire preparation to one condition. A pilot reveals whether the site is accessible and whether the target remains soluble at the selected concentration.
5. Separate products after cleavage. The released target, tag, and GST-fused protease can often be resolved by a second affinity step or size-exclusion strategy, depending on construct architecture. Confirm that residual PSP is removed before functional assays because the protease fusion can contribute extra protein mass or alter sample composition.
6. Verify both chemistry and function. SDS-PAGE can indicate disappearance of the fusion band and appearance of a lower-molecular-weight target, but it does not prove a native terminus. Add intact-mass analysis, N-terminal sequencing, or another identity test when the cleavage junction is central to the experiment. For dKeap1 work, compare condensation behavior at matched protein concentrations after normalization.

Protocol Parameters

• Temperature: Perform the cleavage reaction at 4 °C and equilibrate the fusion-protein sample and cleavage buffer for 15 min before adding PSP. This is a practical starting condition aligned with the product-described low-temperature activity profile.
• Pilot reaction: Prepare a 200 µL screening reaction containing 0.5–2 mg/mL fusion protein and test 0.5, 1, and 2 µL PSP in parallel. Treat these volumes as starting-point optimization conditions rather than a universal activity specification.
• Time course: Remove 10 µL aliquots at 2, 4, 8, and 16 h, immediately place them on ice, and analyze them by SDS-PAGE. Stop extending the incubation when cleavage is complete and the target remains soluble.
• Storage: Dispense unused enzyme into 20 µL aliquots and store them at −80 °C. If short-term working storage is necessary, keep aliquots at −20 °C for no longer than 6 months, consistent with the product information.
• Buffer exchange: Before condensate testing, exchange the cleaved sample through at least 10 column volumes of the assay buffer or perform an equivalent validated dialysis step. Use the same exchange process for uncleaved controls.

The article PreScission Protease: Precision Tag Cleavage for Protein Purification complements this workflow by focusing on low-temperature tag removal and purification logic. It is useful as a general purification companion, whereas the present strategy extends the same principle to matched condensate and domain-function assays.

Advanced Applications and Comparative Advantages

For dKeap1 research, tag cleavage is most valuable when the experimental question concerns material properties rather than expression or capture. A fluorescent fusion may be indispensable for visualization, but a large affinity tag can affect multivalent interactions, steric accessibility, solubility, or the threshold for condensation. Testing the cleaved protein alongside the tagged version helps identify which observations are intrinsic to dKeap1 domains.

The same logic applies to chromatin-binding studies, protein interaction assays, and reconstitution experiments involving disordered regions. The site-specific action of HRV 3C protease limits the risk of cutting at unrelated positions, while the 4 °C workflow can be gentler than extended incubation at room temperature for labile proteins. This is the central comparative advantage over a generic digestion strategy: the goal is not merely to reduce molecular weight, but to recover a defined target with a controlled terminus.

PSP can also support modular construct comparisons. For example, investigators may prepare full-length dKeap1, an isolated C-terminal domain, and a Kelch-domain deletion with the same cleavage architecture. Processing each construct under a matched schedule makes it easier to attribute differences in nuclear foci or in vitro condensates to domain composition instead of unequal purification history.

Why this cross-domain matters, maturity, and limitations

The bridge from protein purification to condensate biology is experimentally useful but remains a sample-preparation strategy, not a new mechanistic claim. The reference study directly supports dKeap1 nuclear foci, reduced mobility within those foci, CTD-YFP condensate formation, and domain-dependent behavior. PSP can improve the comparison of tagged and cleaved proteins, but it does not establish that tag removal alone reproduces the cellular state.

Condensation is sensitive to concentration, temperature, salt, crowding, fluorescent labeling, and incubation history. Cleavage may change any of these variables through dilution, buffer exchange, or removal of the tag. Therefore, a mature workflow should include matched protein concentrations, identical handling times, cleavage-site controls, and orthogonal readouts such as microscopy plus biochemical fractionation. The article Drosophila Keap1 Forms Nuclear Condensates in Oxidative Stress provides the biological context; PSP serves as an experimental extension for controlling protein form, not as evidence that the original study used this enzyme.

Troubleshooting and Optimization Tips

Little or no cleavage

First confirm the exact junction sequence. The site must contain LEVLFQGP in the correct orientation, with cleavage expected between Q and G. If the sequence is correct, test whether the site is buried by the folded tag or target. Lowering the fusion-protein concentration, increasing the accessible linker length in a new construct, or extending the 4 °C incubation can help. A time course is more informative than simply adding more enzyme because excessive protease can increase downstream cleanup.

Partial cleavage

Partial processing often reflects limited accessibility, aggregation, or an incompatible buffer. Compare a fresh buffer-exchanged sample with the original sample and inspect the reaction by soluble-versus-pellet analysis. If the target remains intact but the fusion band persists, run a small PSP input series. Keep the temperature at 4 °C while changing one variable at a time.

Target degradation or loss of activity

Reduce the incubation duration and PSP input, and collect early time points at 2 and 4 h. Confirm that the target is not precipitating during cleavage; a clear supernatant can conceal a substantial loss if the protein has pelleted. For functional assays, compare cleavage-treated and buffer-treated controls that have experienced the same mixing, dilution, and exchange steps.

Unexpected condensate differences

Do not interpret a changed droplet or foci phenotype until residual tag, PSP, and buffer differences have been excluded. Normalize samples by intact target concentration rather than total absorbance, and use the same temperature during cleavage and the first assay measurement. If low temperature preserves the protein but suppresses assembly kinetics, split the experiment into a controlled 4 °C processing phase followed by a separately defined assay-temperature phase.

Future Outlook

The reference findings place dKeap1 domain architecture and condensate behavior at the center of questions about nuclear stress regulation. Future experiments can build on that foundation by comparing precisely processed full-length and domain-specific proteins, then integrating mobility measurements with biochemical and chromatin-associated assays. In this setting, PreScission Protease is best viewed as an enabling control: it helps researchers ask whether a tag is merely a visualization aid or an active determinant of the observed phenotype.

Because no universal cleavage rate or yield is specified here, laboratories should establish their own enzyme-input and time-response curves for each construct. A careful pilot, low-temperature handling, aliquot-based storage, and matched cleaved controls will produce more defensible conclusions than assuming that one condition suits every fusion protein.