Lena Beckman, Product Manager
Johan Öhman, R&D Scientist
This guide walks you through how to experimentally determine the optimal hold-step time so you can maximize yield, reduce processing time, and achieve reproducible performance.
Introduction
Affinity-based purification with Cytiva™ Protein Select™ technology is designed to help you simplify recombinant protein purification by combining capture and tag removal in a single chromatography step.
A defining feature of this technology is the on-column, self-cleaving mechanism that releases your target protein in a tag-free form. Because this cleavage reaction occurs during a defined pause in flow, known as the hold step, selecting the right hold-step duration is essential for high yield and consistent process performance.
If the hold step is too short, cleavage may be incomplete. This means tagged protein remains bound and is lost during regeneration, which leads to reduced recovery. If the hold step is too long, you increase process time without gaining yield and may introduce challenges when scaling. The goal isn’t to maximize hold time, it's to identify the shortest hold-step that reliably delivers your required yield under process-relevant conditions.
This article outlines how to determine a hold-step for your specific protein. We summarize the factors that influence cleavage behavior and walk through two practical experimental approaches—a pulse-elution screening method and a parallel confirmation method. You’ll also see an example study and guidance on interpreting the results.
Fig 1. Workflow showing binding and wash, flow pause for tag cleavage (hold step), and elution of tag-free protein.
What is the hold step in Cytiva Protein Select purification?
Purification using our Cytiva Protein Select technology follows three simple steps:
- Binding and wash: The tagged protein binds specifically to the Cytiva Protein Select resin, while impurities are removed.
- Hold step: Flow is paused to allow on-column tag cleavage.
- Elution: The cleaved, tag-free protein is released from the resin.
During the hold step, the Cytiva Protein Select tag and the immobilized ligand form a self-cleaving complex. Cleavage happens spontaneously and does not require a protease or additional excipients, and the target protein elutes without residual tag-derived amino acids. Folding of the complex is the rate-limiting step since binding is very fast and only limited by diffusion.
For more background, see our related article on factors affecting cleavage efficiency when using Cytiva Protein Select technology.
Why the hold step must be defined experimentally
Cleavage kinetics are protein specific. Each new target protein can have a unique cleavage rate and yield depending on:
- its N‑terminal amino acid sequence
- temperature
- hold duration
- protein size or conformation (including post‑translational modifications)
- pH
Because of this variation, you cannot reliably predict hold-step time from platform knowledge. You should always determine it experimentally under conditions that reflect your intended process.
Key inputs that influence cleavage time
Several parameters influence how quickly cleavage proceeds during the hold step. These factors are discussed in detail in the related article and are summarized here.
N-terminal amino acid sequence
The identity of the first one to three amino acids is the strongest determinant of cleavage kinetics. Proline in the first or second position often results in extremely slow cleavage.
Temperature and pH
Cleavage slows at lower temperatures. pH can also shift cleavage efficiency, so it’s helpful to assess conditions that match your process.
Protein structure and modifications
Protein size, conformation, and any post‑translational modifications (such as glycosylation) can influence cleavage and reinforce the need for empirical hold-step determination.
How to determine hold-step time for your specific protein
You can use two complementary approaches (see Table 1):
- Pulse-elution method: A fast, material efficient way to define your hold-step window.
- Parallel screening method: A more precise confirmation step to validate your selected hold-step time.
The following sections explain each method.
Table 1. Comparison of pulse-elution and parallel screening methods
| Pulse-elution method | Parallel screening method |
| Quick, high‑level assessment of when the protein elutes and how much is released. Method: Perform cleavage studies on an ÄKTA™ system with varied hold-step durations and repeated elution cycles. |
Provides more precise confirmation. Method: Run multiple columns in parallel with identical sample loads and different hold-step times. |
Method 1: pulse-elution method (fast screening)
What it tells you
The pulse-elution method provides a quick, practical readout of when tag-free protein begins to elute and how yield increases as hold time increases. It’s well‑suited for early development, construct comparisons, and distinguishing fast, medium, or slow cleavage profiles.
Pulse-elution steps
- Load the tagged protein onto a Cytiva Protein Select column and perform binding and wash.
- Pause the flow for a defined hold-step time.
- Elute the cleaved, tag-free protein.
- Repeat the hold–elute sequence multiple times within one run, increasing the hold time each cycle.
- Collect fractions separately.
- Analyze fractions (for example, by size exclusion chromatography [SEC]) to quantitate cleaved protein.
This gives you a time-resolved cleavage profile from a single chromatography run.
Example study design- Four IL-1β (interleukin 1-beta) protein variants expressed in E. coli with different N-terminal substitutions.
- 1 mL HiTrap™ Protein Select™ column run on an ÄKTA pure™ system.
- After sample loading and wash, flow is stopped for a defined hold time, followed by elution; this stop–elute sequence is repeated in sequence (see Table 2).
Table 2. Hold times used in example kinetics study
| Peak | Step | CV |
| Equilibration | 10 | |
| Sample application and wash | 6 | |
| B | Elution 0 to 15 min | 2 |
| C | Elution 15 to 30 min | 2 |
| D | Elution 30 to 60 min | 2 |
| E | Elution 1 to 2 h | 2 |
| F | Elution 2 to 3 h | 2 |
| G | Elution overnight 12 h | 5 |
Each elution peak corresponds to one hold‑step interval. Figure 2 shows a chromatogram obtained for one of the proteins.
Fig 2. Example chromatogram with multiple elution peaks representing different hold‑step durations.
Results
Fractions were analyzed by SEC. The amount of cleaved and eluted tag-less protein was plotted in a graph showing the cumulative yield over time:
Fig 3. Cumulative yield vs hold time showing plateau points for hold‑step time selection.
How to interpret the results
Plot cumulative yield against hold time and watch for the plateau:
- If the curve plateaus within ~ 4 to 6 h → fast cleavage, shorter hold steps sufficient.
- If it continues increasing up to ~ 6 to 8 h → medium cleavage, extended hold may increase yield.
- If it rises slowly and improves overnight → slow cleavage, possible overnight hold step required.
In the example:
- AF, RF, and SS reached acceptable yields with hold times around 8 h.
- SE required an overnight hold.
Practical decision rule
- Plateau < 4 h → fast cleavage → recommended hold step: 2 to 4 h.
- Plateau 4 to 8 h → medium → recommended hold step: 4 to 8 h.
- Plateau > 8 h → slow → recommended hold step: overnight.
Choose the shortest hold-step that achieves your yield target with confidence. Then confirm it using the parallel screening method if needed for scale‑up or tech transfer.
Method 2: parallel sreening method (confirmation)
What it tells you
Parallel screening gives you a more accurate, quantitative assessment because each hold time is tested under identical conditions.
How to run it
- Prepare identical columns (same resin, same bed volume).
- Apply the same sample load (same concentration and volume).
- Use different hold times on each column (for example, 2 h, 4 h, 6 h, 8 h, overnight).
- Elute and quantitate tag-free protein for each condition.
How to interpret the results
Select the shortest hold-step that meets your yield requirements with reliable performance. This method is especially useful when defining a process for scale‑up or transfer.
Practical recommendations: Summary
- Start with process‑relevant temperature and pH.
- Use pulse elution to quickly define your hold‑step window and identify fast, medium, or slow cleavage.
- Use parallel screening to confirm your final choice when higher confidence is needed.
- Avoid extending hold time beyond the point where yield no longer improves.
RELATED RESOURCES
ADDITIONAL PRACTICAL TIPS
- Factors affecting the cleavage efficiency when using Cytiva Protein Select technology
- Optimizing protein recovery in the purification step using Cytiva Protein Select resin technology
- AxiChrom™ column packing with Cytiva Protein Select resin
- Design flexibility of protein construct with Cytiva Protein Select tag
- Integrating Cytiva Protein Select tag into an expression vector
- Instructions for use for Cytiva Protein Select resin
- Instructions for use for HiTrap Protein Select columns
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