1. What Are Inclusion Bodies?
Inclusion bodies (IBs) are dense, insoluble aggregates of misfolded recombinant protein that accumulate in the cytoplasm of E. coli during high-level expression. Under phase-contrast microscopy, they appear as bright, refractile particles typically 0.5–1.3 µm in diameter—large enough to be visible at 400× magnification.
The scale of the problem is significant: 30–50% of all recombinant proteins expressed in E. coli form inclusion bodies, and for some proteins—particularly those with complex tertiary structures, multiple disulfide bonds, or eukaryotic post-translational modifications—the figure approaches 100%. This makes inclusion body processing one of the most common challenges in recombinant protein production and a critical skill for any bioprocess engineer.
Inclusion bodies are not simply waste. These dense protein aggregates consist of 50–90% of the target protein in a highly enriched form, which can actually be advantageous: the target protein within inclusion bodies is protected from proteolytic degradation, toxic proteins sequestered in inclusion bodies cannot harm the host cell, and the dense aggregates are easy to separate from soluble host cell proteins by centrifugation. The challenge is recovering biologically active protein from the insoluble inclusion body state.
Inclusion bodies are not a failure—they are a validated manufacturing route. Several approved biologics, including insulin, growth hormones, and interferons, are produced via inclusion body solubilization and refolding at industrial scale. In fact, the inclusion body pathway often provides higher volumetric yields than soluble expression.
2. Why Inclusion Bodies Form
Inclusion body formation is fundamentally a kinetic competition between productive folding and non-productive aggregation. When the rate of protein synthesis overwhelms the cell's folding capacity, partially folded intermediates accumulate and aggregate through exposed hydrophobic surfaces. Several factors tip the balance toward aggregation:
- High expression rate overwhelms chaperone capacity. Strong promoters (T7, tac) with full induction (1 mM IPTG) can produce recombinant protein at 30–50% of total cell protein, driving inclusion body formation. The E. coli chaperone machinery (GroEL/ES, DnaK/J/GrpE, trigger factor) simply cannot process this volume of nascent polypeptides fast enough.
- Hydrophobic aggregation. Partially folded intermediates expose hydrophobic regions that are normally buried in the native structure. These hydrophobic surfaces drive intermolecular association, forming amorphous aggregates or sometimes amyloid-like fibrillar structures.
- Missing disulfide bonds. The E. coli cytoplasm is strongly reducing (glutathione ratio GSH:GSSG > 200:1). Proteins requiring disulfide bonds for structural stability cannot form them in this environment, leaving the protein conformationally unstable and prone to inclusion body aggregation.
- Missing post-translational modifications. Glycosylation, phosphorylation, and other PTMs that stabilize the native fold in eukaryotic cells are absent in E. coli. Without these stabilizing modifications, the protein may not achieve a stable tertiary structure.
- Temperature-dependent folding. Higher temperatures (37°C) increase both the rate of protein synthesis and the rate of inclusion body aggregation. The folding intermediates are less stable at higher temperatures, broadening the window for non-productive interactions that drive inclusion body formation.
- High local concentration on ribosomes. The extremely high local concentration of nascent chains near polysome clusters favors intermolecular aggregation over intramolecular folding.
3. Inclusion Body Prevention Strategies
Before committing to an inclusion body refolding process, it is worth attempting to express the protein in soluble form. The following strategies for preventing inclusion body formation have proven effective—often in combination:
Lower Temperature
Reducing growth temperature to 18–25°C after induction is the single most effective strategy for reducing inclusion body formation and increasing soluble expression. Lower temperature slows both translation and aggregation, giving chaperones more time to assist folding before protein aggregates into inclusion bodies. The trade-off is lower total yield and longer induction times (16–24 h at 18°C vs. 3–4 h at 37°C).
Lower Inducer Concentration
Reducing IPTG from the standard 1 mM to 0.05–0.1 mM decreases the rate of protein synthesis without fully eliminating it. This gentler induction allows the folding machinery to keep pace. For T7-based systems (BL21(DE3)), even 0.01 mM IPTG can provide substantial expression with improved solubility.
Co-express Chaperones
Plasmid-based co-expression of molecular chaperones can dramatically improve soluble yields. The most effective combinations:
- GroEL/ES — barrel-shaped chaperonin that encapsulates proteins up to ~60 kDa and provides a protected folding environment
- DnaK/DnaJ/GrpE — Hsp70 system that binds exposed hydrophobic segments and prevents aggregation
- Trigger factor (TF) — ribosome-associated chaperone that interacts with nascent chains co-translationally
- Combination sets — Takara’s chaperone plasmid set (pG-KJE8, pGro7, pKJE7, pG-Tf2, pTf16) allows systematic screening
Fusion Tags
Solubility-enhancing fusion tags act as folding nuclei that promote correct folding of the downstream target protein:
| Tag | Size (kDa) | Solubility Enhancement | Notes |
|---|---|---|---|
| MBP | 42 | Very high | Best overall; can be cleaved with TEV |
| SUMO | 11 | High | Cleaved by SUMO protease (native N-terminus) |
| Thioredoxin (Trx) | 12 | High | Promotes disulfide formation in Origami strains |
| GST | 26 | Moderate | Dimerizes; can cause aggregation for some targets |
| NusA | 55 | High | Large tag; good for difficult proteins |
Specialized Strains
- SHuffle (NEB) — Engineered E. coli with oxidizing cytoplasm (trxB/gor deletions + cytoplasmic DsbC). Enables disulfide bond formation in the cytoplasm. Excellent for antibody fragments, growth factors, and other disulfide-rich proteins.
- Origami/Origami 2 — Similar concept with trxB/gor mutations. Compatible with thioredoxin fusion tags.
- ArcticExpress — Co-expresses cold-adapted chaperonins (Cpn10/Cpn60 from Oleispira antarctica). Designed for expression at 10–12°C.
Slower Growth Rate
Using minimal media instead of rich media (LB, TB) reduces the overall growth rate and protein synthesis rate, decreasing the tendency toward inclusion body formation. Auto-induction media (Studier formulation) provide gradual induction as lactose is consumed after glucose depletion, avoiding the sudden burst of expression that drives inclusion body aggregation.
4. Inclusion Body Isolation & Washing
If inclusion bodies are the chosen production route—or prevention strategies have failed—the first step is isolating clean inclusion body pellets. A clean preparation is critical for efficient solubilization and refolding of inclusion body proteins.
Standard IB Isolation Protocol
Washed inclusion bodies can be stored at −80°C for months without degradation. Weigh the inclusion body pellet before freezing so you know the starting mass for solubilization. Typical inclusion body yield is 50–200 mg per gram of wet cell paste.
5. Inclusion Body Solubilization
Inclusion body solubilization uses chaotropic denaturants to unfold the aggregated protein and bring it into solution. The goal is to completely dissolve the inclusion body pellet and achieve full protein unfolding, which provides a “clean slate” for the subsequent refolding step.
Denaturant Comparison
| Property | Urea (6–8 M) | GdnHCl (6 M) |
|---|---|---|
| Denaturing strength | Milder | Stronger |
| Solubilization efficiency | Good for most proteins | Better for very stable aggregates |
| Compatibility with ion exchange | Good (non-ionic) | Poor (ionic, high conductivity) |
| Cost | Low | Higher |
| Carbamylation risk | Yes (modify Lys residues at high temp/pH) | No |
| Typical use | First choice | When urea fails |
Standard Solubilization Protocol
Urea solutions contain isocyanate (from urea decomposition) that can carbamylate lysine residues and protein N-termini. To minimize this: (1) prepare urea solutions fresh, (2) deionize with mixed-bed ion exchange resin, (3) never heat urea solutions above 37°C, and (4) keep incubation time under 2 hours.
Line chart with X axis showing protein concentration during refolding on a logarithmic scale from 0.01 to 5 mg/mL and Y axis showing refolding yield from 0 to 100 percent. The curve shows approximately 85% yield at 0.05 mg/mL, declining to around 70% at 0.2 mg/mL, then sharply dropping to about 20% at 2 mg/mL. A shaded green region between 0.05 and 0.2 mg/mL marks the optimal range. Text annotation above 1 mg/mL reads Aggregation dominant.
6. Refolding Strategies for Inclusion Body Proteins
Refolding is the critical step in inclusion body processing where denatured protein is guided back toward its native conformation. The fundamental principle is to gradually remove the denaturant while providing conditions that favor intramolecular folding over the intermolecular aggregation that originally produced the inclusion bodies.
6a. Rapid Dilution
The simplest refolding method. Denatured protein is rapidly diluted into a large volume of refolding buffer, instantly dropping the denaturant concentration below the threshold for protein unfolding.
- Dilution factor: 10–50× (typically 20×)
- Final protein concentration: 0.1–0.5 mg/mL
- Method: Add denatured protein dropwise to rapidly stirred refolding buffer at 4°C
- Pros: Simple, reproducible, fast
- Cons: Requires large buffer volumes; low final protein concentration
6b. Pulse/Dropwise Dilution
An improvement over simple dilution. Denatured protein is added in small pulses (5–10% of total) over several hours, allowing each aliquot to refold before the next is added. This avoids the transient high protein concentration that occurs with single-bolus dilution.
- Pulse interval: Every 30–60 min
- Aliquot size: 5–10% of total denatured protein per pulse
- Typical improvement: 20–50% higher refolding yield vs. single dilution
6c. Dialysis
Gradual removal of denaturant by dialysis against refolding buffer. This method provides a slow, controlled transition through intermediate denaturant concentrations.
- Stepwise dialysis: 4 M → 2 M → 1 M → 0 M urea (4–6 h per step)
- Advantages: Good for small-scale, gentle on protein
- Limitations: Slow, difficult to scale up, protein can aggregate at intermediate denaturant concentrations
6d. On-Column Refolding
Denatured His-tagged protein is bound to Ni-NTA resin in 8 M urea, then refolded by running a linear gradient from denaturing to native buffer through the column. The immobilized protein cannot form intermolecular aggregates because molecules are physically separated on the resin.
- Gradient: 8 M urea → 0 M urea over 10–20 column volumes
- Advantages: Very high refolding yields (50–90%), simultaneous purification
- Limitations: Requires His-tag, limited by column capacity (5–10 mg/mL resin)
6e. SEC-Based Refolding
Size exclusion chromatography can be used for refolding by running denatured protein on a column equilibrated with refolding buffer. As the protein migrates through the column, the denaturant is separated, and the protein refolds. Aggregates are simultaneously separated from correctly folded monomers.
7. Redox Pairs for Disulfide Bonds
Proteins with disulfide bonds require a controlled redox environment during refolding to allow correct disulfide pairing. The refolding buffer must contain both a reduced and oxidized thiol species—the reduced form breaks incorrect disulfides, and the oxidized form drives correct disulfide formation.
| Disulfide Bonds | GSH (reduced) | GSSG (oxidized) | GSH:GSSG Ratio | Total Glutathione |
|---|---|---|---|---|
| 0 (none) | No redox pair needed — add 1 mM DTT to prevent non-specific oxidation | |||
| 1 | 5 mM | 1 mM | 5:1 | 6 mM |
| 2 | 3 mM | 1 mM | 3:1 | 4 mM |
| 3–4 | 2 mM | 1 mM | 2:1 | 3 mM |
| >4 | 1 mM | 1 mM | 1:1 | 2 mM |
Alternative redox pairs:
- Cysteine / cystine: Similar ratios to GSH/GSSG. Cystine has limited solubility—keep total cystine <1 mM.
- DTT + GSSG: DTT (0.5–1 mM) provides the reducing component; GSSG (1–2 mM) provides the oxidizing component. The DTT rapidly reduces to establish the redox baseline.
- Cysteamine / cystamine: Small-molecule thiols with faster kinetics than glutathione. Useful for proteins with slow disulfide rearrangement.
The optimal GSH:GSSG ratio is protein-dependent. Screen ratios from 10:1 to 1:1 in small-scale refolds (1–5 mL) before scaling up. Monitor refolding yield by analytical SEC or reversed-phase HPLC.
8. Refolding Additives
Refolding additives improve yield by stabilizing folding intermediates, preventing aggregation, or assisting with specific structural features. The table below lists the most commonly used additives with recommended concentrations:
| Additive | Concentration | Mechanism | When to Use |
|---|---|---|---|
| L-Arginine | 0.4–0.8 M | Suppresses aggregation by interacting with exposed hydrophobic surfaces; mildly chaotropic | Default additive for most refolds; most effective single agent |
| Glycerol | 10–20% (v/v) | Stabilizes native-like compact states; excluded volume effect | General stabilizer; combine with arginine |
| PEG 3350 | 0.05% (w/v) | Molecular crowding agent; shifts equilibrium toward compact folded states | Proteins with hydrophobic surfaces |
| Tween-20 | 0.01–0.05% | Non-ionic detergent that blocks hydrophobic aggregation at very low concentrations | Membrane-associated proteins; hydrophobic aggregation-prone targets |
| Sucrose | 0.5 M | Osmolyte; preferentially excluded from protein surface, stabilizing the native state | Alternative to glycerol; proteins with large solvent-exposed surfaces |
| Proline | 0.5–2 M | Natural osmolyte; suppresses aggregation without affecting folding kinetics | Proteins prone to kinetically-trapped intermediates |
| NDSB-201 | 0.5–1 M | Non-detergent sulfobetaine; disrupts protein-protein interactions without denaturing | Highly aggregation-prone proteins |
| Low [urea] | 1–2 M | Sub-denaturing urea concentration keeps protein soluble during early folding steps | When rapid dilution causes immediate precipitation |
9. Common Inclusion Body Processing Mistakes
Attempting to refold at >1 mg/mL almost always leads to aggregation. Intermolecular contacts scale with the square of concentration. Keep final protein concentration at 0.1–0.5 mg/mL for initial screens, and never exceed 1 mg/mL without extensive optimization.
Too much reducing agent prevents disulfide formation entirely. Too much oxidizing agent locks in incorrect disulfides before the protein can explore the correct fold. Always screen the GSH:GSSG ratio systematically rather than using a default recipe.
Rapid denaturant removal (especially with dialysis) can trap the protein at intermediate urea concentrations (2–4 M) where it is partially unfolded and maximally aggregation-prone. Use stepwise dialysis or controlled dilution to transit through this danger zone quickly.
Refolded samples always contain some aggregated protein. If not removed by centrifugation (20,000 × g, 30 min) or filtration (0.22 µm) before chromatography, aggregates will foul the column and reduce resolution. Always clarify before loading.
A protein that is soluble after refolding is not necessarily correctly folded. Confirm proper folding with at least two orthogonal methods: analytical SEC (monomer vs. aggregate ratio), circular dichroism (secondary structure), activity assay (if available), or thermal shift assay (thermal stability).
10. Worked Example: Inclusion Body Refold of a 30 kDa Protein
Scenario: You have 100 mg of washed inclusion bodies containing a 30 kDa target protein with 2 disulfide bonds. You want to solubilize and refold these inclusion bodies by rapid dilution and recover active protein for structural studies.
Generate Your Refolding Protocol
Enter your protein properties, disulfide count, and target scale. Get a customized refolding protocol with buffer recipes and step-by-step instructions.
Refolding Protocol Generator →For more on E. coli expression systems and inclusion body management, see our companion guides:
- E. coli Expression Systems Guide — Strain selection, promoters, and induction strategies.
- E. coli Expression Optimizer — Interactive tool to optimize your expression conditions.
- Bioprocess Formulas Cheat Sheet — All the key equations you need for process development.
References
- Singh, S.M. & Panda, A.K. (2005). “Solubilization and refolding of bacterial inclusion body proteins.” Journal of Bioscience and Bioengineering, 99(4), 303–310. doi:10.1263/jbb.99.303
- Vallejo, L.F. & Rinas, U. (2004). “Strategies for the recovery of active proteins through refolding of bacterial inclusion body proteins.” Microbial Cell Factories, 3(1), 11. doi:10.1186/1475-2859-3-11
- Tsumoto, K. et al. (2003). “Practical considerations in refolding proteins from inclusion bodies.” Protein Expression and Purification, 28(1), 1–8. doi:10.1016/S1046-5928(02)00641-1
- de Marco, A. et al. (2007). “Solubility engineering of proteins: an overview.” Current Protein and Peptide Science, 8(4), 346–358.
- Lobstein, J. et al. (2012). “SHuffle, a novel Escherichia coli protein expression strain capable of correctly folding disulfide bonded proteins in its cytoplasm.” Microbial Cell Factories, 11, 56. doi:10.1186/1475-2859-11-56