Insight

Strict standard operating procedures are implemented for industrial upstream processing of recombinant proteins. However, insufficient professional expertise and systematic training in academic settings lead to poor experimental reproducibility. Accordingly, quality control guidelines for recombinant protein purification assessment have been issued to enhance data repeatability and standardize operational practices. Table 1 summarizes QC specifications and applicable purification strategies for proteins with distinct biochemical properties or specialized biological applications.

Nucleic acid elimination is an indispensable step in purifying nucleic acid-binding proteins. Nucleases including Benzonase®, DNase and RNase are commonly supplemented during cell lysis, yet such treatment hardly removes tightly bound nucleic acids. Alternative decontamination approaches encompass nucleic acid precipitation using polyethyleneimine (PEI) or streptomycin sulfate, heparin chromatography and ion exchange chromatography. Nucleic acid contamination is monitored via the A260/A280 absorbance ratio; a ratio below 0.6 indicates high protein purity and negligible nucleic acid residues. For proteins with strong nucleic acid affinity, urea denaturation followed by protein refolding is adopted. High-grade reagents, freshly prepared sterile buffers and thoroughly cleaned chromatographic columns pre-washed with 0.5 M sodium hydroxide are required throughout the workflow.

Mouse ferritin heavy chain 1 serves as a research object for high-resolution single-particle cryo-electron microscopy (cryo-EM). Untagged protein is expressed in Escherichia coli. Cells are lysed by ultrasonication without nuclease addition, followed by thermal treatment at 70 °C, ammonium sulfate precipitation, dialysis and size-exclusion chromatography (SEC). Initial samples contain abundant impurities observed in cryo-EM micrographs. Process optimization is conducted by supplementing Serratia marcescens nuclease prior to cell lysis and post-precipitation dialysis, combined with anion exchange chromatography. The A260/A280 ratio declines from 1.4 to 0.8, and reaches 0.56 after SEC purification, confirming high-purity protein samples with effective impurity removal.

dsRBEC is a fusion protein consisting of the double-stranded RNA-binding domain (dsRBD) of human protein kinase R and human epidermal growth factor (hEGF). Most recombinant proteins exist as insoluble precipitates after cell lysis, and residual RNA interferes with protein binding to immobilized metal-ion affinity chromatography (IMAC) resins. Conventional RNA removal methods including PEI precipitation, streptomycin sulfate treatment, RNase digestion and high-salt buffer conditioning fail to eliminate contaminants. Cell lysis is performed in buffer containing 4 M urea, and supernatants are loaded onto IMAC columns. On-column refolding is carried out overnight with linear urea gradient elution from 4 M to 0 M. Target protein is eluted with refolding buffer supplemented with imidazole, and final polishing is achieved via Superdex 75 gel filtration, yielding functionally active dsRBEC.

Exogenous supplementation of corresponding divalent cations such as Zn²⁺, Fe²⁺ and Cu²⁺ is critical during protein expression for cation-binding proteins. Equivalent trace cations are maintained in purification buffers to sustain protein stability and bioactivity. Chelating agents including EDTA and EGTA, as well as chelating reducing agents like DTT and DTE must be excluded. Chelator-free protease inhibitor cocktails are applied to prevent cation depletion. Atomic absorption spectroscopy and other spectrometric assays verify residual cation and cofactor contents, guaranteeing proper protein folding and biological functionality.

Plant-type iron-sulfur proteins harbor a single [2Fe-2S] cluster and act as electron acceptors of Photosystem I. The target protein is expressed in E. coli BL21(DE3) strain cultured in Terrific Broth medium supplemented with 10 mM FeCl₃ and antibiotics. Elution from IMAC columns adopts histidine-containing buffer at 10 mM instead of imidazole to avoid structural damage to [2Fe-2S] clusters. Further purification is completed via anion exchange chromatography and SEC. Final protein concentrates reach 12 mg/mL for crystallization screening.

Gel filtration profiles of wild-type LLT1 present two distinct peaks, corresponding to homogeneous non-covalent dimers and broad aggregated fractions. Mass spectrometry identifies misfolding caused by unpaired cysteine residues. Two LLT1 mutants are constructed: one with specific cysteine deletion and the other with cysteine residue insertion to reconstruct native disulfide bonds. The reconstructed mutant exhibits favorable stability and high expression yield, and successfully crystallizes. Its three-dimensional structure validates correct protein folding and intact artificial disulfide bond formation. Biophysical characterization and site-directed mutagenesis are effective strategies to optimize disulfide bond assembly and protein folding in recombinant production.

Cdc-like kinase 1 (CLK1) is a dual-specificity kinase capable of autophosphorylation at tyrosine residues and substrate phosphorylation. Although its crystal structure has been resolved, experimental reproducibility remains challenging, a common issue in kinase research. Homogeneous CLK1 without heterogeneous phosphorylation modification is essential for repeatable crystallization and co-crystallization assays. CLK1 is co-expressed with lambda phosphatase to eliminate phosphate groups. His-tagged CLK1 is isolated by IMAC, and eluates are immediately stabilized with 50 mM arginine, 50 mM glutamic acid and 10 mM DTT to mitigate aggregation. Concentrated samples are incubated with TEV protease overnight at 4 °C. SEC with buffer containing 5% glycerol and 5 mM β-mercaptoethanol separates native oligomers from soluble aggregates, followed by final polishing through anion exchange chromatography.

Native conformational integrity is not mandatory for antigens applied in conventional antibody production, as most antibodies recognize linear epitopes. Denatured antigens can induce potent immune responses, indicating that three-dimensional conformation exerts limited influence on immunogenicity. Rigorous purity assessment is required, since trace highly immunogenic impurities may trigger off-target immune reactions. For camelid antibody screening relying on conformational epitope recognition, antigens must retain natural tertiary structures to elicit antibodies targeting physiologically accessible epitopes. Library panning demands identical antigen conformation matching practical detection scenarios to avoid non-specific binder acquisition. Eukaryotic expression systems are preferred for proteins with intra- or intermolecular disulfide bonds, and reducing agents are prohibited during purification to preserve native conformation. Reductants are indispensable for proteins with free cysteine residues to prevent artificial disulfide crosslinking. DTT, β-mercaptoethanol (β-ME) and TCEP are mainstream reducing agents. β-ME is recommended for IMAC resins incompatible with DTT or TCEP, with subsequent reductant substitution. Buffers with zero reductant or low-concentration β-ME (2 mM) are suitable for proteins containing both disulfide bonds and free cysteines. Bioinformatic prediction based on amino acid sequences facilitates disulfide bond identification.

Proteins may exist as insoluble precipitates, partial soluble fractions or aggregates due to multiple factors, bringing substantial obstacles to downstream purification. Aggregation inhibition strategies include host strain screening, cultivation temperature reduction, optimized medium formulation and solubility-enhancing fusion tag fusion. Small-scale expression and purification trials monitor monomer-aggregate ratios to determine optimal conditions for maximum native protein yield and minimal aggregation. Rapid purification protocols and optimized storage conditions are designed to suppress late-stage protein aggregation.

Endotoxin contamination poses severe threats to bioprocessing. Common elimination techniques comprise anion exchange chromatography with positively charged stationary phases, affinity chromatography utilizing polycationic ligands such as poly-L-lysine and polymyxin B, and Triton X-114 surfactant extraction. Endotoxin-free water is used for buffer preparation, and dedicated chromatographic columns are adopted. Fluidic systems and pumps are decontaminated via overnight incubation in 0.5 M NaOH or 4-hour treatment in 1.0 M NaOH. Limulus amoebocyte lysate (LAL) assay quantifies residual lipopolysaccharide (LPS) to meet application-based safety thresholds.

Optimal expression and purification conditions vary individually for protein complexes. Subunits can be expressed separately for in vitro assembly or co-expressed intracellularly to form functional complexes. Rational vector design ensures affinity tags do not interfere with complex assembly. Homogeneity and molecular weight are monitored throughout purification to assess complex integrity and stability. Antigen-antibody complexes serve as typical examples; co-expression stabilizes labile antigens via antibody binding. SDS-PAGE and gel filtration chromatography are standard methods for complex quality evaluation.

Recombinant protein production initiates with strategic planning based on target protein biochemical properties and downstream applications. Valid gene sequences are inserted into appropriate expression plasmids, followed by expression parameter optimization including host strain, culture medium, growth temperature and incubation duration. After establishing conditions for maximal soluble protein acquisition, large-scale purification is implemented with matched chromatographic techniques and buffer systems. Continuous quality control maintains protein stability, solubility and native conformation. Purified proteins are applicable to biophysical characterization, molecular interaction analysis, structural elucidation and cellular functional assays.