Protein A and Protein G are bacterial immunoglobulin-binding proteins that bind the Fc region of antibodies, enabling rapid, single-step purification of polyclonal and monoclonal antibodies from serum, ascites fluid, or cell culture supernatants.
Principle
Protein A (from Staphylococcus aureus) and Protein G (from Streptococcus sp.) bind specifically to the Fc region of IgG antibodies without recognizing the antigen-binding Fab region. This preserves antibody functionality after purification. The bacterial proteins are immobilized on agarose or magnetic beads and used as affinity capture reagents. Antibodies bind at physiological pH and are eluted at low pH (2.5–3.0), which disrupts the Fc-Protein A/G interaction. Eluted fractions are immediately neutralized to prevent acid-catalyzed denaturation.
Protein A vs. Protein G
Protein A binds human IgG subclasses with varying affinity: strong to IgG1, IgG2, and IgG4 but weak to IgG3. It also binds rabbit, pig, dog, and guinea pig IgG well but poorly binds goat, sheep, rat, and mouse IgG1. Protein G binds a broader range of IgG subclasses across species, including all human IgG subclasses, mouse IgG1, goat, sheep, and rat IgG. Protein A/G is a recombinant fusion protein combining the binding domains of both, providing the broadest species and subclass coverage. The choice depends on the antibody species and isotype. For most polyclonal sera, Protein A/G agarose offers the best yield.
Resin Formats
Protein A, Protein G, and Protein A/G are available coupled to agarose, Sepharose, and magnetic beads. Protein A Sepharose (Cytiva) and MabSelect SuRe (an alkali-stabilized Protein A derivative) are standards for monoclonal antibody purification. Recombinant Protein A variants with improved alkaline stability allow cleaning with 0.1–0.5 M NaOH for endotoxin removal. Binding capacity ranges from 5–20 mg human IgG per mL of resin, depending on the resin and flow conditions. High-capacity resins (30–50 mg/mL) are used for bioprocessing.
Buffer System
Loading buffer: 20 mM sodium phosphate, 150 mM NaCl, pH 7.4 (PBS) or 20 mM Tris-HCl, pH 7.5–8.0. Wash buffer: same as loading, sometimes with 0.1% Tween-20 or 1 M NaCl for stringent washing. Elution buffer: 0.1 M glycine-HCl, pH 2.5–3.0, or 0.1 M citrate, pH 3.0. Neutralization buffer: 1 M Tris-HCl, pH 8.5–9.0 (add 50–100 µL per mL of eluate). For gentle elution, 2–3 M MgCl₂ or 3.5 M KSCN can be used, though these chaotropes may affect some antibodies.
Protocol
Equilibrate Protein A/G resin with loading buffer. Load clarified serum (diluted 1:1 in loading buffer) or culture supernatant (concentrated 10-fold if low titer) at 0.5–1 mL/min or batch rotate for 30 min at 4 °C. Wash with 10–15 column volumes of loading buffer until A₂₈₀ reaches baseline. Elute with 5–10 column volumes of elution buffer, collecting 0.5–1 mL fractions into tubes containing neutralization buffer. Pool the A₂₈₀ peak fractions and dialyze or buffer-exchange into PBS or storage buffer. SDS-PAGE under reducing conditions shows 25 kDa (light chain) and 50 kDa (heavy chain) bands.
Affinity Maturation and Engineered Variants
Recombinant Protein A variants (MabSelect SuRe, ProSep Ultra Plus) are engineered for higher binding capacity, improved alkaline stability (>0.5 M NaOH tolerance for cleaning-in-place), and reduced ligand leaching. These resins are essential for biopharmaceutical manufacturing where column reuse (>100 cycles) and strict purity requirements apply.
Applications
Protein A/G purification is the gold standard for antibody purification in research and industry. Monoclonal antibodies from hybridoma supernatants, polyclonal antibodies from rabbit or goat sera, and human therapeutic monoclonal antibodies from CHO cell cultures are routinely purified by this method. Immunoprecipitation of antibody-antigen complexes uses Protein A/G magnetic beads to capture the antibody, along with its bound antigen. For high-throughput formats, Protein A/G is used in multiplexed affinity capture and 96-well plate antibody screening.
