Affinity capture encompasses a set of biochemical techniques that purify or isolate a target protein by exploiting a specific, reversible binding interaction between the target (or a genetically fused affinity tag) and an immobilized capture reagent. These methods are the practical implementation of affinity chromatography principles at the benchtop or microscale.
General Workflow
All affinity capture methods follow the same three-stage sequence. Binding: a crude lysate or biological fluid is incubated with the affinity resin under conditions that favor the specific interaction. Washing: the resin is washed extensively with buffer to remove non-bound and weakly bound contaminants while preserving the target interaction. Elution: the target is released using competitive displacement, pH shift, or reducing conditions, then collected in a purified form. The entire process is performed in batch (batch binding), column (gravity flow or FPLC), or magnetic bead format.
Capture Reagents
The capture reagent is immobilized on a solid support. Common supports include agarose beads (Sepharose 4B, 6B), magnetic beads (Dynabeads, MagnaBind), and paramagnetic agarose beads. The capture reagent is typically coupled via primary amines (NHS ester chemistry) or thiol groups (maleimide chemistry). The choice of support affects binding capacity, non-specific binding, magnetic handling convenience, and scalability. Magnetic beads enable rapid processing in microcentrifuge tubes without columns or centrifugation.
Affinity Tag Systems
Recombinant affinity tags are the most common approach. The polyhistidine tag (6×His–10×His) binds immobilized Ni²⁺ or Co²⁺ ions and is eluted with imidazole or low pH (His-tag/IMAC purification). The GST tag (26 kDa glutathione S-transferase) binds immobilized glutathione and is eluted with reduced glutathione (GST-tag pull-down). The Strep-tag II (8 amino acids) binds Strep-Tactin (engineered streptavidin) and is eluted with biotin or desthiobiotin (Strep-tag purification). The MBP tag (42 kDa maltose-binding protein) binds amylose resin and is eluted with maltose. FLAG tag (8 amino acids, DYKDDDDK) binds anti-FLAG monoclonal antibody and is eluted with FLAG peptide or low pH. Each tag has trade-offs in size, cost, elution conditions, and compatibility with downstream applications.
Native Affinity Methods
Native proteins can also be captured without tagging. Antibody-based capture uses immobilized antibodies to immunoprecipitate endogenous proteins (Protein A/G). Lectin affinity captures glycoproteins via carbohydrate-lectin interactions. Biotin-avidin systems exploit the streptavidin-biotin interaction (K_d ≈ 10⁻¹⁴ M) for ultra-high-affinity capture (biotin-avidin systems).
Applications
Affinity capture methods purify recombinant proteins for structural and functional studies, isolate protein complexes for interactome mapping (affinity purification–mass spectrometry, AP-MS), enrich post-translationally modified proteins, and deplete abundant proteins from clinical samples. The choice of method depends on the required purity, yield, scale, cost, and downstream application.
