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#affinity tags

3 public questions tagged with this topic.

Nucleic acid probe and labelling

Which protein has high affinity for biotin in labeling systems?

Biotin-labeled nucleic acid probes are visualized through exceptionally strong non-covalent protein interactions. Avidin, a tetrameric glycoprotein from egg white, and streptavidin from Streptomyces avidinii bind biotin with a dissociation constant near 10^-15 M, among the strongest known biological affinities. This stability allows efficient capture of biotinylated probes followed by detection using enzyme-conjugated or fluorophore-conjugated avidins. Amplification steps improve sensitivity in blots and histochemistry. Antibodies, alkaline phosphatase, and T4 ligase do not exhibit specific high-affinity biotin binding, making avidin-streptavidin essential for indirect non-isotopic detection systems.

Ref: NCERT Biology Class XII Principles on Klenow fill-in labeling, Lehninger Chapter 9 DNA cloning techniques, and Molecular Cloning by Sambrook Chapter 10 documenting end-labeling of cohesive termini.

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Protein-Protein Interaction

What is a common tag used for affinity purification?

Affinity purification of multiprotein complexes for interaction studies needs small, specific peptide epitopes that enable immunoprecipitation under mild conditions with minimal interference. FLAG octapeptide DYKDDDDK recognized by high-affinity anti-FLAG M2 antibody permits gentle elution with free FLAG peptide, preserving partner proteins for mass spectrometry. FITC and TRITC are fluorescent isothiocyanate dyes used for labeling, not affinity isolation. EGFP is 27 kDa fluorescent reporter useful for localization and FRET but bulky tag may sterically hinder interactions and is not optimized for resin-based purification.

Ref: NCERT Biology Class XII Principles on Klenow fill-in labeling, Lehninger Chapter 9 DNA cloning techniques, and Molecular Cloning by Sambrook Chapter 10 documenting end-labeling of cohesive termini.

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Hybridization

What is the function of TEV protease in TAP?

Tandem Affinity Purification employs TEV protease as a highly specific cleavage tool to release protein complexes between successive affinity steps. TAP tag typically contains Protein A IgG-binding domains, a TEV recognition site, and Calmodulin Binding Peptide. After first purification on IgG Sepharose, TEV protease, derived from Tobacco Etch Virus, recognizes consensus sequence ENLYFQG and cleaves specifically, liberating target complex under mild conditions. This enzymatic cleavage preserves native interactions and eliminates contaminants. Function is not tag synthesis, immobilization, or labeling, but precise sequence-specific proteolysis enabling second purification step.

Ref: NCERT Biology Class XII Principles on Klenow fill-in labeling, Lehninger Chapter 9 DNA cloning techniques, and Molecular Cloning by Sambrook Chapter 10 documenting end-labeling of cohesive termini.

View Details