#protein purification

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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Protein-Protein Interaction

Which protease is used in TAP?

Tandem Affinity Purification requires a highly specific, mild protease to release first affinity complex without disrupting interacting partners. Tobacco Etch Virus protease recognizes seven amino acid sequence ENLYFQG and cleaves between Q and G with high specificity under low temperature, physiological pH, and in presence of mild detergents that preserve protein-protein interactions. This orthogonal cleavage site is engineered between Protein A and CBP modules. Trypsin, caspase, and collagenase are more promiscuous or require conditions that would dissociate or degrade complexes, unsuitable for preserving interactome.

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

The first step in TAP is:

TAP workflow is designed for high purity through two orthogonal affinities. Lysate containing TAP-tagged bait first incubates with IgG sepharose, where Protein A domain binds heavy chain efficiently. Unbound proteins are washed extensively. Bound material is then released by TEV protease cleavage that separates Protein A from CBP-bait, ensuring specific elution. Second affinity over calmodulin beads in presence of calcium captures CBP. Calmodulin binding never occurs first, and mass spectrometry follows after final EGTA elution. This order minimizes contaminants before identification.

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

Which tag in TAP binds calmodulin?

Tandem Affinity Purification tag is engineered for sequential, highly specific purification of native complexes under mild conditions. Typical TAP contains Protein A repeats that bind IgG and calmodulin-binding peptide that binds calmodulin resin in calcium-dependent manner, separated by TEV protease site. Calmodulin-binding peptide is the module that interacts with calmodulin-coated beads in second affinity step. ProtA, GST, and His bind IgG, glutathione, and nickel respectively, not calmodulin. This dual orthogonal purification markedly enhances specificity and preserves physiological interactions for downstream mass spectrometry identification with reduced background contaminants.

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

In pull-down assays, polyhistidine tag binds to:

Polyhistidine tag, commonly hexahistidine, interacts with immobilized divalent cations through coordination chemistry. Histidine imidazole side chains donate electrons to vacant coordination sites of nickel or cobalt ions chelated by nitrilotriacetic acid resin. This interaction remains stable under physiological pH but reversible by competition with high imidazole or low pH. In pull-down and affinity chromatography, His-tagged bait is immobilized on Ni-NTA beads to capture prey interactors. The small tag minimally perturbs folding and enables IMAC purification under native or denaturing conditions unlike streptavidin or glutathione 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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Hybridization

Which tag is commonly used in TAP for high-purity complex isolation?

Tandem Affinity Purification relies on Calmodulin Binding Peptide or CBP as second affinity module for high-purity complex isolation under near-physiological conditions. CBP binds calmodulin resin in calcium-dependent manner after first purification on IgG Sepharose and TEV cleavage. Calcium chelation with EGTA allows gentle elution preserving native complexes for downstream mass spectrometry. TRITC is fluorescent dye, biotin requires streptavidin matrix, and GFP is reporter for localization, not an affinity handle in classic TAP design. CBP provides low background and reversible binding essential for sequential purification strategy maximizing purity.

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

Polyhistidine-tagged proteins bind to:

Polyhistidine tags, typically hexahistidine, exploit coordination chemistry between imidazole side chains of histidine and immobilized divalent transition metal ions such as Ni2+ or Co2+ chelated by nitrilotriacetic acid resin. At physiological pH, histidine residues donate electrons to vacant orbitals of metal, forming stable complex that allows selective retention of tagged protein from crude lysate. Elution is achieved with high imidazole concentration or low pH. Avidin, GST, and Protein A are affinity matrices for biotinylated, GST-tagged, and antibody Fc-containing proteins respectively, unrelated to metal affinity chromatography principle.

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.

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Gene Cloning Process

Which vector is suitable for producing fusion proteins?

Expression vectors are specifically engineered for efficient transcription and translation of cloned genes in host cells, unlike simple cloning vectors designed only for propagation. They contain strong regulated promoters like T7, lac, or tac, ribosome binding site, translation initiation signals, and often sequences encoding affinity tags such as His6, GST, or MBP upstream or downstream of multiple cloning site to generate in-frame fusion proteins. These tags facilitate solubility enhancement and purification. Cosmids and yeast artificial chromosomes are high-capacity cloning systems for large genomic fragments, not for routine fusion protein production.

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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