Tagging and Fusion Proteins

A protein tag is a known protein sequence attached to a protein of interest to facilitate its purification and detection. Researchers insert the target protein's nucleotide sequence into an expression vector containing a fusion tag, then introduce it into a host cell to produce a recombinant or fusion protein. Tags enable researchers to isolate specific proteins from the complex mixture of proteins in cells for further analysis.

Epitope tags are known peptide sequences, such as c-Myc, that can be fused to proteins of interest. When a protein is tagged with c-Myc, it becomes detectable using commercially available antibodies against c-Myc. This approach allows researchers to study and detect proteins for which no natural antibodies exist, enabling identification through enzyme linked immunosorbent assay or other antibody-based methods.

Histidine tags contain 2 to 10 histidine residues fused to a target protein. Polyhistidine-tagged proteins are purified using metal ion affinity chromatography columns containing divalent nickel or cobalt ions. The histidine residues bind specifically to these metal ions, allowing selective separation of the tagged protein from other cellular proteins during purification.

Fluorescent tags, such as green fluorescent protein (GFP), emit fluorescence when exposed to light. A GFP-tagged protein can be visualized inside live cells using fluorescence microscopy without requiring antibodies or chemical staining. This allows real-time observation of protein localization and dynamics within intact cellular environments.

Glutathione S-transferase (GST) is a 211 amino acid protein commonly used to tag recombinant proteins. An expression vector containing the gene of interest and GST DNA sequence is expressed in a host like E. coli. GST tags increase fusion protein solubility and bind specifically to glutathione, enabling purification by affinity chromatography using glutathione-coated beads.

Protein tags can hinder further analysis or alter protein function. Tags are removed using proteolytic enzymes that cleave at specific sites engineered between the target protein and tag. Alternatively, self-splicing protein segments called inteins can be used to remove tags without additional enzymes, triggered by specific conditions like thiol compounds or particular pH and temperature.

Self-splicing inteins are protein segments positioned between the tag and target protein that automatically splice out under specific conditions such as thiol compound presence or defined pH and temperature. This method removes tags without requiring proteolytic enzymes, allowing researchers to obtain pure target proteins for peptide identification using tandem mass spectrometry or other downstream analyses.