What types of protein tags are commonly used?

Protein tags are peptide sequences attached to recombinant proteins to facilitate easy purification and detection of expressed proteins. A wide variety of protein tags can be used in research and commercial production, which can be classified into three main groups.

• Affinity tags: These tags allow purification of recombinant proteins from their crude biological source using affinity-based techniques such as affinity chromatography. Examples are Glutathione-S transferase (GST), Poly-Histidine tag (His), Calmodulin Binding Protein (CBP) and Maltose-binding protein (MBP).
• Epitope tags: These tags are short peptide sequences that are readily recognized by antibodies. Since the peptide sequences are usually very short, they have little or even no effect on the structure of the expressed protein of interest. Some popular epitope tags are Myc tag, Human influenza hemagglutinin (HA) tag and FLAG tag.
• Fluorescent tags: Green fluorescent protein (GFP) and its variants are most commonly used as fluorescent tags, which can give a visual readout of the tagged proteins in both live and fixed cells.