DYKDDDDK

DYKDDDDK, commonly known as the FLAG tag, is an eight-amino-acid epitope used as an affinity tag in recombinant protein expression and purification. The sequence is aspartic acid–tyrosine–lysine–aspartic acid–aspartic acid–aspartic acid–aspartic acid–lysine (D-Y-K-D-D-D-D-K). The tag is recognized by specific anti-FLAG antibodies and is widely used to detect and purify tagged proteins in bacteria, yeast, and mammalian cells.

In practical use, the FLAG tag is typically appended to the N- or C-terminus of a protein of interest. Purification is accomplished via anti-FLAG affinity reagents, often employing the M2 monoclonal antibody bound to resin to capture FLAG-tagged proteins. Detection methods include Western blotting, immunoprecipitation, immunofluorescence, and immunohistochemistry using anti-FLAG antibodies. The FLAG system also supports various reagents and protocols for rapid tagging and analysis.

Variants of the FLAG tag exist to increase sensitivity or alter properties. The most common is 3xFLAG, which concatenates three FLAG units to improve signal strength in detection assays. Researchers may choose different tag placements or linker sequences to balance expression, solubility, and tag accessibility.

Considerations include the tag’s potential impact on protein folding, localization, or function, particularly when placed at sensitive regions or in proteins that form tight structures. While tightly accessible in many contexts, the FLAG tag may be obscured in some conformations or by interactions with other domains, which can reduce antibody binding. Overall, the FLAG tag is a versatile and widely used tool for tagging, purifying, and detecting recombinant proteins.