Influenza hemagglutinin epitope tag (HA tag)
Nine-residue epitope tag, YPYDVPDYA, derived from human influenza hemagglutinin and recognized by commercial anti-HA antibodies. Fused to a protein of interest it allows detection by western blot and immunofluorescence and capture by immunoprecipitation.
Origin: Human influenza hemagglutinin epitope
Characteristics
Encodes YPYDVPDYA (9 amino acids, 27 nucleotides). The peptide is short and hydrophilic and is typically tolerated at either terminus or in a flexible internal loop. Detection depends on antibodies against the epitope, so it works independently of the cargo. Tandem repeats such as 3xHA are used where a single copy gives too little signal.
Applications: Detection and immunoprecipitation of tagged proteins expressed from plasmid or viral vectors in mammalian cells. A common choice for receptors and channels, where a small epitope is less likely to disturb function than a large fusion.
Limitations: Antibody recognition depends on the epitope being accessible, so a tag buried in a folded region may not be detected. A single copy has modest sensitivity. HA is also the name of the influenza signal peptide in this library (ha-sp), so the two should not be confused.