Human influenza hemagglutinin (HA) is a surface glycoprotein required for the infectivity of the human influenza virus. The HA-tag is derived from the HA-molecule corresponding to amino acids 98-106. HA-tag has been extensively used as a general epitope tag in expression vectors. ^[1] Many recombinant proteins have been engineered to express the HA-tag, which does not generally appear to interfere with the bioactivity or the biodistribution of the recombinant protein. This tag facilitates the detection, isolation and purification of the protein of interest.^[2]

The HA-tag is not suitable for detection or purification of proteins from apoptotic cells since it is cleaved by Caspase-3 and / or Caspase-7 after its sequence DVPD, causing it to lose its immunoreactivity.^[3] Labeling of endogenous proteins with HA-tag using CRISPR was recently accomplished in-vivo in differentiated neurons.^[4]

Sequence

The DNA sequences for the HA-tag include: 5'-TAC-CCA-TAC-GAT-GTT-CCA-GAT-TAC-GCT-3' or 5'-TAT-CCA-TAT-GAT-GTT-CCA-GAT-TAT-GCT-3'. The resulting amino acid sequence is YPYDVPDYA (Tyr-Pro-Tyr-Asp-Val-Pro-Asp-Tyr-Ala).^{[citation needed]}