Protein tag

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Protein tags are biochemical indicators. Affinity tags appended to recombinant expressed proteins can serve several purposes. They have traditionally been used as a way of purifying proteins using standard conditions rather than developing individual biochemical purifications based on each protein’s physical characteristics. More recently, their role as an aid to solubilization of a fusion partner has been exploited and maltose binding protein (MBP), glutathione-S-transferase (GST) and thioredoxin have proved useful in this regard. Some tags have also been used as an indicator of fusion protein folding – most notably the Green Fluorescent Protein (GFP). In this case, if the N-terminal fusion partner of GFP fails to acquire a stable folded structure, GFP and its fusion partner will aggregate and/or be degraded and the GFP fluorophore will not form. Tags are also useful in providing a common epitope, allowing a single antibody to recognize each fusion protein. Some tags are multifunctional, combining two or more of these roles: For example, the his-tag both permits purification on Ni2+-NTA and is used as a common epitope, whilst GST solubilizes some fusion partners, often increases expression levels, permits purification on glutathione-sepharose and provides a common epitope. Recently, the Biotin Carboxyl Carrier Protein (BCCP) tag has found particular utility in array fabrication protocols. This tag is a 79-residue polypeptide derived from the biotin carboxyl carrier domain of the E. coli ACCB protein and is efficiently biotinylated in vivo at a single surface-exposed lysine residue by both prokaryotic and eukaryotic biotin ligases.