Protein tags are peptide sequences genetically grafted onto a recombinant
protein. Often these tags are removable by chemical agents or by enzymatic
means, such as proteolysis or intein splicing. Tags are attached to proteins
for various purposes. Affinity tags are appended to proteins
so that they can be purified from their crude biological source using an
affinity technique. These include chitin binding protein, maltose binding
protein, and glutathione-S-transferase. The poly(His) tag is a widely used
protein tag; it binds to metal matrices. Solubilization tags are used, especially
for recombinant proteins expressed in chaperone-deficient species such as E.
coli, to assist in the proper folding in proteins and keep them from
precipitating. These include thioredoxin and poly(NANP). Some affinity tags have
a dual role as a solubilization agent, such as MBP, and GST.
Chromatography tags are used to alter chromatographic properties of the
protein to afford different resolution across a particular separation
technique. Often, these consist of polyanionic amino acids, such as
FLAG-tag. Epitope tags are short peptide sequences
which are chosen because high-affinity antibodies can be reliably produced in
many different species. These are usually derived from viral genes, which
explain their high immunoreactivity. Epitope tags include V5-tag, Myc-tag,
and HA-tag. These tags are particularly useful for western blotting,
immunofluorescence and immunoprecipitation experiments,
although they also find use in antibody purification.
Fluorescence tags are used to give visual readout on a protein. GFP and its
variants are the most commonly used fluorescence tags. More advanced
applications of GFP include using it as a folding reporter.
Protein tags find many other usages, such as specific enzymatic modification
and chemical modification tag. Often tags are combined to produce
multifunctional modifications of the protein. However, with the addition of
each tag comes the risk that the native function of the protein may be abolished
or compromised by interactions with the tag. Therefore, after purification, tags
are commonly removed by specific proteolysis
List of protein tags (See
einogenic_amino_acid#Chemical_properties for the A-Z amino-acid codes)
= Peptide tags = AviTag, a peptide allowing biotinylation
by the enzyme BirA and so the protein can be isolated by streptavidin
Calmodulin-tag, a peptide bound by the protein calmodulin
polyglutamate tag, a peptide binding efficiently to anion-exchange resin such
as Mono-Q E-tag, a peptide recognized by an
antibody FLAG-tag, a peptide recognized by an
antibody HA-tag, a peptide recognized by an
antibody His-tag, 5-10 histidines bound by a
nickel or cobalt chelate Myc-tag, a short peptide recognized by
an antibody S-tag
SBP-tag, a peptide which binds to streptavidin
Softag 1, for mammalian expression Softag 3, for prokaryotic expression
Strep-tag, a peptide which binds to streptavidin or the modified
streptavidin called streptactin TC tag, a tetracysteine tag that is
recognized by FlAsH and ReAsH biarsenical compounds
V5 tag, a peptide recognized by an antibody
VSV-tag, a peptide recognized by an antibody
Xpress tag = Covalent peptide tags =
Isopeptag, a peptide which binds covalently to pilin-C protein
SpyTag, a peptide which binds covalently to SpyCatcher protein
= Protein tags = BCCP, a protein domain biotinylated by
BirA enabling recognition by streptavidin
Glutathione-S-transferase-tag, a protein which binds to immobilized glutathione
Green fluorescent protein-tag, a protein which is spontaneously fluorescent and
can be bound by nanobodies Halo-tag, a mutated hydrolase that
covalently attaches to the HaloLink™ Resin
Maltose binding protein-tag, a protein which binds to amylose agarose
Nus-tag Thioredoxin-tag
Fc-tag, derived from immunoglobulin Fc domain, allow dimerization and
solubilization. Can be used for purification on Protein-A Sepharose
Designed Intrinsically Disordered tags containing disorder promoting amino
acids = Others =
Ty tag Applications
Affinity purification Protein array
Western blotting References