Strep-tag is a widely used affinity tag in protein purification systems. It consists of two types Strep-tag II and Twin Strep-tag II. Strep-tag II is a short peptide tag consisting of 8 amino acids (WSHPQFEK) that can be fused to proteins as an N-terminal or a C-terminal tag with minimal effect on recombinant proteins. The further improved Twin Strep-tag II is a sequence of two Strep-tag II sequences in a sequential order (linked by internal amino acids), and this tag is capable of gentle and rapid purification like Strep-tag II. The two tags are free to bind either of the ligands in Streptactin and STarm Streptactin. Tag/ligand binding depends on the desired binding strength and
application. The affinity of these two tags to Streptaction and STarm Streptaction is in the μg/ml range, a high affinity that is not achieved by any other existing affinity labeling system. In addition, this flexibility of combining tags and ligands allows purification of recombinant proteins under physiological conditions. Strep-tag labeling technology can be used to purify functional strep-tagged proteins from a variety of expression systems, including
baculoviruses, mammalian cells, yeast, and bacteria. In general, both of these tags do not interfere with the folding or biological activity of the target proteins, do not react with heavy metal ions, do not have ion-exchange properties, and do not cause protein aggregation. Therefore, there is no need to remove Strep-tag II and Twin Strep -tag II after purification. The ligand protein for STarm Streptactin Beads 4FF is Streptactin Mutant coupled to highly cross-linked 4% agarose microspheres. Low concentrations (50 μmol/L) of D-Biotin in the sample do not affect the binding of the target protein to STarm Streptactin Beads 4FF. The product can be regenerated with an equilibrium solution after use or cleaned with 10 mM NaOH.