Streptags

Streptags are short peptide tags used in molecular biology to tag recombinant proteins for purification and detection. The system relies on a high-affinity, highly specific interaction between a small peptide and engineered streptavidin variants immobilized on chromatography resin, typically Strep-Tactin. This setup enables gentle, reversible purification under mild conditions.

The most widely used variant is Strep-tag II, an eight-amino-acid sequence with the motif Trp-Ser-His-Pro-Gln-Phe-Glu-Lys (WSHPQFEK). The original Strep-tag is of similar length and function; both are designed to be minimally disruptive to protein folding and function. The tag can be fused to either end of a protein, or placed in a permissive internal loop if needed, and may be left on after purification or removed by including a protease site between the tag and the protein when removal is desired.

Purification proceeds by binding of the tagged protein to a Strep-Tactin resin, followed by washing to remove contaminants. Elution is achieved with a buffer containing desthiobiotin, which competes with the tag for binding to Strep-Tactin, allowing recovery of the tagged protein under mild, near-physiological conditions. The approach offers advantages such as gentle elution, small tag size, and compatibility with a variety of expression systems, but commonly has lower binding capacity than some alternative tags and requires specific resin and reagents.

In practice, the Strep-tag system is used for affinity purification and downstream applications where preserving protein activity is important. It is accompanied by anti-Strep-tag antibodies and detection methods for verification and analysis.