streptag

Strep-tag is a family of short peptide affinity tags used in molecular biology to purify and detect recombinant proteins. The tag is designed to bind specifically to engineered streptavidin proteins, such as Strep-Tactin, enabling purification under mild, near-physiological conditions. The most commonly used form, Strep-tag II, is an eight-amino-acid peptide that is fused to either end of the target protein.

Purification typically involves attaching the tag to the protein of interest and expressing it in a suitable system. The lysate is passed through a column containing immobilized Strep-Tactin. The tagged protein binds to the resin while most other proteins are washed away. Elution is achieved with desthiobiotin, a biotin analog, which competes with the tag for binding to Strep-Tactin. Desthiobiotin can be removed later by dialysis or desalting, yielding purified protein in a gentle elution.

Variants and compatibility are an important aspect of the system. Strep-tag II is the widely adopted form and often provides improved binding properties to Strep-Tactin resin compared with earlier variants. The system also includes engineered forms of streptavidin with enhanced affinity for Strep-tags. The tags are small, which minimizes interference with protein folding and function, and they support both purification and downstream detection methods, including western blotting and immunoassays.

Applications and limitations: Strep-tag is used for recombinant protein purification, functional studies, and interaction analyses in research and industry. Its effectiveness depends on expression level, tag placement, and buffer conditions. Some downstream assays may require removal of the tag, and not all proteins purify efficiently with this method. Overall, it provides a gentle and specific alternative to other affinity tags.