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Unlock the potential of high-affinity purification with Strep-Tactin®XT 

Strep-Tactin® and its high-affinity counterpart, Strep-Tactin®XT, are streptavidin mutants, which bind strep-tagged proteins reversibly. The differences lie in the binding strength of the possible tag-ligand combinations.
Strep-Tactin®XT stands out as the high-affinity option tailored for purifying strep-tagged fusion proteins. With binding affinities reaching the picomolar range for Twin-Strep-tag®, it retains reversible binding properties and ensures gentle recovery of immobilized proteins. This versatility extends to various protein classes and sizes, enabling efficient purification of challenging and low-abundance proteins alike. Strep-Tactin®XT fused to magnetic beads, MagStrep® Strep-Tactin®XT beads, is the ideal tool for fast small-scale batch purification in reaction tubes or 96-well plates, and protein complex identification via pull-downs. 
In particular, Strep-Tactin®XT fused to 4Flow® agarose, Strep-Tactin®XT 4Flow® high capacity resin, offers exceptional efficacy in isolating strep-tagged proteins from diluted cell extracts, enabling thorough washes with substantial volumes of buffer. The binding affinity is stable across a broad pH spectrum and compatible with diverse buffer conditions, ensuring versatility. Moreover, the high stability of the resin ensures effective binding over long periods of time.

Higher protein yields with Strep-Tactin®XT

Several proteins tagged with Strep-tag®II or Twin-Strep-tag® were purified with Strep-Tactin® (ST HC) and Strep-Tactin®XT (STXT HC). In all examples, Strep-Tactin®XT shows the highest binding capacity. The observed binding capacities were 1.5 to 3.4-fold higher in experiments using diluted cell extracts, and 1.8-fold higher using concentrated extracts. Remarkably, lower protein concentrations have no significant influence on Strep-Tactin®XT total binding capacity. In contrast, a significantly decreased binding capacity is observed for diluted cell extracts with Strep-Tactin®.

Cost-effective protein purification

Strep-Tactin®XT resins are the most cost-effective resins due to their higher binding capacity. As compared to Strep-Tactin® resins, Strep-Tactin®XT resin yields more than 2-fold more protein per ml resin and costs up to 54% less per milligram of purified protein. Furthermore, Strep-Tactin®XT prevents the unwanted leakage of the target protein during the washing steps.

Most efficient binding: Strep-Tactin®XT:Twin-Strep-tag®

Strep-Tactin® and Strep-Tactin®XT were loaded with twin-strep-tagged red fluorescent protein mCherry. Strep-Tactin®XT binds the protein directly in the upper part (right column), while Strep-Tactin® resin binds over nearly the entire resin bed (left column). The lower affinity of the tag to Strep-Tactin® causes the protein to detach more easily, resulting in a lower binding capacity as the protein flows out of the column.

During the washing steps, the protein is tightly bound by Strep-Tactin®XT and no protein is lost during purification. In contrast, significant mCherry loss is observed with Strep-Tactin® resin. This result highlights that Strep-Tactin® resins are not compatible with large samples or washing volumes. In the purification steps, due to the lower affinity, the protein detaches and runs off the column as the column volume is increased.    

High stability: Strep-Tactin®XT outperforms Strep-Tactin®

A significant contrast between Strep-Tactin® and Strep-Tactin®XT resins lies in their regeneration durability. Following just a few regeneration cycles with 0,1 M NaOH, Strep-Tactin® binding decreases, resulting in lower protein yields. In contrast, the high stability of the Strep-Tactin®XT 4Flow® high capacity allow long-term use without performance loss.
Long-term use of Strep-Tactin®XT is possible due to its high stability

High affinity of Strep-Tactin®XT enables SPR analysis and BLI

Strep-Tactin®XT can be efficiently immobilized on SPR and BLI sensors and enables capturing of TST-fused ligands with exceptionally high affinity in pM range. The system allows kinetic analyses of strong binding analytes with long dissociation times and thus overcomes the current limitations of other affinity tag-based capture systems, such as the His-tag. The possibility to capture diverse ligands even directly from culture media and a simple regeneration procedure of the biosensors add major value to the application of Strep-Tactin®XT in optical biosensor assays.

Read more about Strep-Tactin®XT technology in downstream applications in this application note. Also, find out why Twin-Strep-Tag® in combination with Strep-Tactin®XT is a better alternative to the commonly used Avi-tag in SPR here.

SPR chips were coated with Strep-Tactin® or Strep-Tactin®XT and used for immobilizing a CD45 nanobody tagged with Twin-Strep-tag®. The results show that immediately after capture, the nanobody dissociates from Strep-Tactin®. In contrast, with Strep-Tactin®XT the nanobody is bound stably over a long-period of time. These data demonstrate that Strep-Tactin®XT can be used to analyze high-affinity interactions with slow dissociation rates. 

Convenient assays and high-throughput screenings with Strep-Tactin®XT

Strep-Tactin®XT coated microplates ensure convenient assays and high-throughput screenings for strep-tagged biomolecules. Especially, the combination Twin-Strep-tag®:Strep-Tactin®XT is highly stable with a T1/2 of 13 hours and affinity in low pM range. The tag-ligand combination is an efficient and elegant option for an antibody-free immobilization of proteins. 

Moreover, the immobilized biomolecules are presented to interaction partners in a uniform manner, which results in reliable and highly reproducible assay formats. This minimizes non-specific binding concomitant with minimal coefficients of variation.  Hence, the Strep-Tactin®XT coated microplates are a precise but cost-effective tool for high-throughput screenings and diagnostic assays.

Efficient immobilization of (Twin-)Strep-tag®II fusion proteins on Strep-Tactin®XT
(A) Schematic of the oriented binding of recombinant proteins with N-terminal or C-terminal Strep-tag® during an fluorometric microtiter plate (MTP) assay. (B) Using a fluorometric MTP assay the maximum binding capacity of Strep-Tactin®XT was examined and compared with Strep-Tactin®. Bacterial alkaline phosphatase (BAP, BAP-Strep-tag®II and BAP-Twin-Strep-tag®) was applied onto microplates each coated either with Strep-Tactin® or Strep-Tactin®XT. After washing, the amounts of bound BAP were determined demonstrating that Strep-Tactin®XT recovers significantly more protein. (C) Comparison of the relative maximal binding capacity showing the efficient immobilization of proteins on Strep-Tactin®XT.

References for Strep-Tactin®XT

Protein purification

High-throughput purification


Strep-Tactin®XT products

Strep-Tactin®XT 4Flow® column
Content: 5 x 0.2 ml
Gravity flow column to purify Strep-tag®II or Twin-Strep-tag® fusion proteins
Content: 5 ml
Strep-Tactin®XT 4Flow® FPLC column
Content: 5 ml
FPLC column to purify Strep-Tag®II or Twin-Strep-tag® fusion proteins
Strep-Tactin®XT 4Flow® high capacity column
Content: 5 x 0.2 ml
Gravity flow column to purify Strep-tag®II or Twin-Strep-tag® fusion proteins
Content: 5 ml
Strep-Tactin®XT 4Flow® high capacity FPLC column
Content: 5 ml
FPLC column to purify Strep-tag®II or Twin-Strep-tag® fusion proteins
Content: 1000 ml
Strep-Tactin®XT 4Flow® high capacity resin
Content: 1000 ml
50% suspension to purify Strep-tag®II or Twin-Strep-tag® fusion proteins
Strep-Tactin®XT 4Flow® high capacity Spin Column Kit
For rapid purification of Strep-tag®II and Twin-Strep-tag® fusion proteins
Strep-Tactin®XT 4Flow® resin
Content: 4 ml
50% suspension to purify Strep-tag®II or Twin-Strep-tag® fusion proteins
Strep-Tactin®XT 4Flow® Starter Kit
All reagents required for purification and detection of Strep-tagII® and Twin-Strep-tag® fusion proteins
Strep-Tactin®XT BLI Coupling Kit
Coating of BLI biosensors with Strep-Tactin®XT for reversible capture and analysis of Twin-Strep-tag® proteins.
Strep-Tactin®XT coated microplate
Ready-to-use microplate for assay applications