Protein Purification - Application Examples for Strep-Tactin®
Strep-Tactin® Sepharose® was deployed in Yan et al., 2020. The study reveals the cryo-EM structure of the receptor protein ACE2, the target for the SARS-CoV-2 virus into human cells.
To infect a host cell, the Coronavirus Sars-CoV-2 must bind to the receptor protein angiotensin-converting enzyme 2 (ACE2) via its surface spike proteins. Yan et al. were able to reveal the structure of the human ACE2 with and without the binding domain of the SARS-CoV-2 spike protein. They found that ACE2, which was formerly difficult to measure, could be stabilized by the presence of the membrane protein B0AT1. Thus, the authors co-expressed Strep- and FLAG-tagged B0AT1 together with ACE2 in a human cell line. Then they purified the protein complex through a tandem affinity approach using anti-FLAG affinity resin and Strep-Tactin® Sepharose followed by size exclusion chromatography. After mixing a part of the purified samples with the SARS-CoV-2 binding domains, the resulting complexes were studied by cryo–electron microscopy. The researchers could present the structure of ACE2 forming a dimer which is enclosed by B0AT1. A further structure with the bound SARS-CoV-2 spike binding domain suggests that two virus spike proteins bind to one ACE2 dimer. The remarkable findings of Yan and colleagues could be helpful for the development of therapeutics that interfere with the virus-host cell interaction and thus prevent infection.
|Read the research article here
The Strep-Tactin®XT is the recommended purification resin of choice for Strep-tag®/Twin-Strep-tag® proteins. Nevertheless in some cases purification of Strep-tag®/Twin-Strep-tag® proteins via Strep-Tactin® might be an alternative:
- When elution of Strep-tag®/Twin-Strep-tag® proteins from Strep-Tactin®XT does not work
- The target protein is a multimer
Unparalleled purity after one single purification step
After purification with the Strep-tag® system, there is no need for an additional purification step like size exclusion or ion exchange chromatography.
A 36 kDa enzyme and a mutant, both with C-terminal Strep-tag®II, were expressed in the cytoplasm of E. coli. The crude lysate was chromatographically separated on Strep-Tactin® Sepharose® (5 mg/ml) under gravity flow and physiological conditions (100 mM Tris-Cl, pH 8.0). The purification is documented on a Coomassie stained SDS gel where samples from the crude lysate (lane 1), from the flow through (lane 2), and from the elution with 2.5 mM desthiobiotin (lane 3) had been applied. The wild type enzyme is shown to be over 99 % pure (lane 3 contains 50 µg protein vs 0.5 µg protein per band in the molecular size standard). The mutant - although expressed at low level only - could be obtained at high purity under the same conditions.
Strep-Tactin® Superflow® for intact virus like particle (VLP) 4.8 MDa purification
1 liter culture was induced at an OD600 of 0.6 using anhydrotetracycline (AHT) and protein expression was performed at 37°C for 3 hours at 200rpm. Cells were then pelleted, resuspended in 20 ml Buffer W (100 mM Tris-Cl, 150 mM NaCl, 1mM EDTA) and sonicated. The insoluble material was pelleted and the crude lysate was loaded on a Strep-Tactin® Superflow® column at a flow rate of 1ml/minute. After washing off the host proteins, the VLPs fused to Strep-tag® II were eluted using 2.5mM desthiobiotin in Buffer W. pASK-IBA7 was used for cloning.
For the purification of intact VLPs, the careful choice of both an ideal affinity tag system and the resin support is important:
- The affinity chromatography process should be gentle in order to keep the VLPs intact, as with Strep-tag®.
- The resin support packing within the column must allow penetration of the large VLPs.
While MacroPrep® carriers did not meet criterion (2), since the viral particles accumulated on top of the column, Sepharose® and Superflow® enabled purification. Superflow® can be used in FPLC systems, whereas Sepharose® is only suitable for gravity flow purification.
For some recombinant proteins, purification characteristics on Strep-Tactin® MarcoPrep® differ from those on Strep-Tactin® Sepharose®
The Coomassie stained SDS gel shows the purification of a recombinant protein which exhibits exceptionally high non-specific protein binding. In this case, we have been able to remove the contaminants nearly quantitatively using the Strep-Tactin® MacroPrep® resin while the contaminants co-eluted with the recombinant protein after the same purification protocol using the Strep-Tactin® Sepharose® resin. (Please note: normally behaving recombinant proteins can be purified in a single step to homogeneity using Strep-Tactin® Sepharose®). This example shows that it may be reasonable to change the resin if non-specific protein binding occurs.
MacroPrep® is particularly advantageous, e.g. for plant proteins, where purification with MacroPrep® leads to better protein yields
Lane 1, molecular weight standard (kDa); Lane 2, Strep-Tactin® MacroPrep purified protein sample; Lane 3, Strep-Tactin® Sepharose® purified protein sample; Lane 4, soluble raw lysate as applied on each column