Purification Of An N-Terminal Histidine-Tagged Protein Using ÄKTA start™

ÄKTA start eliminates the hassles of manual protein purification handling, such as loading large sample volumes and continually monitoring UV readings, and can give you consistency of yield and purity. Madhu here demonstrates how ÄKTA start aided him in purifying a recombinant N-terminal histidine-tagged protein.

Introduction
Expression of recombinant protein with affinity tags to aid purification is a common strategy. Histidine tags are the most common and versatile of these, as histidine is small and can bind to nickel-chelating chromatography media under denaturing conditions. This property is important because recombinant proteins expressed in E. coli sometimes form inclusion bodies, which are insoluble protein aggregates that lack biological activity. Denaturing (chaotropic) agents, such as urea or guanidine hydrochloride, are used to solubilize inclusion bodies, making the extraction process easier. However, slow removal of the denaturing agents is often required for proper refolding of these proteins to their functional form.

Madhu was faced with a challenge of obtaining good yields with high purity of one such N-terminal histidine-tagged protein. The expressed protein formed inclusion bodies and to extract it, 6 M urea was used in the extraction buffer. When attempting to purify the protein Madhu could only get 60% to 80% pure protein with many closely associated impurities. After some initial tests, Madhu realized that if he diluted his original sample ten times to start the refolding process, protein purity improved. However, manually handling large sample volumes was cumbersome and time-consuming. Madhu switched to ÄKTA start and a prepacked HisTrap™ FF crude column and saw dramatic improvements both in his purification results and in his workload.

HisTrap FF crude is a ready-to-use column prepacked with precharged Ni Sepharose™ 6 Fast Flow, an immobilized metal affinity chromatography (IMAC) medium (resin). It has low nickel ion (Ni2+) leakage and is compatible with a wide range of reagents used in protein purification. The column also enables the loading of unclarified cell lysate, thus providing fast, simple, and convenient purification.

Methods
Using the Method editor option in the UNICORN™ start 1.0 control software, Madhu selected the HisTrap FF crude 1 ml column from the column list provided in the software. Using the predefined affinity template he optimized the run parameters to suit his needs (Table 1). He followed the  workflow illustrated in Figure 1 to complete the purification.

Madhu solubilized the inclusion bodies from E. coli containing the histidine-tagged protein in 50 mM Tris-HCl, 0.5 M NaCl, 6 M urea, pH 7.9. He then slowly diluted (15 min) the samples 10-fold with binding buffer (25 mM sodium phosphate, 150 mM NaCl, 20 mM imidazole, pH 7.4) and kept them at room temperature for 2 h before loading onto the column.

Madhu loaded 25 ml of 1:10 diluted sample onto a HisTrap FF crude 1 ml column. To accommodate large sample volumes and to reduce manual intervention, he loaded the sample through the system pump. He washed the column with 10 column volumes (CV) of binding buffer and eluted in a single step with 5 CV of 80% elution buffer. He collected 1 ml fractions using Frac30 fraction collector. Finally, Madhu washed the column with 15 CV of 100% elution buffer to ensure the removal of all tightly bound proteins. He reequilibrated the column with 5 CV of binding buffer so that the column was ready for the next chromatography run.

Madhu pooled the peak fractions and removed the imidazole using a 5 ml HiTrap™ Desalting column. He used the predefined Desalting template available in UNICORN start.  He analyzed the purity of the protein using 12.5% SDS-PAGE, and performed ELISA and immunodetection to confirm activity and specificity of the protein.

Results
Madhu purified the N-terminal histidine-tagged protein in a single purification step using affinity chromatography on ÄKTA start with UNICORN start control software and Frac30 fraction collector. Using UNICORN start Evaluation module, Madhu was able to identify and pool the peak fractions (Fig 2A). He used a desalting method to remove imidazole from the pooled fractions (data not shown).

The desalting step greatly reduced his time when compared to the conventional dialysis method.

Madhu evaluated the results using UNICORN start and generated a report containing information about the protein peak, such as retention time,  area and height of the peak. He also used the report to identify and pool the peak fractions, which he later analyzed for purity using 12.5% SDS-PAGE (Fig 2B). The affinity purification method Madhu developed yielded excellent separation of the target protein from contaminating cellular proteins. Using ÄKTA start he could use the same method to consistently obtain a high level of purity (> 95%) of the target protein. Madhu achieved 1.6-fold higher purity of protein when compared to his conventional manual method. The quality of the purified protein was found to be high as revealed by SDS-PAGE (Fig 3A). He further analyzed the specificity of the protein by immunoblot (Fig 3B) and ELISA (data not shown).

Conclusions
Purification of the protein on ÄKTA start using a prepacked HiTrap FF crude column, Frac30 fraction collector, and UNICORN start software reduced Madhu’s time and effort, giving him higher yield and purity with greater consistency.