Affinity chromatography: Which tag to use?

Content:

Affinity chromatigraphy explanation
Fig. 1: Workflow of an affinity chromatography using a drip-column.
A: Cell Lysate including a protein of interest, also known as mobile phase.
B: Schematic depiction of an affinity bead corresponding to the affinity tag attached the protein of interest, also known as stationary phase.
C: A drip-column filled with affinity agarose resin.
D: While the cell lysate is flowing through the column, the proteins of interest binds to the matching affinity beads, while the rest is flowing through.
E: Removing the protein of interest from the affinity beads by changing the binding conditions. Examples here are the changing of the pH value or adding an excessive amount of a competitive peptide to the protein of interest.
F: Purified & active protein collected in a tube after affinity chromatography.
Purification via affinity chromatography is a powerful technique to separate your protein of interest from your sample. The proteins are marked with "tags" (Fig. 3, Table 4) which give them specific binding properties. A suitable immobilized ligand binds to the tagged protein (Fig. 1). So your protein of interest is separated from the rest of your sample.

Affinity chromatography: Principles

In general affinity chromatography is composed of a stationary phase (solid phase) and a mobile phase (Fig. 1). The mobile phase is your cell lysate or any mixture that contains biomolecules. A ligand that binds the target molecule is attached covalently to the solid phase. The interaction between the solid and the mobile phase are exploited by affinity chromatography to get your desired substance in a pure form. The target molecule binds to the ligand, whereas most of the other molecules flow through. The target biomolecule is eluted by changing conditions (pH or salt concentrations) or by competition with a free ligand.
The most important property which the solid phase should have is ligand immobilization. Various materials like acrylates or silica gels are appropriate. To prevent steric interference of the target molecule to the ligand, an inhibitor is attached to the solid phase. This inhibitor is known as the spacer (Fig. 2). The classic spacer is an inhibitor containing a hydrocarbon chain (CH2 spacer). Chemicals like cyanogen bromide or epoxide can functionalize the solid phase with hydrocarbon chains which result in various carbon chain lengths depending on the chemical (Table 2).

The Stationary Phase

The stationary phase of an affinity chromatography, also called the solid phase, consists of the
 
  • Core
  • Spacer
  • Ligand
  • and on occasion a metal ion that is coupled to the ligand (e.g. His-tag purification).

The Core

Table 1: Possible Cores for a stationary (solid) phase
  Features
Agarose
  • hydrophilic
  • almost no unspecific bonds
  • the gold standard for protein purification
Silica gel
  • nanoporous (leads to unspecific bonds)
  • functionalized via silane
  • silanes are washed away by alkaline buffer → reduced stability
  • applications: bound nucleic acids chaotropic
Aluminium oxide
  • acidic surface
  • bounds amines irreversibly
  • used to reduce the amount of specific substances
Acrylate
  • partially hydrophobic (unspecific bounds possible)
  • monodisperse particles
  • used for cell separation
Organic polymers
  • partially hydrophobic (unspecific bounds possible)
  • monodisperse particles
  • can be used for ligand coupling
  • not recommended for protein purification because of unspecific bounds
The preferred solid phase for protein purification are porous agarose gels. They have crucial advantages:

  • easily dispensed to fill and "pack" columns with resin beds of any size
  • large enough that biomolecules (proteins, etc.) can flow as freely into and through the beads
  • Ligands are covalently attached to the bead polymer (external and internal surfaces) by various means
  • Beaded agarose is good for routine applications as it allows easy setup, making it suitable for gravity-flow, low-speed-centrifugation, and low-pressure procedures

The Spacer

The spacer of an affinity bead can vary in size and length. Longer chain lengths have a higher mobility and are thus crucial for stericaly hindered tags.
Table 2: Overview of different spacers for affinity beads.
ChemicalChain length
Cyanogen bromide C1
Epoxide C3
Epoxide with C6 acid C10
Diamin C10
spacer affinity chromatography
Fig. 2: A functionalized bead

The Ligand

The Ligand is responsible for the affinity of the bead.
In some cases the affinity of the ligand can be further modified. A noteworthy example here are His-tag purifications. In this case the ligand is a chelator coupled with a metal ion. The metal ion is responsible for the balance between affinity and specificity of the purification.
Table 3: Typical ligands used in affinity chromatography
LigandTarget
Antibody Antigen
Iron-, aluminium-ions Phosphoproteins
Avidin Biotin
Glutathione GST
Chelator + Ni-, Co-ions His-tagged proteins

The Mobile / Liquid Phase

protein with and without tag
Fig. 3: A protein with a 6xHis-tag (A) and the native protein without the tag (B).
Affinity chromatography relies on the presence of an affinity tag - typically a peptide or protein sequence which can be added to the protein of interest on the DNA level. For small tags, this can be done by a two-step PCR, but in most cases dedicated cloning vectors are available. Once equipped with a suitable affinity tag, the gene of interest is expressed and the recombinant protein carries the respective additional amino acids.
Affinity chromatography resins or matrices are typically agarose or magnetic agarose beads that are covalently coupled to a molecule that specifically binds to the affinity tag. There is a great variety in tag-resin partner chemistries and interaction types. See table 3 for a first overview.
Table 4 on the other hand lists the most commonly used affinity tags and their individual features.
Table 4: Features of some commercially available affinity tags
  Amino acid sequenceDNA SequenceSizeSpecificity of interaction (KD)Protein Yield per ml resinElution conditions
Poly His-tag 6-14 Histidine residues CAT CAC CAT CAC CAT CAC 840.8-1,937.9 Dalton 10 µM up to 80 mg/ml (1) Imidazole or histidine, or at low pH
GST-tag GST-tag protein (whole protein) CLICK HERE 26 kDa protein 1 µM 10-12 mg/ml Reduced glutathione
Strep®-tag WSHPQFEK TGG TCG CAT CCG CAG TTC GAG AAG 1,058.1 Dalton 300 nM up to 9 mg/ml Desthiobiotin or biotin
Rho1D4 / 1D4-tag TETSQVAPA ACC GAG ACT TCC CAG GTG GCG CCA GCT 902.9 Dalton 20 nM (4) 3-4 mg/ml) Rho1D4 peptide, low pH, or protease digest
FLAG®-tag DYKDDDDK GAC TAC AAA GAC GAT GAC GAC AAG 1,012.9 Dalton 100 nM 0.6-1 mg/ml DYKDDDDK peptide

Bind- Wash - Elute: The three steps of affinity purification

Most affinity purification protocols follow the same three steps:
1. Binding:
A complex solution containing the tagged protein is applied to the column and binds based on the affinity tag - matrix interaction
2. Wash:
Other proteins which bind unspecifically are washed away with suitable buffers
3. Elution:
Specifically bound protein is eluted from the column, typically by competitive binding of a similar molecule (e.g. histidine and imidazole), by cutting off the tag with a protease or by destabilization of the affinity tag - matrix interaction e.g. by a change of pH
Binding of proteins
Washing of bound proteins
Elution of bound proteins
Fig. 4: Bind wash elute principle exemplified by an antibody-based affinity matrix (e.g. Rho1D4)

Affinity tag applications

While the term "affinity chromatography" implies that the protein of interest is being purified via the affinity tag, there are a number of applications that can be done in addition to purificaton. These include:
 
  • Detection: Specific antibodies are available for most affinity tags, so that tagged proteins can be detected in Western Blots, via immunostaining, in ELISA assays or other antibody-based applications.
  • Immobilization: Affinity tags can be used to immobilize tagged proteins, e.g. on surface plasmon resonance chips, on ELISA plates or other surfaces. The immobilized proteins can then be assessed e.g. for their ligand binding kinetics.
  • Pulldown: Affinity-tagged proteins can be pulled down from complex solutions e.g. via affinity magnetic beads. They can also be immobilized via affinity beads and used to pull down interaction partners from complex mixtures, such as cell lysates.

Different affinity chromatography methods in the lab.

Magnetic Beads

 
Ligands bound to magnetic beads are a fast and smart way to purify proteins (Read the complete guide here). To perform the purification you need a magnetic bead separator which purifies your protein of interest. Our scientist Roland Fabis PhD made a complete MagBead tutorial on YouTube (You can watch it here).
 
 
magbead separator
 
Fig. 5: A tube with magnetic beads. Put your sample into the tube and mix well. Your protein of interest binds to the magnetic beads. With a magbead separator you can isolate your protein of interest from the rest of the sample.

Fast protein liquid chromatography (FPLC)

 
FPLC is the standard method for protein purification via affinity chromatography. The advantages of FPLC is that the buffer flow rate is controlled by a positive-displacement pump and the total flow rate of the buffer is kept constant. So FPLC is very suitable for method development and the results are reproducible. The used pressure is with typically less than 5 bar relativley low. The drawback of the method is that you need to buy a very expensive chromatography system.
 
 
FPLC
 
Fig. 6: FPLC systems need many different solvents for protein purification.

Batch Spin

 
The batch spin method is done at ambient pressure. The solid phase is packed onto a column and the sample is added to bind the protein. It is a very simple method to get purified proteins in a short time.
 
 
Batch Spin
 
Fig. 7: This batch spin is done on ice to prevent protein denaturation.

Drip Columns

 
With drip columns you can work directly at your bench. They are small disposable plastic columns packed with chromatography affinity resin. In contrast to the FPLC method you cannot control pressure and flow rate.
 
 
Drip column
 
Fig. 8: A drip columns filled with our INDIGO agarose. The agarose filled in this columns have big diameters to prevent the column from clogging.

Limitations of affinity chromatography

Since affinity chromatography solely relies on the interaction of a tag-matrix interaction, it is important to be aware of certain limitations:
 
  • Tag accessibility: Interaction between the protein tag and the purification matrix is only possible if the tag is accessible and not buried within the folds of the target protein. Other buffer components such as detergents can also bury the tag sequence and have a negative effect on the tag/matrix interaction. In many cases, moving the tag to the other terminus of the protein - or even to an accessible loop within the protein - can help. If this proves unsuccessful, protein purification via the His-tag (but not via other tags) can also be done under denaturing conditions, thereby exposing the tag to the matrix surface.
  • Influence of tag on expression Depending on the protein, tags may interfere with expression rates, folding, and activity. This effect is rather inpredictable unless structural information is already available. Therefore, screening of different tags and tag positions can be useful.
  • Discrimination between protein conformations / folding variants: Affinity matrices purify all proteins which carry the affinity tag, regardless of correct folding or conformation. To discriminate between different protein conformations, multimers, or to separate correctly folded from inactive protein, protein-specific purification matrices are a useful alternative or additional method.

Further reading

1. IDA vs NTA: A tale of two ligands, Cube Biotech 2013
2. Angelo DePalma, PhD: Polyhis-Tags Improve Protein Purification. The most popular protein tagging method has many advantages and few caveats. Genetic Engineering and Biotechnology News May 8, 2014
3. Angelo DePalma, PhD: Getting a Sure Hold on Protein Purification by Attaching Convenient Handles. Genetic Engineering and Biotechnology News Mar 1, 2015, Vol. 35, No. 5
4. Locatelli-Hoops, S.C. et.al. 2013. Expression, surface immobilization, and characterization of functional recombinant cannabinoid receptor CB2. Biochim. Biophys. Acta 1834 (10):2045-56.
5. PureCube Rho1D4 Agarose, Cube Biotech 2013
Trademarks and disclaimers: StrepTactin® and Strep-tag®(IBA), FLAG® (Sigma-Aldrich Co. LLC)
Viewed