Affinity tags and competitive elution

An affinity tag is a handle you built into the protein so a ligand on a resin can choose it out of a lysate. Competitive elution is the decision to give that ligand a soluble partner it prefers, so the protein lets go without a brutal solvent. This page is a way to pick the class of handle and the class of competitor together. The wider set of chromatography modes is in chromatography methods in life-science labs. A worked view of capture followed by a charge step is in affinity and ion-exchange protein purification.

Four classes, four competitors

Chromatography on an affinity resin is still a distribution, here between a bound ligand and the mobile phase, biased hard by a specific contact. The histidine-tag class uses an immobilised metal, often nickel or cobalt. Several histidine side chains coordinate that metal. Imidazole in solution competes for the same coordination. A low-millimolar imidazole wash is a widely cited way to discourage weak binders. A higher-millimolar imidazole step or gradient elutes the tagged protein. The numbers are method-specific. Follow the resin protocol. A ballpark you can use to read a protocol, not to invent one, is a low-millimolar wash and a higher-millimolar elution.

GST-tagged fusions bind immobilised glutathione. The competitor is glutathione in solution. The tag is a whole domain, large enough to change solubility and to need a removal plan if the native protein is the real goal. Strep-tag peptides bind a streptavidin variant. Elution uses a biotin analogue such as desthiobiotin, chosen because it competes and still lets you regenerate the resin. Biotin itself can bind so tightly that regeneration becomes the problem. FLAG epitope tags are recognised by an immobilised antibody. Elution is commonly a FLAG peptide competitor or a mild pH shift the insert specifies. Peptide elution is gentle and adds a peptide you may have to remove before an assay.

These competitors are not a menu you can swap. Imidazole does not release GST. Glutathione does not release a histidine tag. Write the pair in the method title so a later HPLC step is not surprised by the wrong additive.

Sequence is the check that the pair still exists. UniProt is a public place to confirm domain boundaries and mass for the protein you think you cloned. Your construct's actual tag lives in your plasmid record. A purification of a protein whose tag was removed by a protease last Tuesday will flow through, correctly, and the chromatogram will look like failure.

Load, wash, elute, and the branch

Clarify the lysate. Equilibrate the resin in the buffer the insert names. Load at a contact time the flow and the bed allow. A rush load on a short residence time is a classic reason a tagged protein is found in the flow-through of a resin that "should have worked". Assay that flow-through. If the target is there, slow the load or add resin before you change the tag.

Wash with the protocol's wash, including the low competitor level if the class uses one. Then elute with the competitor at the protocol's elution strength, either as a step or as a gradient. Collect fractions. Pool only the fractions a gel, a blot, or an activity assay supports. Ultraviolet absorbance at 280 nanometres is a locating tool and also sees the competitor when the competitor itself absorbs. Imidazole absorbs in the ultraviolet. A tall peak at the elution step can be imidazole plus protein. The gel tells them apart.

If the elution is empty and the flow-through is empty, the protein may be in the pellet, unfolded on the resin, or lost to a protease. Check the lysate and the pellet before you buy a different tag. If the elution is full of host proteins, the wash was too meek or the tag class is shared by native proteins, which is a known histidine-tag limitation. Tighten the wash inside the protocol's range, or add an ion-exchange polish. Do not jump to a denaturing elution unless you are prepared to refold.

Chelators such as EDTA strip nickel and cobalt. Many inhibitor cocktails contain them. A "protected" lysate that never binds is sometimes a lysate that dissolved the ligand. Read the lysis recipe against the resin note.

After the competitor

The elution buffer is now part of the protein. Imidazole, glutathione, desthiobiotin, or a peptide will travel into ion exchange, into an activity assay, and into a mass spectrometer unless you remove them. Ion exchange often will not bind a protein that is still in high imidazole or high salt. HPLC may precipitate that salt when organic solvent arrives. Plan the exchange before you scale the capture. A small scout that includes the exchange teaches you more than a large elution you cannot inject.

If the tag must come off, the protease and the cut tag are new contaminants. A second pass over the same ligand can bind the leftover tag and the uncleaved fusion while the cut protein flows through. That second pass fails if the protease buffer contains a chelator and the ligand is a metal. Read both inserts together.

Identity of the band on the gel is still a claim. Peptide mass spectrometry, interpreted with the care HUPO promotes, is one way to support it. The competitor peak is not that claim.

Research-use and biosafety

Tag elution is a research purification step. It is not a clinical preparation and not evidence the protein is safe to administer. Lysates are biological materials. Your institution sets the containment. The WHO laboratory biosafety manual is a public frame for that decision. Imidazole and peptide solutions are chemicals. Handle them under your chemical assessment. Follow the resin's pressure limit. A stopcock column and an HPLC affinity column do not share a flow rate.

What to put in the enquiry

Name the tag class, the host, the lysis additives, the competitor you can accept downstream, the scale in bed volume, and whether ion exchange or HPLC follows. Ask for the ligand chemistry and the elution range the manufacturer publishes for that resin. Hardware and column classes are listed in the scientific instruments catalogue. Send the scientific requirement with the quote request. The method can be discussed from the custom protein expression and purification reference. That page is an enquiry reference. It does not mean a purification is being carried out for you. ---