GFP antibodies and reporter and tag reagents: detecting what you introduced

Using a GFP antibody and the other tag reagents properly

1. Run the untransduced line every time. A cell line lacking the construct is the ideal negative control and it costs one lane. Any signal there is background, and knowing its size makes every subsequent interpretation quantitative.
2. Check cross reactivity across the family. Fluorescent proteins share substantial sequence, and a reagent raised against one frequently detects several. If two colours are used in the same experiment, confirm the reagents distinguish them before relying on either.
3. Confirm the fusion is intact. A tag antibody reports the tag, and a cleaved or degraded fusion still carries it. Run the tag reagent and a reagent against the partner and check the bands agree in size before treating a tag signal as the protein.
4. Choose the tag for the application. Small peptide tags are less likely to perturb function and are harder to detect at low abundance. Larger fusion tags detect easily and are more likely to interfere. Decide against the experiment rather than by habit.
5. Keep detection chemistry separate in your thinking. Reagents against labels such as biotin, digoxigenin or a fluorophore are part of a detection layer. Treating them as target reagents leads to protocols where nobody can say what the signal reports.

The best control in the building

Almost no antibody experiment has a true negative sample available. Tag and reporter work does, because the parental line genuinely lacks the antigen. Laboratories that habitually run it get a clean measurement of background that transfers to every other stain on the same instrument.

Make it part of the protocol rather than an occasional check. It costs a lane and it settles arguments before they start.

Tags change the protein

A fusion tag can alter folding, localisation, turnover and interactions, and the effect depends on position. A construct that works for detection may not behave like the endogenous protein.

Where the conclusion depends on the protein behaving normally, compare tagged and untagged versions once, or use an endogenous tagging approach. It is the difference between a reporter and a proxy.

An anti cas9 antibody and the nuclease as an epitope

Editing experiments need to show the nuclease was expressed and then that it is gone, so an anti cas9 antibody is used for both, and the orthologue matters: a clone raised against the Streptococcus protein may not see the Staphylococcus one. Most constructs carry a tag as well, which gives a second detection route and a control when the antibody and the tag disagree.

A crispr antibody and what that phrase can mean

There is no antibody against an editing method, so a crispr antibody in a catalogue means an antibody against a component: the nuclease, a base editor's deaminase, a tag on the construct, or a marker of the modified protein. The order has to name the component, because a request against the acronym cannot be filled. The tag route is usually the more reliable detection.

A gal4 antibody and a yeast protein used as a tool

GAL4 and its DNA binding domain are used as transcriptional tools in systems that carry no endogenous copy, so a gal4 antibody is expected to give no signal in an untransfected control and that absence is the cleanest specificity evidence available. Many constructs express only a fragment, so the epitope has to lie inside it, and the expected mass is the construct's rather than the full protein's.

An anti tdtomato antibody and a red reporter that is a dimer

tdTomato is a tandem dimer, so its mass is about twice that of a single fluorescent protein and a blot looking for the smaller band finds nothing. An anti tdtomato antibody often cross reacts with related red proteins derived from the same ancestor, which is useful or misleading depending on the experiment. The protein's own fluorescence is the control the antibody is checked against.

A cre antibody and the recombinase after the event

A recombination that has happened leaves a permanent genetic mark while the enzyme itself may be transient or restricted, so a cre antibody reports where the enzyme is now and a reporter allele reports where it acted. The two images are different and both belong in a lineage figure. A line carrying no recombinase is the negative control, and it is easy to include.

A beta galactosidase antibody and the endogenous background

The bacterial enzyme is a classic reporter, but mammalian tissue carries its own acidic galactosidase activity, so an enzymatic assay has background that an antibody avoids: a beta galactosidase antibody detects the bacterial protein specifically if the clone says so. An untransfected control run through the same protocol is what separates reporter signal from tissue activity.

A zsgreen antibody and a reporter that resists denaturation

Several fluorescent reporters are tetrameric or unusually stable, so a zsgreen antibody may see an incompletely denatured species running above the expected mass, and a longer boil changes the band. Because the protein is bright, its own fluorescence is available as an independent check, and a blot that disagrees with the microscope usually means the sample preparation rather than the antibody.

Common questions

Do I still need controls for a tag antibody?

Yes, and they are unusually easy: an untransduced line, and where possible a construct with the tag in another position. The convenience of the reagent is not a reason to skip the evidence.

Will an EGFP antibody against one fluorescent protein detect another?

Frequently, because the family shares sequence. Test it explicitly if two are used together, because a cross reacting reagent will quietly merge the two populations.

Why does my tag blot show the wrong size?

Degradation of the fusion, an unintended start site, or cleavage at a linker. Blot the partner as well and compare, because a tag signal at an unexpected size is information rather than noise.

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