Choosing an egfr antibody: why total and phosphorylation-specific reagents answer different questions, what a phospho antibody needs in sample handling that a total one does not, and the stimulated and inhibited controls that make a signalling blot mean something

Running a receptor signalling experiment

1. Decide whether the question is amount or activity. A total receptor antibody reports how much is present, which changes over hours to days. A phosphorylation-specific antibody reports activation at a residue, which changes in seconds to minutes. They answer different questions and a study usually needs both, on the same membrane.
2. Put phosphatase inhibitors in the buffer before you lyse. Phosphatases are active the moment a cell breaks. Inhibitors go into the lysis buffer fresh, the plate goes on ice, and the lysate is handled cold and quickly. Most missing phospho signal is lost in the first minute, and no amount of antibody optimisation recovers it.
3. Include a stimulated positive and an inhibited negative. Ligand-stimulated cells give a signal the antibody must detect, and cells treated with a selective inhibitor give a signal it must lose. That pair demonstrates both the reagent and the experiment, and a blot without them cannot show much.
4. Normalise phospho signal to total, not to a housekeeping protein. The informative quantity is the fraction of receptor that is phosphorylated, so the phospho signal is read against total receptor on the same samples. Normalising to a loading control alone confuses a change in activation with a change in abundance.
5. Check the residue and the species the antibody was raised against. Phospho antibodies are specific to a residue, and different residues report different downstream consequences. The residue number should be in your methods, and the sequence around it has to be conserved in your species for the reagent to work.

Time courses rather than single points

Receptor phosphorylation peaks within minutes and falls as the receptor is internalised and dephosphorylated, so a single time point can sit anywhere on that curve. A short time course is what makes a comparison between conditions interpretable.

Where the comparison is between cell lines, the kinetics themselves frequently differ, so matching a single time point between them compares the timing rather than the response.

Other readouts of the same question

Immunoassay formats measure phosphorylation quantitatively across many samples, and cytometry measures it per cell, which reveals whether a population responds uniformly or a fraction responds strongly. Both answer questions a blot cannot.

For surface receptor amount, flow cytometry on intact cells with an extracellular-epitope antibody is more informative than a blot, because it separates surface from total pools.

Buying the pair together

Buy total and phospho reagents from the same supplier where possible, so that the pair is validated together and the species reactivity claims are consistent. Record clone or catalogue number, lot and the residue for the phospho reagent.

Bank one lot of the phospho antibody for a study. These reagents are among the most lot-variable there are, and a change mid-study is indistinguishable from a change in biology.

Common questions

Why is there no phospho signal on my blot?

Almost always sample handling. Phosphatases act within seconds of lysis, so inhibitors have to be in the buffer before lysis and everything kept cold and fast. A stimulated positive control tells you immediately whether the problem is handling or biology.

Should phospho signal be normalised to total receptor or to a loading control?

To total receptor on the same samples. The meaningful quantity is the fraction activated; normalising only to a housekeeping protein cannot separate more activation from more receptor.

Does the residue matter?

Yes. Different phosphorylation sites recruit different partners and report different downstream events, so the residue belongs in the methods and a result about one site should not be generalised to the receptor being active.

Can I strip and reprobe for total after phospho?

Commonly done and it costs signal and risks uneven stripping. Running duplicate membranes, or a fluorescent two-colour detection on one membrane, is more reliable where the ratio is the measurement.

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