Choosing antibodies against transporters and surface receptors: what a glut1 antibody, an ldlr antibody, an mrp1 antibody, a ferroportin antibody and an sr-bi antibody each demand of the sample, why multipass membrane proteins defeat a default blot, and the controls that separate signal from background

Preparing the sample for a membrane target

1. Do not boil a multipass membrane protein. Heating in sample buffer aggregates multipass proteins, and the aggregate stays in the well. Incubating at thirty seven or seventy degrees instead is the single change that rescues most apparently failed transporter blots, and it should be the first thing tried.
2. Choose the detergent for solubility and for the epitope. A detergent that extracts the protein may denature the epitope, and a gentle one may leave the target in the pellet. Test two or three on a known positive lysate and look at both the pellet and the supernatant before concluding anything.
3. Expect glycosylation to broaden the band. Heavily glycosylated transporters run as a smear well above the predicted mass. Enzymatic deglycosylation collapses it and confirms identity, and it is the fastest way to show that a smear is your protein rather than background.
4. Use a genetic negative, and an overexpression positive. For a membrane protein at native abundance, a knockout or knockdown lysate is the control that settles specificity. An overexpression lysate as a positive tells you the antibody can see the protein at all, which is worth knowing before hunting for it at endogenous level.
5. Check whether the epitope is intracellular or extracellular. For surface staining of intact cells the epitope must be on an extracellular loop, which for a multipass transporter is a short and awkward region. Antibodies against intracellular loops are common and will not stain an intact cell whatever the protocol.

Membrane preparation rather than whole lysate

Enriching membranes before electrophoresis raises the target relative to abundant soluble protein and makes a low expressed transporter visible. It is a centrifugation step and it frequently makes more difference than any change of antibody.

Keep the whole lysate lane alongside the membrane fraction. Showing the target enriched where it should be is itself a specificity argument.

Immunohistochemistry on transporters

Membrane staining should look like a membrane: a rim around the cell or a polarised face, not a diffuse cytoplasmic haze. A diffuse pattern for a transporter is a warning sign regardless of intensity.

Retrieval conditions matter and differ between these targets, so test two buffers on a known positive tissue. A transporter with a strongly polarised distribution also gives you a built-in internal control: the wrong face is wrong.

Quantifying transporter expression

Blot signal is not proportional to protein over a wide range, so a claim about a fold change needs a dilution series showing linearity in the range measured. For membrane proteins this is skipped more often than for soluble ones.

Where absolute levels matter, a targeted mass spectrometry method with a labelled standard is the honest measurement, and an antibody-based one is a screen.

Common questions

Why does my transporter antibody show nothing on a blot?

Most often the sample was boiled and the protein aggregated in the well, or the detergent left it in the pellet. Try thirty seven or seventy degrees and check the pellet before changing reagent.

Why is the band a smear?

Glycosylation. Heavily glycosylated transporters migrate broadly and above the predicted mass; enzymatic deglycosylation collapses the smear to a band and confirms the identification.

Can I stain a transporter on intact cells?

Only with an antibody against an extracellular loop. Most catalogue reagents for these targets are raised against intracellular regions, which is visible on the datasheet and is the commonest reason a surface stain fails.

What control settles specificity?

A knockout or knockdown sample. For membrane targets at native abundance the background is high enough that an isotype control proves very little, and a genetic negative is the only strong evidence.

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