Dot blot is a simplified, rapid immunoassay used to detect the presence of a specific protein or antigen in a liquid sample. Unlike Western blot, it does not involve gel electrophoresis for separation, so it provides no information about molecular weight. It is ideal for screening multiple samples quickly or for detecting antigens that may be denatured by SDS-PAGE.
The entire procedure can be divided into five sequential stages:
(Workflow Overview)
Stage 1: Sample Preparation & Protein Extraction
The goal is to obtain a clarified protein lysate where target proteins are soluble, intact, and represent their in vivo state.
1.1 Sample Collection
Adherent Cells: Wash with ice-cold PBS, then scrape or trypsinize into a collection tube on ice.
Suspension Cells: Pellet by centrifugation and wash with PBS.
Tissues: Snap-freeze in liquid nitrogen, then pulverize. Alternatively, homogenize directly in cold lysis buffer.
1.2 Cell/Tissue Lysis
1.2.1 Resuspend sample in appropriate, ice-cold lysis buffer.
1.2.2 Common Buffer Examples:
RIPA Buffer: General purpose, for total cell lysates.
Laemmli (SDS) Buffer: For direct denaturation and loading.
NP-40/Triton Buffer: Milder, for native protein complexes.
1.2.3 Essential Buffer Additives (add fresh):
Protease Inhibitor Cocktail
Phosphatase Inhibitor Cocktail (if studying phosphorylation)
PMSF (serine protease inhibitor)
1.3 Clarification
1.3.1 Incubate lysate on ice for 20-30 minutes.
1.3.2 Centrifuge at >12,000 x g for 15 minutes at 4°C.
1.3.3 Transfer supernatant (cleared lysate) to a new tube. Discard pellet.
1.4 Protein Quantification & Denaturation
Quantify: Measure protein concentration using BCA or Bradford assay.
Normalize: Dilute all samples to the same concentration.
Samples can be used immediately or stored at -20°C/-80°C.
Stage 2: Coating with samples
2.1 Choose a Membrane
Typically Nitrocellulose or PVDF
2.2 Cut Membrane
Cut to the desired size. For PVDF, pre-wet in 100% methanol for a few seconds, then rinse in distilled water, and equilibrate in transfer buffer or PBS.
2.3 Mark Grid
Lightly use a pencil to draw a grid on the membrane to identify sample locations. Do not use pen.
2.4 Spot Samples
2.4.1 Place the membrane on a clean, dry surface or on top of a stack of absorbent paper.
2.4.2 Using a pipette, spot 1-5 µL of each sample directly onto the center of a grid square.
2.4.3 Allow the spot to air-dry completely (5-15 minutes) at room temperature. This fixes the protein to the membrane.
Stage 3: Blocking
Place the dried membrane in a container (e.g., Petri dish, tray). Incubate with blocking buffer (e.g., 5% non-fat dry milk in TBST or PBS-T) for 1 hour at room temperature with gentle rocking. This step saturates non-specific protein-binding sites to prevent background noise.
Stage 4: Immunodetection
Target protein is specifically identified using antibodies.
4.1 Primary Antibody Incubation
4.1.1 Dilute specific primary antibody in blocking buffer or antibody diluent.
Incubate membrane:
Option A: 1-2 hours at room temperature with agitation.
Option B (Recommended for sensitivity): Overnight at 4°C with gentle shaking.
4.1.2 Wash membrane 3 x 5-10 minutes with TBST.
4.2 Secondary Antibody Incubation
4.2.1 Incubate with HRP- or Fluorophore-conjugated secondary antibody (targeting the host species of the primary antibody) for 1 hour at RT.
4.2.2 Wash membrane thoroughly 3 x 5-10 minutes with TBST.
Stage 5: Signal Development & Analysis
5.1 Signal Detection
For HRP-conjugated Antibodies (Chemiluminescence):
Mix ECL substrate components.
Incubate membrane with substrate for 1-5 minutes.
Image using X-ray film or a digital CCD imager.
For Fluorescent Antibodies:
Rinse membrane.
Image directly using a fluorescence/scanner imager at appropriate wavelengths.
5.2 Membrane Stripping & Reprobing (Optional)
To detect a loading control (e.g., β-actin, GAPDH) on the same membrane, use a mild stripping buffer to remove antibodies, then re-block and re-probe.
5.3 Data Analysis
5.3.1 Estimate protein size using the molecular weight ladder.
5.3.2 Quantify band intensity using software (e.g., ImageJ, Image Studio Lite).
5.3.3 Normalize target protein signal to the loading control signal.
5.3.4 Compare relative expression levels between samples.