Unit 6: Detection of proteins by western blotting - Subjective Questions

Western blotting (also called immunoblotting) is a widely used analytical technique used to detect specific proteins from a complex mixture of proteins extracted from cells or tissues.

Principle:

• Proteins are first separated based on their molecular weight using SDS-PAGE (Sodium Dodecyl Sulphate Polyacrylamide Gel Electrophoresis).
• The separated proteins are then transferred (blotted) onto a solid membrane such as nitrocellulose or PVDF (polyvinylidene difluoride).
• The membrane is probed using a specific primary antibody that recognizes the target protein.
• A secondary antibody conjugated with an enzyme or fluorophore binds to the primary antibody.
• Detection is achieved through a substrate reaction (e.g., chemiluminescence) that produces a visible or measurable signal at the position of the target protein.

Thus, Western blotting combines the resolving power of gel electrophoresis with the specificity of antibody-antigen recognition to identify and quantify a particular protein.

The Western blotting procedure involves the following major steps:

1. Sample Preparation: Cells or tissues are lysed to extract proteins. Protease inhibitors are added to prevent degradation. Protein concentration is estimated.

2. Gel Electrophoresis (SDS-PAGE): Proteins are denatured with SDS and separated according to molecular weight by applying an electric field.

3. Transfer (Blotting): Separated proteins are transferred from the gel onto a membrane (nitrocellulose or PVDF) using electroblotting.

4. Blocking: The membrane is incubated with a blocking agent (e.g., BSA or non-fat dry milk) to prevent non-specific antibody binding.

5. Primary Antibody Incubation: The membrane is treated with a specific primary antibody that binds to the target protein.

6. Washing: Unbound antibody is washed away using buffers such as TBST.

7. Secondary Antibody Incubation: An enzyme- or fluorophore-conjugated secondary antibody binds to the primary antibody.

8. Washing: Again, excess secondary antibody is removed.

9. Detection: A substrate is added, producing chemiluminescent, colorimetric, or fluorescent signals.

10. Visualization & Analysis: The signal is captured on X-ray film or an imaging system and analyzed for protein size and quantity.

SDS-PAGE (Sodium Dodecyl Sulphate Polyacrylamide Gel Electrophoresis) is the first and crucial step in Western blotting used for separation of proteins.

Role and Function:

• SDS is an anionic detergent that binds to proteins and imparts a uniform negative charge proportional to their length, masking the intrinsic charge of proteins.
• SDS also denatures proteins into linear polypeptides by disrupting non-covalent interactions.
• A reducing agent like β-mercaptoethanol or DTT breaks disulfide bonds.
• As a result, proteins migrate through the polyacrylamide gel purely based on their molecular weight, with smaller proteins moving faster.

Importance in Western blotting:

• Provides clear separation of proteins by size before transfer.
• Enables accurate estimation of the molecular weight of the target protein using molecular weight markers.
• Ensures that individual proteins are resolved so they can be specifically detected by antibodies.

The two commonly used membranes in Western blotting differ as follows:

Conclusion: PVDF membranes are more durable and have higher binding capacity, making them ideal for reprobing, while nitrocellulose membranes offer lower background and are cheaper.

Purpose of Blocking:
After protein transfer, the membrane still has many free binding sites where antibodies could attach non-specifically. If these sites are not blocked, antibodies bind everywhere, resulting in a high background and false signals.

The blocking step saturates these unoccupied sites with irrelevant proteins, ensuring that antibodies bind only to the specific target protein, thereby improving the signal-to-noise ratio.

Commonly Used Blocking Agents:

• Non-fat dry milk (5%): Inexpensive, widely used; not suitable for phosphoprotein detection as milk contains casein (a phosphoprotein).
• Bovine Serum Albumin (BSA): Preferred for detecting phosphorylated proteins.
• Normal serum: Used to reduce non-specific binding.
• Casein / Commercial blocking buffers: Provide consistent blocking.

Key point: The choice of blocking agent depends on the antibody and the nature of the target protein.

Transfer of proteins from gel to membrane can be achieved by the following methods:

1. Electroblotting (most common):
An electric current drives negatively charged proteins from the gel onto the membrane.

• Wet (Tank) Transfer: The gel-membrane sandwich is submerged in transfer buffer. Provides efficient and reliable transfer, ideal for large proteins. Time-consuming.
• Semi-dry Transfer: The sandwich is placed between buffer-soaked filter papers with electrodes. Faster but less efficient for very large proteins.

2. Capillary Blotting:
Proteins are transferred by capillary action of buffer moving through the gel into the membrane. Slow and rarely used for proteins.

3. Vacuum Blotting:
A vacuum draws buffer and proteins from the gel onto the membrane. Faster than capillary transfer.

Summary: Electroblotting is the preferred method because it is rapid and efficient, ensuring quantitative transfer of proteins onto the membrane.

Antibodies are the key components that provide specificity in Western blotting.

Primary Antibody:

• Specifically recognizes and binds to a particular epitope on the target protein (antigen).
• Can be monoclonal (highly specific, single epitope) or polyclonal (recognizes multiple epitopes).
• It is raised against the protein of interest in a host animal.

Secondary Antibody:

• Recognizes and binds to the primary antibody (specifically to the Fc region of the host species' antibody).
• Is conjugated with an enzyme (like HRP – Horseradish Peroxidase or AP – Alkaline Phosphatase) or a fluorophore.
• Enables signal amplification, as multiple secondary antibodies can bind to a single primary antibody.

Detection Concept:
The enzyme on the secondary antibody reacts with a substrate to generate a detectable signal (color, light, or fluorescence) precisely at the target protein location. This two-antibody system enhances sensitivity and specificity.

The final step of Western blotting is detection of the antibody-bound target protein. The main detection methods are:

1. Colorimetric Detection:

• Uses enzyme-substrate reactions producing a colored insoluble precipitate on the membrane.
• Example: HRP with DAB or AP with BCIP/NBT.
• Simple but less sensitive.

2. Chemiluminescent Detection (most common):

• Enzyme (HRP) reacts with a substrate (e.g., luminol) to emit light.
• Detected using X-ray film or CCD imaging systems.
• Highly sensitive and widely used.

3. Fluorescent Detection:

• Secondary antibody is conjugated to a fluorophore.
• Excitation with specific light produces fluorescence detected by an imager.
• Allows multiplexing (detecting multiple proteins simultaneously).

4. Radioactive Detection (rarely used now):

• Uses radioactively labeled antibodies (e.g., ).
• Detected by autoradiography; sensitive but hazardous.

Conclusion: Chemiluminescence and fluorescence are preferred today due to high sensitivity and safety.

Western blotting is a versatile technique with numerous applications:

In Medicine and Diagnostics:

• Confirmatory test for HIV infection (after ELISA screening).
• Diagnosis of Lyme disease and Hepatitis infections.
• Detection of prion diseases such as Creutzfeldt-Jakob disease.

In Research and Biotechnology:

• Detection of specific proteins in a sample.
• Quantification of protein expression levels.
• Study of post-translational modifications (phosphorylation, glycosylation).
• Confirmation of recombinant protein expression.
• Analysis of protein-protein interactions.
• Verification of antibody specificity.

In Quality Control:

• Checking purity and identity of protein products in the pharmaceutical industry.

Conclusion: Its specificity and sensitivity make Western blotting invaluable for both diagnostic and research purposes.

These three blotting techniques are used to detect different biomolecules:

Note: Southern blotting was named after its inventor Edwin Southern, while Northern and Western were named as a play on words following the same convention.

Molecular weight markers, also called a protein ladder, are a mixture of proteins of known molecular weights loaded alongside the sample during SDS-PAGE.

Importance:

• Size Determination: They allow estimation of the molecular weight of the target protein by comparison.
• Transfer Verification: Pre-stained markers indicate whether the protein transfer from gel to membrane was successful.
• Orientation Reference: Help identify the correct band position on the membrane.
• Standardization: Enable comparison across different experiments and gels.

Types:

• Pre-stained markers: Visible without staining; used to monitor separation and transfer.
• Unstained markers: Provide more accurate molecular weight estimation.

Conclusion: Protein ladders act as a reference scale, essential for accurate interpretation of Western blot results.

Several problems can arise in Western blotting, along with their solutions:

1. High Background:

• Cause: Insufficient blocking, high antibody concentration, inadequate washing.
• Solution: Increase blocking time, dilute antibodies, wash thoroughly.

2. No Signal / Weak Signal:

• Cause: Low protein amount, inactive antibody, inefficient transfer.
• Solution: Increase protein loading, use fresh antibody, verify transfer.

3. Non-specific Bands (Multiple Bands):

• Cause: Antibody cross-reactivity, protein degradation.
• Solution: Use specific antibody, add protease inhibitors.

4. Uneven / Patchy Bands:

• Cause: Air bubbles during transfer, uneven contact.
• Solution: Remove air bubbles, ensure proper sandwich assembly.

5. "Smiling" Bands:

• Cause: Overheating during electrophoresis.
• Solution: Run gel at lower voltage, keep cool.

6. White (Reverse) Bands:

• Cause: Too much antibody or excess signal.
• Solution: Reduce antibody concentration.

Conclusion: Careful optimization of each step is essential to obtain clear, reproducible results.

Proper sample preparation is essential for accurate protein separation in Western blotting.

Role of SDS (Sodium Dodecyl Sulphate):

• An anionic detergent that binds to the hydrophobic regions of proteins.
• Denatures proteins, unfolding them into linear polypeptide chains.
• Confers a uniform negative charge proportional to protein length.
• Ensures proteins separate strictly by molecular weight, not by native charge or shape.

Role of Reducing Agents (β-mercaptoethanol / DTT):

• Break disulfide bonds () between and within polypeptide chains.
• Convert oligomeric proteins into their individual subunits.
• Ensure complete denaturation and accurate size-based separation.

Additional Components:

• Bromophenol blue: Tracking dye to monitor migration.
• Glycerol: Increases sample density so it settles into wells.

Conclusion: SDS and reducing agents together fully denature proteins, ensuring reliable separation for subsequent detection.

Western blotting is used as a confirmatory test for HIV because of its high specificity, which reduces false-positive results obtained from screening tests like ELISA.

Reason and Procedure:

• The ELISA test is highly sensitive but can give false positives.
• To confirm, the patient's serum antibodies are tested against specific HIV viral proteins (antigens) that are separated by SDS-PAGE and blotted onto a membrane.
• Key HIV antigens include gp120, gp41 (envelope glycoproteins) and p24 (core protein).
• If the patient's serum contains antibodies against these HIV proteins, they bind to the corresponding bands.
• A secondary antibody-enzyme conjugate detects this binding.

Interpretation:

• Positive result: Presence of bands corresponding to specific HIV proteins (e.g., p24, gp41, gp120).
• Negative result: No specific bands.

Conclusion: Since it detects antibodies against multiple specific viral proteins, Western blotting provides definitive confirmation of HIV infection.

Various buffers play critical roles at different stages of Western blotting:

1. Sample/Loading Buffer (Laemmli buffer):

• Contains SDS, glycerol, β-mercaptoethanol, Tris-HCl, and bromophenol blue.
• Denatures proteins and prepares them for loading.

2. Running Buffer (Tris-Glycine-SDS):

• Used during electrophoresis.
• Maintains pH and provides ions for current conduction.

3. Transfer Buffer (Tris-Glycine-Methanol):

• Facilitates transfer of proteins from gel to membrane.
• Methanol aids protein binding to the membrane; SDS may be reduced for efficient transfer.

4. Blocking Buffer:

• Contains blocking agents (milk or BSA) in TBS/PBS.
• Prevents non-specific antibody binding.

5. Washing Buffer (TBST / PBST):

• TBS or PBS + Tween-20 (a detergent).
• Removes unbound antibodies and reduces background.

Conclusion: Each buffer is specifically formulated to optimize its corresponding step, ensuring efficient and specific protein detection.

Both types of antibodies can serve as primary antibodies but differ in their properties:

Application in Western Blotting:

• Monoclonal: Preferred when high specificity and reproducibility are needed.
• Polyclonal: Preferred for detecting low-abundance proteins due to higher sensitivity through multiple epitope binding.

Western blotting can provide semi-quantitative data about the relative amount of a target protein.

Principle:
The intensity of the band is proportional to the amount of target protein present, within a certain linear range.

Procedure for Quantification:

1. Densitometry: The band intensity is measured using imaging software (e.g., ImageJ) to obtain a numerical value.
2. Normalization: Target protein intensity is normalized against a loading control (housekeeping protein such as β-actin, GAPDH, or tubulin) to correct for loading differences.
3. Relative Quantification: The ratio is compared across samples.

Limitations:

• It is only semi-quantitative, not absolute.
• Accurate only within the linear range of detection.
• Signal saturation and variability can affect results.

Conclusion: By combining densitometry with proper loading controls, Western blotting allows comparison of relative protein expression between samples.

Loading controls are proteins used as internal references to ensure accurate and reliable interpretation of Western blot results.

Definition:
They are housekeeping proteins that are expressed at constant levels in all cells regardless of experimental conditions.

Common Examples:

• β-actin
• GAPDH (Glyceraldehyde-3-phosphate dehydrogenase)
• α-tubulin
• Histones (for nuclear proteins)

Significance:

• Verify equal loading: Confirm that the same amount of total protein was loaded in each lane.
• Normalization: Serve as a reference for quantifying target protein expression.
• Transfer efficiency: Confirm uniform protein transfer across lanes.
• Error detection: Reveal errors in loading, transfer, or sample preparation.

Conclusion: Loading controls are essential for validating that any observed differences in target protein levels are real and not due to experimental variation.

Stripping and reprobing is a technique that allows the reuse of a single membrane to detect multiple proteins.

Concept:
After detecting one protein, the bound antibodies are removed (stripped) from the membrane, which is then reprobed with a different primary antibody to detect another target protein.

Stripping Procedure:

• The membrane is incubated in a stripping buffer (containing SDS, β-mercaptoethanol, and Tris-HCl, usually at mild heat).
• This disrupts the antibody-antigen bonds without removing the proteins fixed to the membrane.
• The membrane is washed, re-blocked, and reprobed.

Advantages:

• Saves time, samples, and cost.
• Useful for detecting a target protein and its loading control on the same membrane.
• Allows comparison of multiple proteins from the same sample.

Limitations:

• Repeated stripping may remove proteins or reduce signal intensity.
• PVDF membranes withstand stripping better than nitrocellulose.

Conclusion: Stripping and reprobing maximizes the information obtained from a single blot while conserving resources.

Western blotting is a powerful but not perfect technique. Its merits and drawbacks are:

Advantages:

• High specificity due to antigen-antibody interaction.
• High sensitivity, detecting proteins in nanogram/picogram quantities.
• Provides information about the molecular weight of the protein.
• Can detect post-translational modifications.
• Allows semi-quantitative analysis of protein expression.
• Serves as a confirmatory diagnostic tool (e.g., HIV).

Limitations:

• Time-consuming and labor-intensive (multi-step procedure).
• Requires a specific, high-quality antibody, which may not always be available.
• Only semi-quantitative, not truly quantitative.
• Low throughput — analyzes limited samples at a time.
• Results depend heavily on technical skill and optimization.
• Prone to artifacts (background, non-specific bands).

Conclusion: Despite its limitations, Western blotting remains a gold-standard technique for specific protein detection when performed with proper controls and optimization.