Western Blot- Definition, Principle, Steps, Results, Applications
Western blotting, also known as immunoblotting or protein blotting, is a widely used technique in molecular biology and biochemistry to detect and identify specific proteins in a complex mixture of proteins. The technique is based on the principle of antigen-antibody recognition, where a specific antibody binds to its corresponding antigen (protein) on a membrane. The antibody can then be visualized by various methods, such as colorimetric, chemiluminescent, or fluorescent detection.
Western blotting can provide information about the presence, absence, size, quantity, and post-translational modifications of proteins in a sample. The technique can also be used to compare the expression levels of proteins in different samples or under different conditions. Western blotting is often used as a confirmatory test for other methods of protein analysis, such as enzyme-linked immunosorbent assay (ELISA), mass spectrometry, or immunohistochemistry.
The name western blotting was derived from the analogy with southern blotting and northern blotting, which are techniques for detecting DNA and RNA respectively. The term western refers to the use of antibodies for protein detection, which originated from the Western world. However, western blotting is not limited to antibodies and can also use other probes that bind to specific proteins, such as lectins, aptamers, or nanoparticles.
Western blotting consists of four main steps: sample preparation, gel electrophoresis, protein transfer, and immunodetection. In the sample preparation step, the proteins in the sample are extracted and solubilized in a buffer containing detergents and reducing agents. The sample is then heated to denature the proteins and break any disulfide bonds. In the gel electrophoresis step, the proteins are separated by size using an electric field applied across a polyacrylamide gel. The smaller proteins migrate faster than the larger ones through the pores of the gel. In the protein transfer step, the proteins are transferred from the gel to a membrane made of nitrocellulose or polyvinylidene fluoride (PVDF), where they are immobilized by hydrophobic interactions. The membrane acts as a solid support for the detection of proteins. In the immunodetection step, the membrane is incubated with a primary antibody that recognizes and binds to the target protein. The primary antibody is then detected by a secondary antibody that is conjugated with a reporter molecule, such as an enzyme or a fluorophore. The reporter molecule produces a signal that can be measured by a detection system.
Western blotting is a powerful and versatile technique that can be applied to various fields of research and clinical diagnosis. Some of the applications of western blotting include:
- Identification and characterization of proteins involved in biological processes, such as cell signaling, gene expression, metabolism, differentiation, apoptosis, etc.
- Detection and quantification of proteins related to diseases, such as cancer, neurodegenerative disorders, infectious diseases, autoimmune diseases, etc.
- Validation and verification of antibodies for specificity and sensitivity
- Evaluation of protein-protein interactions by co-immunoprecipitation or pull-down assays
- Assessment of protein modifications by phosphorylation, glycosylation, ubiquitination, etc.
In this article, we will discuss the principle, requirements, procedure, result interpretation, applications, and limitations of western blotting in detail.
The principle of western blotting is based on the specific recognition of proteins by antibodies. Antibodies are proteins that can bind to other proteins with high specificity and affinity, meaning that they can recognize and attach to a particular target protein among many other proteins.
Western blotting involves four main steps: sample preparation, gel electrophoresis, protein transfer, and immunodetection.
- Sample preparation: The protein sample is extracted from the biological source (such as cells, tissues, or fluids) using various methods that break down the cell membranes and release the proteins. The protein sample is then mixed with a buffer that contains detergent, reducing agents, and dye. The detergent helps to solubilize the proteins and prevent them from aggregating. The reducing agents break the disulfide bonds that hold together some proteins. The dye helps to visualize the proteins during electrophoresis.
- Gel electrophoresis: The protein sample is loaded onto a gel matrix that is made of polyacrylamide, a polymer that forms pores of different sizes. An electric current is applied across the gel, which causes the proteins to migrate through the pores according to their size and charge. Smaller and more negatively charged proteins move faster and farther than larger and less negatively charged proteins. The result is a separation of proteins into distinct bands on the gel.
- Protein transfer: The separated proteins on the gel are transferred to a membrane that is made of nitrocellulose or polyvinylidene fluoride (PVDF). The membrane is more durable and easier to handle than the gel. The transfer is done by placing the gel and the membrane in a sandwich-like configuration with filter papers soaked in a buffer that contains methanol. An electric current is applied across the sandwich, which causes the proteins to move from the gel to the membrane. The proteins are immobilized on the membrane by forming covalent or non-covalent bonds with the membrane material.
- Immunodetection: The membrane with the transferred proteins is incubated with a primary antibody that is specific for the target protein of interest. The primary antibody binds to the target protein on the membrane, forming an antigen-antibody complex. The membrane is then washed to remove any unbound primary antibody. The membrane is then incubated with a secondary antibody that is specific for the primary antibody. The secondary antibody is conjugated with a reporter molecule, such as an enzyme or a fluorophore, that can produce a detectable signal when exposed to a substrate or a light source. The secondary antibody binds to the primary antibody on the membrane, forming a sandwich-like complex. The membrane is then washed again to remove any unbound secondary antibody. The membrane is then exposed to the substrate or the light source that activates the reporter molecule, generating a signal that can be visualized by a detection system, such as a camera or a scanner. The signal intensity reflects the amount of target protein present on the membrane.
The principle of western blotting allows for the identification and quantification of specific proteins in complex mixtures by using antibodies as probes that can selectively bind to their target proteins. Western blotting can also provide information about the molecular weight, post-translational modifications, and interactions of proteins by analyzing their mobility, size, and pattern on the gel and the membrane.
Western blotting is a technique that requires various reagents and equipment to perform successfully. Some of the main requirements for western blotting are:
Gel electrophoresis: This is the first step of western blotting, where the proteins are separated by size using an electric current. The main components required for gel electrophoresis are:
- Gel (NuPAGE): This is a precast polyacrylamide gel that provides a matrix for protein separation. Different types of gels are available depending on the molecular weight range and buffer system of the proteins.
- Basic power supply: This is a device that provides a constant voltage and current to run the gel electrophoresis.
- Sample buffer: This is a solution that contains SDS (sodium dodecyl sulfate), a detergent that denatures and coats the proteins with a negative charge, allowing them to migrate through the gel according to their size. The sample buffer also contains reducing agents, such as 2-mercaptoethanol or dithiothreitol (DTT), that break the disulfide bonds in the proteins and prevent aggregation. The sample buffer also contains glycerol, which increases the density of the samples and helps them sink into the wells of the gel, and bromophenol blue, which is a dye that indicates the progress of the electrophoresis.
- Heating block: This is a device that heats the samples to 70°C for 10 minutes before loading them onto the gel. This step ensures complete denaturation and reduction of the proteins.
- Pre-stained protein ladder: This is a mixture of proteins with known molecular weights that are labeled with different colors. The protein ladder is loaded onto one of the lanes of the gel and serves as a reference for estimating the size of the proteins in the samples.
Protein transfer: This is the second step of western blotting, where the proteins are transferred from the gel to a membrane. The main components required for protein transfer are:
- Mini Trans-Blot: This is a device that consists of a tank, a lid, and a blot module. The tank holds the transfer buffer, which is a solution that contains Tris base, glycine, and methanol. The methanol helps to remove SDS from the proteins and enhances their binding to the membrane. The blot module holds the sandwich assembly, which consists of two foam pads, two filter papers, the gel, and the membrane. The sandwich assembly is placed between two electrodes that apply an electric current to transfer the proteins from the gel to the membrane. The blot module also contains cooling ice that prevents overheating during the transfer.
- Nitrocellulose or PVDF membrane: This is a thin sheet of material that binds to proteins by hydrophobic interactions. Nitrocellulose membranes are cheaper and easier to handle than PVDF membranes, but they are more fragile and have lower binding capacity. PVDF membranes are more durable and have higher binding capacity than nitrocellulose membranes, but they require activation with methanol before use.
- Square Pyrex dish: This is a container that holds water or transfer buffer for wetting or equilibrating the membrane before or after transfer.
- Square disposable plastic Petri dish: This is a container that holds water or transfer buffer for rinsing or washing the membrane after transfer.
- Razorblade and gel knife: These are tools that are used to cut or trim the gel and membrane according to the size of interest.
Immunodetection: This is the third step of western blotting, where the proteins on the membrane are detected by specific antibodies. The main components required for immunodetection are:
- Tris-buffered saline with Tween 20 (TBST): This is a solution that contains Tris-HCl buffer, sodium chloride, and Tween 20, which is a detergent that reduces nonspecific binding of antibodies to the membrane. TBST is used for washing and diluting antibodies during immunodetection.
- Shaker: This is a device that provides gentle agitation to facilitate antibody binding and washing steps during immunodetection.
- Nonfat dry milk or bovine serum albumin (BSA): These are protein sources that are added to TBST to make blocking buffer. Blocking buffer is used to cover any remaining sites on the membrane that are not occupied by proteins, preventing nonspecific binding of antibodies.
- Plastic pouches: These are bags that are used to seal and store the membrane with antibodies during incubation steps.
- Impulse heat sealer: This is a device that seals plastic pouches by applying heat and pressure.
- Primary antibody: This is an antibody that recognizes and binds to a specific epitope on the protein of interest. Primary antibodies can be monoclonal or polyclonal, depending on their origin and specificity. Primary antibodies can be obtained from various sources, such as commercial vendors or academic laboratories.
- Secondary antibody: This is an antibody that recognizes and binds to a specific region on the primary antibody. Secondary antibodies are usually conjugated with an enzyme or a fluorophore that allows detection of antibody-antigen complexes on the membrane. Secondary antibodies can be obtained from various sources, such as commercial vendors or academic laboratories.
- Detection reagents: These are substances that react with the conjugate on the secondary antibody to produce a signal on the membrane. The signal can be colorimetric, chemiluminescent, or fluorescent, depending on the type of detection reagent used. Colorimetric detection reagents produce a colored precipitate on the membrane that can be visualized by eye or scanned by an image scanner. Chemiluminescent detection reagents produce light emission on
The process of western blotting consists of the following steps:
The most commonly used samples for western blot are cell lysates which are collected by the process of extraction. The extraction can be achieved by different means like mechanical destruction, chemical extraction, or the use of enzymes. The extraction is often performed at cold temperature in the presence of protease inhibitors in order to prevent the denaturation of the proteins.
The protein sample is diluted with the sample buffer and is heated and shaken for 10 minutes at 70°C. The sample is then centrifuged at 5000g. The gel case is removed from the pouch and is placed in the buffer tank against the rubber seal with the gel walls facing the inside of the tank reservoir. The running buffer is poured onto the upper reservoir while ensuring that no buffer leakage occurs on the lower tank. Each of the wells is then loaded with an equal volume of heat-denatured sample, and one of the lanes is reserved for the protein ladder. The lid is placed on the tank, and it is connected to the power supply. The run is allowed to run at 200 V constant for 50 minutes.
The transfer buffer is prepared by adding 10% methanol to the buffer. The transfer case is taken and laid out. It is then covered with a transfer buffer. A foam sponge is taken and laid on the backside, over which goes the filter paper. These should be placed to ensure that both of them are wet and slightly submerged. The gel is taken out from the tank and placed on the wet filter paper. The nitrocellulose membrane is wet with the transfer buffer and is placed on top of the gel in a way that there are no bubbles between the gel and the membrane. The transfer case is placed into the transfer tank, which is further filled with transfer buffer. The tank is then connected to power at 100V for 1 hour. Once the transfer is complete, the transfer case is removed, and the nitrocellulose membrane is removed from the gel.
The membrane is washed with Tris-buffered saline for 5 minutes in a Petri dish. The 10% nonfat dry milk is mixed with the Tris buffer, and the membrane is covered with the mixture for 30 minutes at room temperature. The membrane is washed with the Tris buffer to remove any excess mixture remaining on the membrane. With the help of forceps, the membrane is transferred to a new Petri dish onto which the primary antibody is added. The membrane with the antibody is incubated for 3 hours at room temperature. The membrane is washed after incubation with the Tris buffer. The membrane is transferred again to a new Petri dish, where a secondary HRP-conjugated antibody is added. The membrane is incubated for 1 hour. The concentration of secondary antibodies often remains at 1 µg/ml, but this also depends on the dilution. The membrane is washed again with Tris buffer to remove excess antibodies from the surface. The membrane is incubated with the substrate for 5 minutes, and then observed using a gel documentation system.
The result of western blotting depends on the type of probes used during the process. If an enzyme-conjugated secondary antibody is used, the reaction between the substrate and the enzyme produces a color or a light signal on the membrane . The soluble dye is converted into an insoluble form, resulting in a different color on the membrane. The light output can be captured using film or a digital imaging instrument. The intensity of the signal should correlate with the abundance of the antigen on the membrane.
Alternatively, fluorescently tagged antibodies can be used, which require detection using an instrument capable of capturing the fluorescent signal. Fluorescent blotting allows for multiplexing, which means detecting multiple proteins on a single blot.
The identity of the target protein is confirmed by comparing its size to molecular weight markers and a positive control. The size of the protein is estimated by its relative mobility in the gel, which is inversely proportional to its molecular weight. The positive control is a sample that is known to contain the target protein and should produce a band at the expected size.
The result of western blotting can be qualitative or semi-quantitative. Qualitative analysis involves determining the presence or absence of the target protein in the sample. Semi-quantitative analysis involves estimating the relative amount of the target protein in different samples by comparing the signal intensity of the bands. However, western blotting is not a precise quantitative method, as there are many factors that can affect the signal intensity, such as antibody affinity, enzyme activity, substrate concentration, exposure time, etc.
Western blotting can also be used for clinical diagnosis of some diseases by detecting specific antibodies or antigens in serum samples. For example, western blotting is used as a confirmatory test for HIV infection by detecting anti-HIV antibodies in serum . However, some challenges may arise in interpreting western blot results, such as indeterminate results, cross-reactivity, false positives or negatives, etc.
Therefore, western blotting is a useful technique for protein analysis, but it requires careful optimization and validation of each step to ensure reliable and reproducible results.
Western blotting is an excellent method with high sensitivity and specificity to detect and analyze proteins in complex mixtures. It has various applications in different fields of biology, medicine, and biotechnology. Some of the applications are:
- Identification of a specific protein in a sample by using an antibody that recognizes and binds to the target protein. This can help to study the expression, localization, modification, and interaction of proteins in different cells, tissues, or organisms.
- Estimation of the size and amount of a protein in a sample by comparing the intensity and position of the protein band on the blot membrane with a standard protein ladder or a known concentration of purified protein. This can help to quantify the relative or absolute abundance of proteins in different samples or conditions.
- Diagnosis of diseases by detecting the presence or absence of specific antibodies or antigens in the serum or other body fluids of patients. For example, western blotting is used as a confirmatory test for HIV infection, where it detects anti-HIV antibodies that react with one or more viral proteins.
- Analysis of protein modifications such as phosphorylation, glycosylation, ubiquitination, acetylation, etc. by using antibodies that recognize specific modified forms of proteins. This can help to study the regulation and function of proteins in different signaling pathways or cellular processes.
- Evaluation of protein fractions during protein purification or separation by using antibodies that recognize specific markers or tags on the proteins. This can help to monitor the quality and yield of the purification process and to identify contaminants or impurities.
- Analysis of biomarkers such as growth factors, cytokines, hormones, enzymes, etc. by using antibodies that recognize specific molecules that are involved in various physiological or pathological processes. This can help to study the role and mechanism of these molecules in health and disease.
Western blotting is a powerful technique for detecting and analyzing proteins, but it also has some limitations that need to be considered. Some of the limitations are:
- Sensitivity and specificity: Western blotting is a very sensitive technique that can detect even low amounts of proteins in a sample. However, this also means that any contamination or cross-reactivity can affect the results. For example, if the antibodies used for detection are not specific enough, they might bind to other proteins that have similar epitopes or structures. This can lead to false-positive or false-negative results. Therefore, it is important to use high-quality antibodies and validate their specificity by using appropriate controls.
- Quantification: Western blotting can provide a semi-quantitative estimate of the relative abundance of proteins in a sample. However, it is not a precise or accurate method for measuring the absolute amount of proteins. This is because the signal intensity on the blot depends on several factors, such as the loading amount, the transfer efficiency, the antibody concentration, the detection method, and the image acquisition and analysis. Moreover, different proteins may have different affinities for the membrane or the antibodies, which can affect their signal intensity. Therefore, western blotting should be complemented by other methods, such as mass spectrometry or ELISA, for accurate quantification of proteins.
- Time and cost: Western blotting is a complex and labor-intensive technique that requires several steps and equipment. It can take several hours or days to complete a western blot experiment, depending on the number of samples and the protocol used. Moreover, western blotting can be expensive, especially if high-quality antibodies and detection systems are used. Therefore, western blotting may not be feasible for large-scale or routine analysis of proteins.
- Size and transfer: Western blotting relies on the separation of proteins by gel electrophoresis based on their size and charge. However, some proteins may not be well resolved by this method due to their molecular weight, shape, or post-translational modifications. For example, small proteins may not be retained by the membrane, whereas large proteins may be difficult to transfer from the gel to the membrane. Moreover, some proteins may form aggregates or complexes that affect their mobility on the gel. Therefore, western blotting may not be suitable for analyzing all types of proteins.
- Availability of antibodies: Western blotting depends on the availability of specific antibodies that can recognize and bind to the target proteins. However, not all proteins have commercially available antibodies, especially if they are novel or rare. Moreover, some antibodies may have low affinity or specificity for their target proteins, which can affect their detection performance. Therefore, western blotting may be limited by the availability and quality of antibodies.
These are some of the limitations of western blotting that should be taken into account when designing and interpreting western blot experiments. Despite these limitations, western blotting remains a valuable technique for studying protein expression and function in various biological contexts.
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