Western Blotting separates proteins by size and labels the protein of interest with an antibody
Western blotting (also called Protein Immunoblotting because an antibody is used to specifically detect its antigen) is a widely accepted analytical technique used to detect specific proteins in the given sample.
It uses SDS-polyacrylamide gel electrophoresis (SDS-PAGE) to separate various proteins contained in the given sample (e.g. to separate native proteins by 3-D structure or denatured proteins by the length of the polypeptide). The
separated
proteins are then transferred or blotted onto a matrix (generally nitrocellulose or PVDF membrane), where they are stained with antibodies (used as a probe) specific to the target protein. By analyzing location and intensity of the
specific reaction, expression details of the target proteins in the given cells or tissue homogenate could be obtained. Western blotting analysis could detect target protein which is as low as 1ng due to high resolution of the gel
electrophoresis and strong specificity and high sensitivity of the immunoassay. This method is used in the fields of molecular biology, biochemistry, immunogenetics and other molecular biology disciplines.
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Electrophoresis separation describes a phenomenon that charged particles move towards opposite electrode under the influence of electric field. It is used to separate proteins according to their
electrophoretic mobility which depends on charge, molecule size and structure of the proteins. Polyacrylamide gel (PAG) is a three-dimensional mesh networks polymer composed of acrylamide and a cross-linker (methylene bisacrylamide)
under the catalyzation of ammonium persulfate. PAG is a versatile supporting matrix due to its stable hydrophily and little adsorption and electroosmosis effect provided by its neutrally charged nature. (It possesses several
electrophoretically desirable features that make it a versatile medium. It is a synthetic, thermo-stable, transparent, strong, chemically relatively inert gel, and can be prepared with a wide range of average pore sizes).
Coats protein with negative charge: In the presence of SDS, electrophoretic mobility is mainly based on molecule weight instead of on charge and size of the proteins. SDS is an anionic detergent which could break hydrogen bond
within and between molecules to unfold proteins and break up secondary and tertiary structures as denaturing agent and hydrotropy agent. Strong reducing agents such as mercaptoethanol and Dithiothreitol (DTT) could disrupt disulfide
linkages between cysteine residues. SDS and reducing agents are applied to protein sample to linearize proteins and to impart a negative charge to linearized proteins. In most proteins, the binding of SDS to the polypeptide chain
imparts an even distribution of charge per unit mass, thereby the intrinsic charges of polypeptides becomes negligible when compared to the negative charges contributed by SDS. This new negative charge is significantly greater than
the original charge of that protein.
The electrostatic repulsion that is created by binding of SDS causes proteins to unfold into a rod-like shape thereby eliminating differences in shape as a factor for separation in the gel. Minor axis of all rods,
the SDS-protein subunit compound are nearly the same, about 1.8nm. And the length of major axis is in proportion to molecular weight of the protein subunit. Thus electrophoretic mobility of the SDS-protein subunit compound is
based on molecular weight, eliminating the influence imposed by size and charge.
The sample to be analyzed is mixed with SDS. And the mixed samples are subsequently treated by related solution. Heating the samples to at least 60°C further promotes protein denaturation and depolymerization,
helping SDS to bind and enabling the rod-shape formation and negative charge adherence. A bromophenol blue dye may be added to the protein solution to allow the experimenter to track the progress of the protein solution through
the gel during the electrophoretic run. An appropriate amount of glycerol is added to increase density and accelerate the migration of sample solution.
A buffer system with different pH values is applied in gel electrophoresis process. A very widespread discontinuous buffer system is the tris-glycine or "Laemmli" system that stacks at a pH of 6.8 and resolves at a pH of ~8.3-9.0. A
drawback of this system is that these pH values may promote disulfide bond formation between cysteine residues in the proteins because the pKa of cysteine ranges from 8-9 and because reducing agent present in the loading buffer
doesn't co-migrate with the proteins. Recent advances in buffering technology alleviate this problem by resolving the proteins at a pH well below the pKa of cysteine (e.g., bis-tris, pH 6.5) and include reducing agents (e.g. sodium
bisulfite) that move into the gel ahead of the proteins to maintain a reducing environment. An additional benefit of using buffers with lower pH values is that the acrylamide gel is more stable at lower pH values, so the gels can be
stored for long periods of time before use.
As voltage is applied, the anions (and negatively charged sample molecules) migrate toward the positive electrode (anode) in the lower chamber, the leading ion is Cl¯ ( high mobility and high
concentration); glycinate is the trailing ion (low mobility and low concentration). SDS-protein particles do not migrate freely at the border between the Cl¯ of the gel buffer and the Gly¯ of the cathode buffer. Because of the
voltage drop between the Cl- and Glycine-buffers, proteins are compressed (stacked) into micrometer thin layer-stacking gel layer.
In resolving gel layer, proteins with more negative charges per unit migrate faster than those with less negative charges per unit. That is, proteins with small molecular weight migrate faster than proteins
with large molecular weight. The boundary moves through a pore gradient and the protein stack gradually disperses due to a frictional resistance increase of the gel matrix. Stacking and unstacking occur continuously in the gradient
gel, for every protein at a different position.
Choosing The Right Gel
How to choose the gel percentage, SDS-PAGE gel percentage calculator, the relationship between gel percentage and pore size
How to choose gel percentage based on protein size
Polyacrylamide gel electrophoresis (PAGE) is used for separating proteins ranging in size from 5 to 2,000 kDa due to the uniform pore size provided by the polyacrylamide gel. Pore size is controlled by controlling the
concentrations of acrylamide and bis-acrylamide powder used in creating a gel. Typically resolving gels are made in 5%, 8%, 10%, 12% or 15%. Stacking gel (5%) is poured on top of the resolving gel and a gel comb (which forms the wells
and
defines the lanes where proteins, sample buffer and ladders will be placed) is inserted. The percentage chosen depends on the size of the protein that one wishes to identify or probe in the sample. The smaller the known weight, the
higher the
percentage that should be used. Changes on the buffer system of the gel can help to further resolve proteins of very small sizes
Check the table below for common protein sizes and their recommended gel percentages
Range of molecular weight (KD)
Concentration of gel (%)
<10
15
10 - 30
12
30 - 100
10
100 - 500
8
> 500
5
WESTERN BLOT WORKFLOW
Five steps are involved in western blotting procedure and detection assay, namely, transfer, blocking, primary antibody incubation, secondary antibody incubation and protein detection, and
western blotting analysis.
Transfer
Proteins are moved from within the gel onto a membrane made of nitrocellulose (NC) or polyvinylidene difluoride (PVDF). Without pre-activation, proteins combine with nitrocellulose membrane based on hydrophobic interaction,
thereby having slight effect on protein activities. Besides, nitrocellulose membrane produces little non-specific staining. It is cheap and ease to use. However, it is easy to erase small molecular proteins while washing. It is
fragile and has poor toughness. With high affinity, the PVDF membrane needs to be sunk in methanol before use to activate positive charge groups on the membrane, promoting combination with negative charged proteins. Specific NC
membrane with different pores should be applied according to the molecular weight of transferred proteins due to the smaller the pore of membrane the tighter the combination between membrane and small molecular weight proteins. NC
membranes of 0.45 µm and of 0.2 µm are used most. The size of 0.45 µm should be applied for proteins with molecular weight over 20KD while the size of 0.2 µm will be chosen for those below 20KD. PVDF membrane is best for the
detection of small molecular weight proteins due to its higher sensitivity, resolution as well as affinity than normal membrane.
Transfer methods that are used most for proteins are semi-dry transfer and wet transfer. Semi-dry transfer describes the method that Gel-Membrane-Filter sandwich is placed between filters loaded with transfer buffer. The transfer
process is based on current conduction produced by the transfer buffer. Semi-dry transfer takes little time with high efficiency as electric current works directly on membrane and gel. While applying wet transfer, the
Gel-Membrane-Filter sandwich is placed in the transfer tank, suspending in transfer buffer vertically. Proteins transfer from the gel to the membrane under the control of high intensity electric field produced by electrode plate
paralleled to the sandwich. While prolonging time to an appropriate extend, proteins could be transferred more effectively. Proteins within several gels could be transferred.
Blocking
In a western blot, it is important to block the unreacted sites on the membrane to reduce the amount of nonspecific binding of proteins during subsequent steps in the assay using inert protein or nonionic detergent. Blocking
buffers should block all unreacted sites. And Blocking buffers should not replace target protein on the membrane, not bind epitope on the target protein and not cross react with antibody or detection reagents. The most typical
blockers
are BSA, nonfat dry milk, casein, gelatin and Tween-20. TBS and/or PBS are the most commonly used buffers.
Inertia protein BSA, nonfat dry milk, casein, gelatin or nonionic detergent Tween-20 reduce nonspecific binding by blocking unreacted sites. Retaining protein structure, Tween-20 can reduce breakup to original interaction among
proteins while is used for protein emulsification.
Nonfat dry milk is the most economic choice
Avoid using nonfat dry milk as a blocking reagent for blots with biotin conjugated antibody because milk contains variable amounts of glycoprotein and biotin.
BSA is appropriate for blots with phosphorylated protein as target. Phosphatase contained in nonfat dry milk leads to dephosphorylation of phosphorylated protein on the membrane while phosphoryltion specific antibody is used
to identify phosphorylated protein. And nonfat dry milk is improper for blots which rely on alkaline phosphatase system.
Avoid adding NaN3 into blocking reagent for blots that base on HRP system because NaN3 is enabled to inactivate HRP.
Casein is recommended for blots with alkaline phosphatase conjugated secondary antibody. TBS buffer instead of PBS buffer should be chosen because PBS interferes alkaline phosphatase.
Primary Antibody incubation
After blocking, primary antibody specific to target protein is incubated with the membrane. And the primary antibody binds to target protein on the membrane.
In western blot, primary antibody should be validated before use. The choice of a primary antibody depends on the antigen to be detected. Both polyclonal and monoclonal antibodies work well for western blot. Monoclonal antibodies
recognize single specific antigenic epitope. Thus, they have higher specificity resulting in lower background. Blot results will be influenced if the target epitope is destroyed. Polyclonal antibodies recognize more epitopes and
they often have higher affinity. Blot results will be stable even though a few epitopes are destroyed.
Secondary antibody incubation
After rinsing the membrane to remove unbound primary antibody, the membrane is exposed to a specific enzyme-conjugated secondary antibody. And the secondary antibody binds to the primary antibody which has reacted with
the target
protein.
The most popular secondary antibodies are anti-mouse and anti-rabbit immune globulin since the host species for primary antibodies are mainly mouse and rabbit. Goat is used widely to raise anti-mouse and anti-rabbit
polyclonal
antibodies. Thus, goat anti-mouse and goat anti-rabbit immune globulin are the most commonly used secondary antibodies. The choice of secondary antibody depends upon the species of animal in which the primary antibody was raised.
For example, if the primary antibody is a mouse monoclonal antibody, the secondary antibody must be an anti-mouse antibody. If the primary antibody is a rabbit polyclonal antibody, the secondary antibody must be an anti-rabbit
antibody.
Protein detection (color development) and analysis of Protein detection (color development)
A substrate reacts with the enzyme that is bound to the secondary antibody to generate colored substance, namely, visible protein bands. The target protein levels in cells or tissues are evaluated through densitometry
and the
location of the visible protein bands.
Alkaline phosphatase (AP) and horseradish peroxidase (HRP) are the two enzymes that are used extensively. Functioned by Alkaline phosphatase (AP) catalyzation, a colorless substrate BCIP will be converted to a blue
product. In
the presence of H2O2, 3-amino-9- ethyl carbazole and 4-chlorine naphthol will be oxidized into brown substance and blue products respectively under the catalyzation of HRP. Enhanced chemiluminescence is another method that employs
HPR
detection. Using HRP as the enzyme label, luminescent substance luminol will be oxidized by H2O2 and will luminesce. Moreover, enhancers in this substrate will enable a 1000-fold increase in light intensity. HRP will be detected
when the blot is sensitized on photographic film.
Western Blotting Analysis
After color development, the pattern of the separated proteins is imprinted onto the film or captured by Western Blot gel imager. By comparing the band position to the protein ladder, one can estimate the size of the
protein.
Recommended reagents
The reagents you will need for each step are listed below.
Western blot control design
5 types of common controls
No controls, no science
Proper control design is essential to western blot. It will guarantee accurate and specific test result by identifying various problems quickly and precisely. There are 5 common types of controls seen in Western blot
experiment design.
Positive control: A lysate from a cell line or tissue sample known to express the protein you are detecting. Positive control is designed to verify working efficiency of the antibodies.
Negative control: A lysate from a cell line or tissue sample known not to express the protein you are detecting. Negative control is to check antibody specificity. Nonspecific binding and false positive result will be
identified.
Secondary antibody control (No primary antibody control): The primary antibody is not added to the membrane. Only secondary antibody is added. This is to check secondary antibody specificity. Nonspecific binding and
false
positive result caused by secondary antibody will be indicated.
Blank control: Both primary and secondary antibody are not added to membrane. This is to check membrane nature and blocking effect.
Loading control: Loading control is used to check sample quality and the performance of secondary antibody system.
More about loading controls
Loading controls are antibodies to "house-keeping proteins", or proteins that are expressed at equivalent levels in almost all tissues and cells.
Loading controls are required to check that the lanes in your gel have been evenly loaded with sample, especially when a comparison must be made between the expression levels of a protein in different samples. They are also
useful to check for even transfer from the gel to the membrane across the whole gel. Where even loading or transfer have not occurred, the loading control bands can be used to quantify the protein amounts in each lane. For publication-quality
work, use of a loading control is absolutely essential
Loading Control
Molecular Weight (KD)
Sample Type
Beta-actin
43KD
Whole Cytoplasmic
GAPDH
30 - 40KD
Whole Cytoplasmic
Tubulin
55KD
Whole Cytoplasmic
VCDA1/Porin
43KD
Mitochondrial
COXIV
16KD
Mitochondrial
Lamin B1
16KD
Nuclear (Not suitable for samples where the nuclear envelope is removed.)
TBP
38KD
Nuclear (Not suitable for samples where the nuclear envelope is removed.)
More western blot resources
sample preparation, protocol, troubleshooting and more.
WB Sample Preparation
The choice of protein extraction method is crucial. It ultimately makes the difference between a blank blot and a beautiful one.
