Griffin:Western Blot Optimization
Your time and labor at the bench is extremely valuable. Below is a guide that will enable you to interpret your western blotting experience in a purposeful way so that each experiment will make a difference. Watch out for futile cycling in your work and focus your energy in a meaningful way.
There is an increasing number of available commercial antibodies from several vendors. Depending on who you buy from, the level of quality control the vendor performs is highly variable and this leaves you with the responsibility to optimize your own protocol in an efficient manner. Because every antibody is different, each one requires a different blocking/incbuation buffer in order to optimize the signal:noise ratio.
There are common themes in the types of incubation buffers that tend to work well for blocking and incubation.
Commercial antibodies have a wide range of specificity and sensitivity for the protein of interest. Effecient blocking of the primary and secondary antibodies can take place in a variety of different blocking agents that contain soluble proteins and nonionic detergent. When developing a new antibody by western blot, it is important to test several different blocking/incubation buffers for the best possible signal:noise ratio in the assay. No single blocking agent is ideal for every primary antiobody, since each antibody-antigen pair has unique binding characteristics.
The importance of the blocking/antibody incubation buffer
PVDF or nitrocellulose membrane has the ability to bind protein of all sorts. Since both the antibodies and the transferred lysate/extract are proteins, steps must be taken to optimze interactions between the membrane and the antibody used for detection of the target protein.
Blocking of non-specific binding is achieved by placing the membrane in a dilute solution of protein containing a low percentage of detergent such as Tween-20. The protein in the dilute solution attaches to the membrane in all places where the target proteins have not attached from the gel transfer to the membrane.
When the primary antibody is added, there is no room on the membrane for it to attach other than on the binding sites of the specific target protein. This reduces "noise" in the final product of the Western blot, leading to clearer results.
Causes of excess signal; multiple bands/too much banding
Theoretical prediction is a primary antibody toward a single gene product should produce a single band. While this is common and in some cases to be expected, there are legitimate exceptions to the rule and other factors may be responsible. For example:
Factors that can produce multiple bands of variable molecular weight
- transcript variation
- multiple start/stop sights
- signal-dependent protein processing/shuttling (ie secretion)
- cell type/differentiation state specific variation
- endoplasmic reticulum and golgi-dependent alternative cofactor additions
Factors that can produce a smearing effect for the band of interest
- post-translational modifications including glycosylation, nitrosylation, phosphorylation, methylation, acetylation, ubiquitination,
- ubiquitin-dependent protein degradation
How do I decrease the background on my blot?
The leading cause of excess background is cross-reactivity between blocking agent and primary or secondary antibody: this will result in an overall membrane staining. The best blocking/incubation buffer for your immunoassay is the one that gives you the most clean bands with minimal background noise.
Concentration of antibody may be too high or incubation time too long. Also, several short washing steps are better than one long one.
The primary antibody you purchased may be too sensitive for specific detection of the target protein. Contact the vendor and explain your results. Value your time and notify the vendor of your observations. Common feedback may include;
1. Further dilute out your HRP or AP conjugated secondary antibody.
2. Instead of primary antibody incubation overnight, try 2 hours at room temp.
3. Instead of 2 hours at room temp, try 1 hour are room remp or 1 hour at 37C.
4. Perform more wash steps between incubation steps. Try 5 shake rinses followed by 4 x5min washes in 1X TTBS. Several short washing steps are better than one long one.
5. Load less protein onto the gel;
whole cell lysate or tissue extract: 20-50 micrograms subcellular fractions (ie nuclear or cytosol extracts): 10-30 micrograms purified proteins (ie recombinant or eukaryotic expression): 5-50 nanograms
What does proteolytic breakdown of the antigen look like?
If additional smear/ladder type banding is of lower apparent molecular mass than the full-length protein, then proteases may be active. The addition of fresh protease inhibitors such as PMSF, pepstatin or leupeptin can resolve this. Proteases can mediate degradation when samples are stored for prolonged time or samples are fractionated from starting cell or tissue preps.
'What is the origin of ghost bands (reverse banding)?
Ghost banding is the negative image of the proteins that have been transferred to the membrane.
Visualize in your mind.... A 15 lane gel that has been run out and coommassie stained. Every lane will have a laddering appearance. Now imagine this same laddering band pattern being transferred to a PVDF or nitrocellulose membrane. The transfer of the proteins from the gel onto the surface of the membrane creates a protein fingerprint on the membrane. Blocking the membrane is an attempt to cover all other unbound sights.
Ghost banding occurs when there is residual background noise around where the protein fingerprint is present. This event suggests;
1. There is no detectable level of the target protein in the samples
2. The primary antibody is not recognizing the target protein
What if the concentration of antigen is too low?
The resolution of SDS-PAGE is limited to 50-100 bands. If the relative concentration of the antigen of interest is too low (less than 0.2% of total protein), it may be difficult to detect (for instance, synaptobrevin/VAMP comigrates with histones in cell homogenates which interfere with its detection). Signal enhancement may then lead to the appearance of artificial bands. Enrichment of the antigen by fractionation or by immunoprecipitation should be considered.
Causes of no signal
Running a parallel Actin or GAPDH positive control western blot that corresponds to the same host species as your experimental primary antibody will conclusively indicate if the issue relates to your protocol on some level OR an issue with the primary antibody/protein expression level.
Antigen is not recognized by primary antibody: this can occur especially with monoclonal antibodies that were raised against a native protein. In some cases, a non-reducing gel system may need to be used. Otherwise contact the vendor technial service.
1. Reagent omitted or improperly prepared. A simple fix yet this becomes more and more rare with experience. Review the protocol.
2. Protein did not transfer from gel to membrane. Try a Ponceau S stain of the membrane to see if there are bands on the membrane.
3. Specificity of HRP secondary antibody not appropriate for primary antibody.
4. Correct orientation of membrane not maintained throughout procedure.
5. Presence of azide in buffer, inhibiting peroxidase activity. Horseradish peroxidase labeled antibodies should not be used in conjunction with sodium azide. A change in the blocking agent or incubation solution will solve this problem.
6. Detergent is too harsh: SDS, Nonidet P-40, and Triton X-100 disrupt binding between proteins. 0.01-0.05% Tween-20 is the most commonly used and recommended detergent for washing and incubation solutions.
Causes of weak signal or poorly defined signal
First and foremost, the primary antibody may have low affinity for target protein. Antibody affinity may also change after denaturation of a cell/tissue sample with SDS.
1. Low antibody concentration. Increase the primary dilution.
2. Incubation times too brief.
3. Insufficient protein loaded onto gel. Load more protein.
5. Exposure of film too brief. Try multiple exposures extending from 1 minute all the way to overnight.
6. Bald Spots: bubbles between gel and membrane: bubbles create points of high resistance that lead to low transfer efficiency, be sure to remove bubbles completely when putting together the transfer sandwich.
7. Incomplete Transfer.
One of several technical errors can be the source of incomplete transfer
I) Proteins not completely eluted out of gel: this often occurs with high molecular weight proteins, especially when using a transfer buffer containing methanol. One way to overcome this phenomenon is by using nitrocellulose, which does not require methanol in the transfer buffer. Adding SDS to the transfer buffer as well as using higher field strengths also improves protein elution.
II) Proteins have transferred through membrane: this may occur when working with proteins of very low molecular weight. Optimizing/shortening transfer times and using a double layer of membrane usually enables retention of small proteins.
III) Inappropriate transfer buffer used: the most stable and commonly used buffers are Tris-Glycine based and contain methanol.
IV) Impurities in the transfer buffer: this will lead to a pattern on the membrane that mirrors the holes in the transfer cassette. Fresh buffer should be prepared prior to each transfer process. and and