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Western blotting Pete Jones
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Outline What is western blotting and what is it use for?
An outline of each of the major steps … Some troubleshooting tips.
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Introduction What is western blotting?
A way of detecting a specific protein in a sample
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Introduction Northern Blotting (RNA) Western Blotting (Protein)
Eastern Blotting (Glycosylation etc) Southern Blotting (DNA)
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Transfer proteins to membrane
Introduction Sample Prep Transfer proteins to membrane Block membrane 1ᵒ antibody Wash 2ᵒ antibody Detection Separate Proteins Proteins are separated by SDS-PAGE and then a specific antibody used to detect the protein of interest. Potential applications: Is my protein expressed? Do the levels change in response to treatment? Detection of PTMs Has my protein expression / purification / immunoprecipitation worked properly? Other ways of detecting proteins: ELISA Mass-spec Protein Array
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Sample Prep and Quantification
Lysis depends on tissue! Culture Cells -> sonicate Tissue samples -> homogenise Centrifuge to remove debris Keep cold and use protease inhibitors and phosphatase inhibitors! Separate Proteins Transfer proteins to membrane Block membrane 1ᵒ antibody Wash 2ᵒ antibody Wash Detection
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Sample Prep and Quantification
If you want to compare samples, you need to load the same amount of protein in each sample. Accurate quantification important Bradford assay BCA and Lowry assays. Less variable, but more sensitive to pH, detergents, EDTA etc in lysis buffer A280. Accurate, but sensitive to DNA contamination. Separate Proteins Transfer proteins to membrane Block membrane 1ᵒ antibody Wash 2ᵒ antibody Wash Detection
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Transfer proteins to membrane
SDS-PAGE Sample Prep Transfer proteins to membrane Block membrane 1ᵒ antibody Wash 2ᵒ antibody Detection Separate Proteins In order to see proteins, we need to separate them out SDS-PAGE -> Separation of proteins in sample according to molecular weight Proteins are denatured before SDS-PAGE. Pick gel, buffer, running conditions appropriate for size of protein. 2D-PAGE.
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Transfer proteins to membrane
Sample Prep Transfer proteins to membrane Block membrane 1ᵒ antibody Wash 2ᵒ antibody Detection Separate Proteins Transfer separated proteins onto a membrane, which can then be probed with antibodies to detect the protein of interest. Membrane can be Nitrocellulose or PVDF Types of transfer: Semi-dry Dry Wet Nitrocellulose -> cheaper, easier to use. PVDF -> bit more faff. Binds most proteins more effectively. Best for proteins >100kDa Quick Even quicker
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Transfer proteins to membrane
Sample Prep Transfer proteins to membrane Block membrane 1ᵒ antibody Wash 2ᵒ antibody Detection Separate Proteins Did the proteins transfer to the membrane? Pre-stained ladder Ponceau staining (reversible)
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Blocking Sample Prep Separate Proteins
Transfer proteins to membrane Block membrane 1ᵒ antibody Wash 2ᵒ antibody Detection Separate Proteins Fill up the space on the membrane to prevent non-specific antibody binding Recommended to block for >1 hour Milk Strong blocking agent Less signal Not-recommended for phospho-proteins Cheap! BSA High signal High background! Diluted in same Buffer use for washing (PBST/TBST)
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Transfer proteins to membrane
Primary Antibodies Sample Prep Transfer proteins to membrane Block membrane 1ᵒ antibody Wash 2ᵒ antibody Detection Separate Proteins Antibody specific to your protein of interest Monoclonal vs Polyclonal Monoclonal – one antigen sequence – highly specific, low background, but low signal Commercial monoclonal ~£200 for 100l Custom monoclonal >£2000 Polyclonal – multiple antigen sequences – more background, high signal Custom monoclonal £ Directly conjugated HRP – fewer steps, lower signal Amount of antibody –determined empirically (1/ /5000) Incubate in wash buffer, or blocking buffer for 1-2 hours at RT or overnight at 4ᵒC
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Transfer proteins to membrane
Washing Sample Prep Transfer proteins to membrane Block membrane 1ᵒ antibody Wash 2ᵒ antibody Detection Separate Proteins Wash off unbound antibody Tris-Buffered Saline or Phosphate-Buffered Saline Tween-20 or Triton-X-100 (or other detergents beginning with T) In most cases, it doesn’t matter which you use … but … for phospho-proteins TBST may be better.
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Transfer proteins to membrane
Secondary Antibody Sample Prep Transfer proteins to membrane Block membrane 1ᵒ antibody Wash 2ᵒ antibody Detection Separate Proteins Antibody against IgG of primary antibody Conjugated to a reporter – HRP, Alexa 488 etc. Dilution in range of 1/10000 – 1/ in blocking buffer Incubate for ~2 hours at RT.
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Transfer proteins to membrane
Detection Sample Prep Transfer proteins to membrane Block membrane 1ᵒ antibody Wash 2ᵒ antibody Detection Separate Proteins Colorimetric – less sensitive Radioactive label Fluorescently labelled secondary antibody – highly quantitative Chemiluminescent – HRP or AP labelled secondary antibody - very sensitive!
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Detection Chemiluminescent detection
Mix 2 solutions together -> pipette onto membrane Wait 5 minutes Image
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Transfer proteins to membrane
Loading Controls Sample Prep Transfer proteins to membrane Block membrane 1ᵒ antibody Wash 2ᵒ antibody Detection Separate Proteins Difference between samples? – is it real? Normalise to either a loading control or total protein stain. Possible loading controls – tubulin, -actin, GAPDH, other housekeeping protein. Total protein – colloidal coomassie, fluorescent stains.
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Quantification ImageQuant software allows quantification of blot results. Bear in mind, signal is not linear!
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Common problems High background Non specific bands Incomplete transfer
Probably too much antibody Or insufficient blocking Probably too much antibody Or insufficient blocking Or insufficient washing Incomplete transfer Blotchy transfer Transfer time too short Transfer current too low Air bubbles between gel and membrane
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Summary A lot of steps … a lot to go wrong …
Everybody does it differently … take specific advice with a pinch of salt … A bit of optimisation can go a long way!
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Further information openwetware.org
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