Protein blot (Western)

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Protein gel (1st step of a Western blot) showing a prestained ladder on both sides and Coomassie-stained protein samples in the centre. PS: You wouldn't stain with Coomassie before labelling with antibodies. This is for better illustration.
Protein gel (1st step of a Western blot) showing a prestained ladder on both sides and Coomassie-stained protein samples in the centre. PS: You wouldn't stain with Coomassie before labelling with antibodies. This is for better illustration.

A Western blot, also known as the immunoblot or protein blot, is a method of semiquantitative determination of protein expression. Crude cell lysates are loaded into a polyacrylamide gel containing a denaturing agent which give all the proteins a net negative charge. A current passing through the gel will then propel the proteins through the gel, with the largest proteins travelling the slowest. This results in the proteins being roughly separated by their mass. The proteins are then transferred from the gel onto a membrane (often nitrocellulose or PVDF), which is then incubated with an antibody directed against the protein of interest. By using a detector conjugated to an antibody one can then specifically detect the protein of interest.

Contents

Standard western blot procedure

Prior to running the western blotting experiment, consult with the vendor in order to determine if they have optimized the titration for the antibody you just purchased. Most vendors should offer some guideline for how to use the antibody. Value your time and effort first and foremost. If the experiment comes out strange, make the call or email the vendor for assistance. Your time is the most valuable element to the procedure!

I) Load 50 ug of whole cell lysate/tissue extract OR 20 ug of nuclear extract prepared from fresh buffers. Resolve proteins on the gel voltage gradient.

II) Transfer proteins to a nitrocellulose or PVDF membrane and block in fresh 5% milk TTBS 2 hr, r.t. or overnight at 4C. Perform 3 shake rinses and drain until no residual milk appears to stream off the wash buffer.

III) Incubate the primary antibody 1:200-20000 in fresh 5% milk TTBS for 120 minutes room temperature. Perform 3 shake rinses followed by wash with cold TTBS 4X for 5 minutes each wash.

IV) Incubate the the secondary antibody 1:2000 anti-mouse, 1:5000 anti-rabbit, 1:5000 anti-goat in fresh 5%milk TTBS for 60 minutes at room temperature. Perform 3 shake rinses followed by wash with cold TTBS 4X for 5 minutes each wash.

V) Perform ECL using a suitable chemiluminescent reagent

VI) Optimize protein signal with multiple exposure times.

Mouse monoclonal western blot

  • Load at least 50 ug of whole cell lysate or 20 ug nuclear extract prepared from fresh buffers. Resolve proteins on the gel voltage gradient.
  • Transfer proteins to a nitrocellulose or PVDF membrane and block in fresh 5% milk TTBS 2 hr, r.t. or overnight at 4C.
  • Incubate the primary antibody 1:200 in fresh 3% BSA TTBS for 60 minutes room temperature.
  • Incubate the the secondary antibody 1:1000 anti-mouse in fresh 3% BSA TTBS for 45 minutes at room temperature.
  • Make sure to wash between steps with ambient or cold TTBS at least 4X for 5 minutes each wash.
  • Perform ECL using a suitable detection reagent.
  • Optimize protein signal with multiple exposure times.

Goat polyclonal western blot

  • Load at least 50 ug of whole cell lysate or 20 ug nuclear extract prepared from fresh buffers. Resolve proteins on the gel voltage gradient.
  • Transfer proteins to a nitrocellulose or PVDF membrane and block in fresh 5% milk TTBS 2 hr, r.t. or overnight at 4C.
  • Incubate the primary antibody 1:200 in fresh 5% Milk TTBS for 60 minutes room temperature.
  • Incubate the the secondary antibody 1:5000 anti-goat in fresh 5% Milk TTBS for 45 minutes at room temperature.
  • Make sure to wash between steps with ambient or cold TTBS at least 4X for 5 minutes each wash.
  • Perform ECL using a suitable detection reagent.
  • Optimize protein signal with multiple exposure times.

Rabbit polyclonal western blot

  • Load at least 50 ug of whole cell lysate or 20 ug nuclear extract prepared from fresh buffers. Resolve proteins on the gel voltage gradient.
  • Transfer proteins to a nitrocellulose or PVDF membrane and block in fresh 5% milk TTBS 2 hr, r.t. or overnight at 4C.
  • Incubate the primary antibody 1:200 in fresh 1% milk/1% BSA TTBS for 60 minutes room temperature.
  • Incubate the the secondary antibody 1:4000 anti-rabbit in fresh 5% Milk TTBS for 45 minutes at room temperature.
  • Make sure to wash between steps with ambient or cold TTBS at least 4X for 5 minutes each wash.
  • Perform ECL using a suitable detection reagent.
  • Optimize protein signal with multiple exposure times.

Phosphorylation state specific western blot procedure

Adjusting certain incubation conditions can improve the detection signal for the phospho-specific antibody. When serine/threonine phosphatase inhibitor Sodium Fluoride (NaF), and tyrosine phosphatase inhibitor Sodium Orthovanadate (Na3VO4) are included in the blocking and incubation buffers, phospho-specific signals can noticeably improve. Including 50 mM NaF and 5 mM Na3VO4 in the blocking and incubation buffers can improve the signal. Using 5% milk diluent for primary and secondary incubations will reduce the nonspecific banding and background.

I) Load 50 ug of whole cell lysate/tissue extract OR 20 ug of nuclear extract prepared from fresh buffers. Resolve proteins on the gel voltage gradient.

II) Transfer proteins to a nitrocellulose or PVDF membrane and block in fresh 5% milk TTBS, 50 mM NaF/5 mM Na3VO4 for 2 hr, r.t. or overnight at 4C. Perform 3 shake rinses and drain until no residual milk appears to stream off the wash buffer.

III) Incubate the primary antibody 1:200-20000 in fresh 5% milk TTBS, 50 mM NaF/5 mM Na3VO4 for 120 minutes room temperature. Perform 3 shake rinses followed by wash with cold TTBS 4X for 5 minutes each wash.

IV) Incubate the the secondary antibody 1:2000 anti-mouse, 1:5000 anti-rabbit, 1:5000 anti-goat in fresh 5% milk TTBS, 50 mM NaF/5 mM Na3VO4 for 60 minutes at room temperature. Perform 3 shake rinses followed by wash with cold TTBS 4X for 5 minutes each wash.

V) Perform ECL using a suitable chemiluminescent reagent

VI) Optimize protein signal with multiple exposure times.

  1. Sharma SK and Carew TJ. . pmid:12137799. PubMed HubMed [Paper1]

Transfer tips for high molecular weight proteins

A standard protein transfer procedure goes as follows;

1) Perform electrophoresis on 8-15% acrylamide SDS-PAGE gel, according to standard procedures.

2) Transfer proteins onto a nitrocellulose membrane, overnight (approximately 18 hours) at 0.1 Amp.

3) Transfer buffer: 50 mM Tris, 380 mM glycine, 0.1% (w/v) SDS, 10% (v/v) methanol. Final pH: 8.3.

Adjustments to consider for high molecular weight proteins;

  • 6% acrylamide gel is preferable with high molecular weight proteins.
  • Use 15-20% methanol in transfer buffer. Methanol strips the SDS off of the proteins, which changes the charge of the proteins. Methanol also causes the gel to shrink.
  • Do not let the gel sit in transfer buffer for prolonged periods prior to transfer.
  • Transfer for 16 hours at 30 Volts, then 1 hour at 100 Volts. Do not exceed 220 mAmps.
  • PVDF may be used instead of nitrocellulose.

Troubleshooting

Western Blot Optimization

Blocking: To block nonspecific sites on the membrane, incubate the membrane with blocking buffer (5% milk TTBS) prior to primary antibody incubation. 2 hour room temperature blocking is sufficient for the membrane to absorb proteins and minimize noise.

Primary antibody: Use a diluent containing a blocking protein (5% milk TTBS or 1%milk/1%BSA TTBS) to dilute the primary antibody.

Secondary antibody: Use a diluent containing a blocking protein (5% milk TTBS or 1%milk/1%BSA TTBS) to dilute the primary antibody. Performing a "Secondary control" control blot where the primary antibody is purposely omitted is useful for identifying whether nonspecific signals are due to the secondary antibody conjugate or the primary antibody.

Washes: All wash steps are critical for reducing general background signal and nonspecific binding to discrete bands. If high background is a problem, the number, length and composition of the washes can all be increased.

General: Handle membranes carefully with forceps to minimize problems with nonspecific signals. Wear gloves for all steps to prevent hand contact with film, membranes or detection reagents.

PVDF versus Nitrocellulose

The western blotting procedures are the same for PVDF or nitrocellulose, however the handling of these membranes are different prior to- and during- transfer of proteins from the SDS-PAGE gel to the membrane.

Nitrocellulose exhibits the highest sensitivity with very low backgrounds in all transfers, especially in protein blotting. Unlike PVDF, nitrocellulose wets out naturally, does not require methanol, and will not turn hydrophobic during the transfer process. Nitrocellulose is very easily blocked and does not need the many blocking steps required with PVDF. Protocols for Western Blotting with PVDF and Nitrocellulose are the same with a few exceptions.

PVDF is hydrophobic and therefore should be prewet in methanol before it is used. Wet the membrane in methanol for 15 seconds. Membrane should uniformly change from opaque to semi-transparent. Carefully place the membrane in ultrapure water and soak for 2 minutes. Then carefully place the membrane in transfer buffer and let equilibrate for at least 5 minutes. Another change to note is that the SDS tolerances are not equivalent for PVDF and Nitrocellulose. The binding of protein to PVDF is much more sensitive to SDS levels. Too much SDS can inhibit the protein's ability to bind to the PVDF and can, in fact, help proteins already bound to the membrane to slip off. SDS levels should never exceed 0.05% for PVDF.

Membrane Stripping

Griffin:Membrane_Stripping_and_Reprobing

Stability of reagents

  • Ammonium persulphate/persulfate (APS) is a very reactive compound (used for radical generation in the polymerisation process). It is prone to oxidation and loss of activity which will increase your polymerisation times. Make small aliquots, keep in the cold. [1]
  • TEMED, the catalyst of polymerisation, is also prone to oxidation especially if mixed with water which it attracts if the container is left open. Seal quickly, keep in cold place. [2] [3]

Toxicity of reagents

  • acrylamide/bis-acrylmide is a neurotoxin. Handle with precaution. [4]
  • TEMED is very corrosive. Most likely exposure in the lab due to inhalation. Reseal the container as quickly as possible. [5]

See also

Curators

User:Korey Griffin

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