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REACTIVITY SENSITIVITY MW (kDa) Isotype
H M R Mk Endogenous 40 Mouse IgG1
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Immunoprecipitation of NIH/3T3 cell lysates, untreated or TNF-α-treated, using Mouse IgG (Sepharose Bead Conjugate) #3420 (Lanes 1 and 2) and Immobilized Phospho-IκBα (Ser32/36) (5A5) Mouse mAb (Bead Conjugate) (Lanes 3 and 4). The blot was probed using Phospho-IκBα (Ser32/36) (5A5) Mouse mAb #9246.

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Immunoprecipitation for Analysis by Western Blotting

This protocol is intended for immunoprecipitation of native proteins for analysis by western immunoblot or kinase activity.

A. Solutions and Reagents

NOTE: Prepare solutions with reverse osmosis deionized (RODI) or equivalently purified water.

  1. 20X Phosphate Buffered Saline (PBS): (#9808).
  2. 10X Cell Lysis Buffer: (#9803) 20 mM Tris (pH 7.5), 150 mM NaCl, 1 mM EDTA, 1 mM EGTA, 1% Triton X-100, 2.5 mM Sodium pyrophosphate, 1 mM β-glycerophosphate, 1 mM Na3VO4, 1 μg/ml Leupeptin

    NOTE: CST recommends adding 1 mM PMSF (#8553) before use*.

  1. 3X SDS Sample Buffer: (#7722) 187.5 mM Tris-HCl (pH 6.8 at 25°C), 6% w/v SDS, 30% glycerol, 150 mM DTT, 0.03% w/v bromophenol blue
  2. 10X Kinase Buffer (for kinase assays): (#9802) To Prepare 1 ml of 1X kinase buffer, add 100 µl 10X kinase buffer to 900 µl dH2O, mix.
  3. ATP (10 mM) (for kinase assays): (#9804) To prepare 0.5 ml of ATP (200 µM), add 10 µl ATP (10 mM) to 490 µl 1X kinase buffer.

B. Preparing Cell Lysates

  1. Aspirate media. Treat cells by adding fresh media containing regulator for desired time.
  2. To harvest cells under nondenaturing conditions, remove media and rinse cells once with ice-cold PBS.
  3. Remove PBS and add 0.5 ml 1X ice-cold cell lysis buffer to each plate (10 cm) and incubate the plates on ice for 5 minutes.
  4. Scrape cells off the plates and transfer to microcentrifuge tubes. Keep on ice.
  5. Sonicate samples on ice three times for 5 seconds each.
  6. Microcentrifuge for 10 minutes at 4°C, 14,000 x g, and transfer the supernatant to a new tube. If necessary, lysate can be stored at –80°C.

C. Immunoprecipitation

  1. Take 200 μl cell lysate and add 20 μl of the immobilized antibody, incubate with rotation overnight at 4°C.
  2. Microcentrifuge for 30 seconds at 4°C. Wash pellet five times with 500 μl of 1X cell lysis buffer. Keep on ice during washes.
  3. Proceed to sample analysis by western blotting or kinase activity (section D).

D. Sample Analysis

Proceed to one of the following specific set of steps.

For Analysis by Western Immunoblotting

  1. Resuspend the pellet with 20 µl 3X SDS sample buffer. Vortex, then microcentrifuge for 30 sec at 14,000 x g.
  2. Heat the sample to 95–100°C for 2-5 min and microcentrifuge for 1 min at 14,000 x g.
  3. Load the sample (15–30 µl) on a 4–20% gel for SDS-PAGE.
  4. Analyze sample by western blot (see Western Immunoblotting Protocol).

NOTE: To minimize masking caused by denatured IgG heavy chains (~50 kDa), we recommend using Mouse Anti-Rabbit IgG (Light-Chain Specific) (L57A3) mAb (#3677) or Mouse Anti-Rabbit IgG (Conformation Specific) (L27A9) mAb (#3678) (or HRP conjugate #5127). To minimize masking caused by denatured IgG light chains (~25 kDa), we recommend using Mouse Anti-Rabbit IgG (Conformation Specific) (L27A9) mAb (#3678) (or HRP conjugate #5127).

For Analysis by Kinase Assay

  1. Wash pellet twice with 500 µl 1X kinase buffer. Keep on ice.
  2. Suspend pellet in 40 µl 1X kinase buffer supplemented with 200 µM ATP and appropriate substrate.
  3. Incubate for 30 min at 30°C.
  4. Terminate reaction with 20 µl 3X SDS sample buffer. Vortex, then microcentrifuge for 30 sec.
  5. Transfer supernatant containing phosphorylated substrate to another tube.
  6. Heat the sample to 95–100°C for 2–5 min and microcentrifuge for 1 min at 14,000 x g.
  7. Load the sample (15–30 µl) on SDS-PAGE (4–20%).

posted December 2007

protocol id: 27

Product Usage Information

Application Dilutions
Immunoprecipitation 1:20

Storage: Supplied in 10 mM sodium HEPES (pH 7.5), 150 mM NaCl, 100 µg/ml BSA, 50% glycerol. Store at –20°C. Do not aliquot the antibodies.

Specificity / Sensitivity

Phospho-IκBα (Ser32/36) (5A5) Mouse mAb (Sepharose Bead Conjugate) detects endogenous levels of IκBα only when phosphorylated at Ser32/36.


Species Reactivity: Human, Mouse, Rat, Monkey
Species predicted to react based on 100% sequence homology: Bovine, Dog, Pig, Guinea Pig

Source / Purification

Monoclonal antibody is produced by immunizing animals with a synthetic phosphopeptide corresponding to residues surrounding Ser32/36 of human IκBα.

Product Description

This Cell Signaling Technology antibody is immobilized via covalent binding of primary amino groups to N-hydroxysuccinimide (NHS)-activated sepharose beads. Phospho-IκBα (Ser32/36) (5A5) Mouse mAb (Sepharose Bead Conjugate) is useful for the immunoprecipitation of phosphorylated IκBα.


The NF-κB/Rel transcription factors are present in the cytosol in an inactive state complexed with the inhibitory IκB proteins (1-3). Activation occurs via phosphorylation of IκBα at Ser32 and Ser36 followed by proteasome-mediated degradation that results in the release and nuclear translocation of active NF-κB (3-7). IκBα phosphorylation and resulting Rel-dependent transcription are activated by a highly diverse group of extracellular signals including inflammatory cytokines, growth factors, and chemokines. Kinases that phosphorylate IκB at these activating sites have been identified (8).


1.  Baeuerle, P.A. and Baltimore, D. (1988) Science 242, 540-6.

2.  Beg, A.A. and Baldwin, A.S. (1993) Genes Dev 7, 2064-70.

3.  Finco, T.S. et al. (1994) Proc Natl Acad Sci USA 91, 11884-8.

4.  Brown, K. et al. (1995) Science 267, 1485-8.

5.  Brockman, J.A. et al. (1995) Mol Cell Biol 15, 2809-18.

6.  Traenckner, E.B. et al. (1995) EMBO J 14, 2876-83.

7.  Chen, Z.J. et al. (1996) Cell 84, 853-62.

8.  Karin, M. and Ben-Neriah, Y. (2000) Annu Rev Immunol 18, 621-63.


Entrez-Gene Id 4792
Swiss-Prot Acc. P25963


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