Co-Immunoprecipitation of the RAF-MEK Complex to Assess Disruption by the MEK Inhibitor Trametinib in BRAF-V600E A375 Melanoma Cells

Two treatment arms (0.1% DMSO vehicle vs 100 nM trametinib, 2 h) in n=3 independent biological replicates. Each replicate includes three IP conditions run in parallel from a shared lysate pool: (a) anti-MEK1 IP, (b) isotype-matched IgG IP, and (c) 5% input. Bait and prey are read on the same membrane. The densitometry analyst is blinded to treatment via coded images. Lysate protein input per IP is fixed at 1 mg.

BSL-2 for A375 human melanoma cells; biosafety cabinet for all open-cell work. Trametinib and DMSO handled with nitrile gloves. Glycerol/NP-40 lysis buffers and methanol transfer buffer require eye protection and chemical-resistant gloves. Magnetic beads and ECL are low-hazard. Cell waste decontaminated in 10% bleach 30 min; sharps and biohazard waste autoclaved. Maintain a designated phospho-protein cold chain to limit dephosphorylation.

Isotype IgG IP = negative control defining non-specific bead/antibody binding (BRAF signal here must be minimal). Input lane (5%) confirms BRAF and MEK1 are present and equivalent across arms. p-ERK on inputs is a positive pharmacodynamic control confirming trametinib engaged the pathway. A bead-only (no antibody) condition controls for direct bead binding. Reciprocal IP (anti-BRAF pulling MEK1) is recommended to confirm interaction directionality.

Anti-MEK1 IP recovers MEK1 strongly and co-precipitates BRAF well above the IgG control (signal-to-IgG ratio >5). The normalized BRAF/MEK1 co-IP ratio should be comparable between DMSO and trametinib arms (within ~30%), while input p-ERK is abolished by trametinib (>90% reduction). IgG lanes show negligible BRAF.

• A375 cells (ATCC CRL-1619), BRAF-V600E melanoma
• DMEM high glucose + 10% FBS + 1% pen/strep
• Trametinib (GSK1120212), 1 mM stock in DMSO
• Native lysis buffer (50 mM Tris-HCl pH 7.5, 150 mM NaCl, 1% NP-40, 10% glycerol, 1 mM EDTA)
• Halt protease + phosphatase inhibitor cocktail (100x)
• Protein A/G magnetic beads
• Anti-MEK1 rabbit mAb (IP-grade), 2 µg per IP
• Normal rabbit IgG isotype control, 2 µg per IP
• Anti-BRAF (D9T6S) rabbit mAb for blotting, 1:1000
• Anti-MEK1/2 (47E6) rabbit mAb, 1:1000
• Anti-phospho-ERK1/2 (Thr202/Tyr204) (D13.14.4E), 1:2000
• Mouse anti-rabbit IgG (conformation-specific, light-chain-sparing) HRP, 1:2000
• 4-12% Bis-Tris gels, PVDF, ECL

To pull down endogenous MEK1 from A375 melanoma lysates and quantify co-precipitating BRAF by immunoblot, comparing DMSO-treated versus trametinib-treated cells, in order to test whether catalytic MEK inhibition perturbs the RAF-MEK scaffold interaction. The protocol uses a native (non-denaturing) lysis to preserve protein-protein contacts and an IgG isotype control to define non-specific binding.

1. Grow A375 in 15-cm dishes to 80% confluence; treat with 0.1% DMSO or 100 nM trametinib for 2 h.
2. Wash 2x with ice-cold PBS; lyse in 1 mL native buffer + inhibitors per dish, 30 min on ice with intermittent agitation.
3. Centrifuge 16,000 x g, 15 min, 4 °C; quantify supernatant by BCA. Reserve 50 µg as input.
4. Pre-clear 1 mg lysate with 25 µL protein A/G beads, 1 h at 4 °C, rotating.
5. Add 2 µg anti-MEK1 (or 2 µg IgG) to pre-cleared lysate; rotate overnight at 4 °C.
6. Add 30 µL washed protein A/G magnetic beads; rotate 2 h at 4 °C.
7. Magnetically separate; wash beads 4x with 1 mL ice-cold lysis buffer (no inhibitors), 5 min each.
8. Elute in 40 µL 2x LDS + reducing agent, heat 70 °C for 10 min.
9. Load IP eluates + 5% input on 4-12% gel; run 150 V, 60 min; transfer to PVDF.
10. Probe for BRAF (prey), then MEK1 (bait), then p-ERK on input lanes.
11. Image by ECL; quantify co-precipitated BRAF normalized to immunoprecipitated MEK1.

Independent variable: treatment (DMSO vs 100 nM trametinib). Dependent variable: co-immunoprecipitated BRAF normalized to immunoprecipitated MEK1 (interaction stoichiometry index). Controlled variables: lysate input mass (1 mg), antibody amount (2 µg), wash stringency (4x), bead volume, lysis detergent (1% NP-40 non-denaturing), treatment time (2 h), and cell confluence (80%).

We hypothesize that the BRAF-MEK1 interaction is largely scaffold/conformation-dependent and that trametinib, which traps MEK in an inactive but RAF-bound conformation, will NOT substantially reduce co-precipitated BRAF (interaction preserved or modestly increased), whereas total phospho-ERK downstream will be abolished—dissociating physical complex integrity from catalytic output.

Quantify BRAF and MEK1 bands from IP lanes by densitometry (ImageJ/Fiji), subtract IgG-lane background, and compute the BRAF/MEK1 ratio per replicate. Express each treated ratio relative to its paired DMSO control. Confirm pathway engagement by quantifying input p-ERK/total-ERK. Report mean ± SD across three biological replicates; present representative blots plus the quantification graph.

1. High BRAF in IgG lane: increase wash count/stringency (add 0.1% NP-40 wash), increase pre-clear time. 2. Weak MEK1 recovery: confirm IP antibody is IP-validated and not denatured; increase antibody to 3 µg. 3. Strong 50/25 kDa antibody bands obscuring prey: use light-chain-specific or HRP-conjugated detection antibodies. 4. Interaction lost in both arms: lysis too harsh—lower NP-40 to 0.5% or switch to digitonin. 5. No p-ERK change: verify trametinib stock activity and treatment time; re-run input immunoblot.

n=3 biological replicates. Compare normalized BRAF/MEK1 co-IP ratios between DMSO and trametinib by a two-tailed paired t-test (alpha = 0.05). Because the hypothesis predicts no change, also report a two-one-sided-test (TOST) equivalence analysis with a ±30% margin. p-ERK reduction is confirmed by paired t-test. Effect sizes (Cohen's d) reported; given the paired design and large expected p-ERK effect, n=3 is powered for the pharmacodynamic control.