RNA Pull-Down Standard Operating Procedure (SOP)
RNA Pull-Down Standard Operating Procedure (SOP)
I. Principle and Scope of Application
1.1 Principle
RNA pull-down is performed by incubating biotin-labeled bait RNA with a protein lysate to form RNA–protein complexes. The complexes are then enriched using the biotin–streptavidin system on streptavidin magnetic beads or streptavidin agarose. After multi-step washing to remove nonspecific binders, proteins are recovered for SDS-PAGE/Western blot or mass spectrometry analysis, thereby identifying proteins that bind a specific RNA.
1.2 Suitable research questions
(1) Screen proteins that bind to mRNA, lncRNA, circRNA, viral RNA, and other RNA species.
(2) Validate whether a candidate RNA-binding protein physically interacts with the target RNA.
(3) Compare differences in RNA–protein interaction profiles under different treatments/conditions.
(4) Generate a candidate interactor list to support downstream RIP, CLIP, and functional perturbation experiments.
II. Reagents and Materials
2.1 Sample types and recommended input
(1) Cell samples
① Total cell number ≥ 1 × 10⁷ (increase as needed depending on target RNA abundance).
② Cells should be healthy, free of contamination, and without obvious apoptosis/necrosis.
(2) Tissue samples
① Fresh or properly cryopreserved animal tissues, typically ≥ 0.5 g.
② Store at low temperature as soon as possible after collection; avoid repeated freeze–thaw cycles.
2.2 Key reagents
(1) RNA bait
① Target RNA generated by in vitro transcription (full length or functional fragment, with partial NTP substitution by biotin-NTP).
② Alternatively, chemically synthesized short RNA probes (typically ≤ 60 nt) with 5′- or 3′-end biotin labeling.
(2) Affinity enrichment reagents
① Streptavidin magnetic beads or streptavidin agarose.
② RNase inhibitor.
③ Protease inhibitor (e.g., PMSF or a protease inhibitor cocktail).
(3) Buffers (typical formulations; optimize as needed)
① Lysis buffer: Tris-HCl, pH 7.5–8.0, with appropriate NaCl, MgCl₂, and a non-ionic detergent (e.g., NP-40); add protease inhibitor and RNase inhibitor immediately before use.
② Incubation buffer: based on lysis buffer, with salt and detergent adjusted as needed.
③ Wash buffer: similar to incubation buffer or with slightly higher salt; optionally increase detergent to reduce nonspecific binding.
④ Elution buffer: SDS loading buffer (for Western blot) or high-salt / competitor-RNA buffer (for MS).
(4) Other reagents
① DEPC-treated water or RNase-free water.
② SDS-PAGE and Western blot reagents.
③ For MS: reagents for protein precipitation and desalting.
2.3 Instruments and consumables
(1) Refrigerated centrifuge (4°C).
(2) Sonicator or tissue homogenizer.
(3) Thermomixer or rotating mixer.
(4) Magnetic rack.
(5) Gel electrophoresis, transfer, and imaging system, or an MS platform.
(6) RNase-free pipette tips, microcentrifuge tubes, centrifuge tubes, ice box, etc.
III. RNA Bait Preparation and Quality Control
3.1 Preparation of biotin-labeled RNA by in vitro transcription (recommended)
(1) Template construction
① Add a T7 promoter sequence to the 5′ end of the forward primer.
② PCR-amplify the DNA template containing the T7 promoter.
(2) In vitro transcription
① Assemble the reaction containing NTPs (with a portion substituted by biotin-NTPs) according to the kit instructions.
② Incubate at 37°C for 1–2 h to ensure sufficient transcription.
(3) RNA purification and quantification
① Add DNase I to digest the DNA template (most in vitro transcription kits include a DNase step after transcription), followed by column purification or phenol/chloroform extraction and precipitation/desalting to obtain RNA free of template DNA.
② Measure OD260/280 and assess integrity by agarose gel electrophoresis.
③ Yield and concentration vary with template length, sequence, and system; quantify and ensure that a single reaction can supply 1–5 μg per tube (long RNA) or 10–100 pmol (short probes) as required.
3.2 Synthesis of short RNA probes
(1) For bait fragments ≤ 60 nt, directly synthesize biotin-labeled RNA probes.
(2) During design, avoid regions with strong secondary structure to improve sequence-specificity and reduce nonspecific binding risk.
3.3 QC of RNA bait
(1) Use agarose gel or denaturing PAGE to check band integrity and purity.
(2) If needed, compare migration of biotin-labeled RNA versus unlabeled RNA.
(3) RNA is typically used in excess in pull-down; ensure consistent input amounts within the same experimental batch.
IV. Protein Sample Preparation
4.1 Lysis
(1) Cell samples
① Gently wash cells with PBS 1–2 times to remove culture medium.
② Add pre-chilled lysis buffer and resuspend by pipetting or brief vortexing.
③ Lyse on ice for 20–30 min.
(2) Tissue samples
① Rinse with PBS to remove blood and contaminants.
② Add lysis buffer and homogenize mechanically, or grind into powder in liquid nitrogen.
③ Continue lysis on ice.
4.2 Sonication and clarification
(1) Sonication in an ice bath
① Use cycles such as “2–3 s ON / 3 s OFF.”
② Total time: ~1–2 min for cells; extend to ~3–5 min for tissues.
③ Avoid obvious foaming and temperature increases.
(2) Clarification by centrifugation
① Centrifuge at 12,000 rpm for 10–15 min at 4°C.
② Collect the supernatant as protein lysate (total protein).
4.3 Protection and storage
(1) Add protease inhibitor and RNase inhibitor to the lysis buffer immediately before use.
(2) Store at 4°C short-term; aliquot and store at −80°C long-term; avoid repeated freeze–thaw cycles.
(3) Optionally assess lysis efficiency and protein integrity by SDS-PAGE or Coomassie staining.
V. RNA Pull-Down Procedure
5.1 Bead pre-treatment and equilibration
(1) Remove streptavidin magnetic beads from 4°C and gently mix.
(2) Aliquot beads per reaction (e.g., 30 μL per tube), place on a magnetic rack for 1 min, and discard supernatant.
(3) Resuspend in pre-chilled equilibration buffer and wash 2–3 times.
(4) Finally resuspend in incubation buffer and adjust volume for use.
5.2 RNA immobilization on beads
(1) Add biotin-labeled RNA to pre-treated beads: short oligos (≤ 60 nt) can be optimized at 10–100 pmol; longer in vitro–transcribed RNA is preferably dosed by mass (e.g., 1–5 μg, increase if necessary).
(2) Bring to 200–500 μL with incubation buffer and mix gently.
(3) Incubate at room temperature with rotation or gentle shaking for 1–2 h.
(4) Magnetically separate, discard supernatant, and wash 2–3 times with incubation buffer.
(5) Obtain the RNA–bead complex.
Recommended controls:
① Biotin-labeled target RNA (experimental group).
② Biotin-labeled irrelevant sequence RNA (scramble) or binding-site mutant RNA (negative control).
③ Beads-only (no RNA) control (background control).
5.3 RNA–protein incubation
(1) Resuspend the RNA–bead complex in incubation buffer (typically 200–500 μL).
(2) Add 300–2000 μg protein lysate.
(3) Supplement with RNase inhibitor and protease inhibitor.
(4) Reserve an aliquot of lysate as the Input control.
(5) Incubate at 4°C with rotation or gentle shaking for 4 h to overnight (12–16 h).
5.4 Washing
(1) After incubation, place on a magnetic rack to collect beads; retain supernatant if needed.
(2) Add pre-chilled wash buffer, resuspend beads, and gently shake for 3–5 min.
(3) Magnetically separate and discard supernatant.
(4) Repeat washing 3–5 times; increase salt or detergent moderately if background is high.
5.5 Protein elution and sample preparation
(1) Elution for Western blot
① Add 1× or 2× SDS loading buffer to the beads.
② Heat at 95–100°C for 5–10 min.
③ Magnetically separate (or briefly centrifuge) and collect the supernatant as the pull-down sample.
④ Treat Input samples similarly by adding loading buffer and boiling.
(2) Elution for mass spectrometry
① Use mild elution with high salt or competitor RNA to avoid SDS interference.
② Precipitate proteins using acetone or TCA and transfer into an MS-compatible buffer.
③ Alternatively, run SDS-PAGE first, perform silver staining, and excise bands for MS.
VI. Detection and Data Interpretation
6.1 Western blot
(1) Run Input, experimental, and control pull-down samples by SDS-PAGE.
(2) Transfer and probe with primary antibody against the candidate RNA-binding protein and the corresponding secondary antibody.
(3) Interpretation principles:
① A clear specific band in the experimental group but not in controls indicates a relatively specific RNA–protein interaction.
② Bands in both experimental and control groups indicate high nonspecific binding; optimize incubation and washing conditions.
③ If Input shows a band but pull-down has no signal, check protein expression level, antibody performance, and bait/incubation conditions.
6.2 Mass spectrometry
(1) Use silver staining or Coomassie staining to assess protein abundance and complexity.
(2) Identify proteins by LC-MS/MS.
(3) Remove background proteins using control datasets and retain specifically enriched proteins.
(4) Prioritize follow-up targets based on unique peptide counts, sequence coverage, and functional annotation.
VII. Common Issues and Troubleshooting
7.1 RNA degradation
(1) Use RNase-free reagents and consumables whenever possible; DEPC treatment should be applied only to “pure water / solutions without amine-containing buffer components.” For buffers containing Tris, prepare with RNase-free reagents or use commercially prepared RNase-free buffers.
(2) Wear gloves throughout to minimize contact with exposed skin.
(3) Perform lysis, incubation, and washing steps on ice whenever possible.
(4) Add sufficient RNase inhibitor to lysis and incubation systems.
7.2 RNA source and strategy selection
(1) In vitro transcription is preferred for longer RNAs, offering better purity and specificity.
(2) Short-probe strategies are suitable for fragments ≤ 60 nt but carry higher nonspecific binding risk.
(3) If the target RNA is > 2000 nt and in vitro transcription is difficult, consider:
① transcribing a functional fragment as bait;
② using biotinylated antisense oligonucleotides to capture endogenous RNA and enrich its binding proteins.
7.3 Low binding yield or no obvious bands
(1) Ensure protein activity: use fresh lysate, maintain low temperature throughout, and include protease inhibitors.
(2) Optimize incubation time and temperature: prefer overnight rotation at 4°C; for strong interactions, test shorter incubations at room temperature.
(3) Increase RNA or protein input to ensure bait is in relative excess.
(4) If necessary, pre-overexpress the target protein in cells/tissues.
7.4 High background and many nonspecific bands
(1) Increase wash-buffer salt concentration moderately, or slightly increase detergent concentration.
(2) Add nonspecific competitor RNA (e.g., tRNA) or nonspecific proteins/peptides to reduce nonspecific adsorption.