Protein-RNA Binding Site Mapping Services: Precision Analysis for Post‑Transcriptional Regulation & RNA Therapeutics

As an independent third-party analytical service provider, we offer comprehensive protein‑RNA binding site detection for studies of RNA‑binding proteins (RBPs), RNA‑based therapeutics, virus‑host interactions, and post‑transcriptional regulation. The interaction between proteins and RNA molecules is fundamental to nearly all aspects of gene expression – including splicing, polyadenylation, stability, transport, translation, and RNA decay. Identifying the precise binding sites on RNA (and reciprocally, the RNA‑binding domains on proteins) is crucial for understanding disease mechanisms (e.g., ALS, myotonic dystrophy, cancer), validating RNA therapeutics (antisense oligonucleotides – ASOs, siRNA, mRNA), and engineering synthetic biological circuits. Our accredited laboratory follows established protocols (CLIP‑seq, RIP‑seq, EMSA, SPR, ITC, crosslinking mass spectrometry) to deliver accurate, reproducible, and publication‑ready binding site data. This article outlines our protein‑RNA binding site detection capabilities – including scope, key test items, and standard methods – to help academic researchers, biotech companies, and pharmaceutical developers unravel the molecular grammar of protein‑RNA recognition.

1. Our Testing Scope for Protein‑RNA Binding Site Detection

We cover a wide range of RNA‑binding proteins (RBPs), RNA substrates, and detection strategies:

By protein type / RNA‑binding domain (RBD) class: Canonical RBDs – RNA recognition motif (RRM), hnRNP K homology (KH) domain, double‑stranded RNA binding domain (dsRBD), zinc finger (CCCH, CCHC, etc.), arginine‑rich motif (ARM), PUF (Pumilio‑FBF) repeat, cold shock domain (CSD), S1 domain, and others. Non‑canonical RBPs – enzymes (RNA editing deaminases, methyltransferases), metabolic enzymes (GAPDH, aconitase), ribosomal proteins, viral RNA silencing suppressors, and intrinsically disordered regions (IDRs) that bind RNA.

By RNA type / substrate: Total RNA (cell or tissue extract); Specific transcripts – in‑vitro transcribed (IVT) RNA, synthetic RNA oligonucleotides (labeled or unlabeled); Endogenous RNA from crosslinked cells; Structured RNA (e.g., stem‑loops, pseudoknots, G‑quadruplexes, aptamers); Modified RNA (pseudouridine, N6‑methyladenosine – m⁶A, 5‑methylcytosine – m⁵C).

By species / organism: Human; Mouse, rat; Virus (HIV, SARS‑CoV‑2, Zika, influenza); Bacteria (E. coli, Mycobacterium); Yeast (S. cerevisiae, S. pombe); Plant (Arabidopsis, rice); Custom species.

By detection platform / experiment type: In vitro binding assays (purified protein + RNA) – electrophoretic mobility shift assay (EMSA), surface plasmon resonance (SPR), biolayer interferometry (BLI), microscale thermophoresis (MST), isothermal titration calorimetry (ITC), fluorescence anisotropy, filter binding, and RNA pull‑down (biotin‑labeled RNA + streptavidin beads). In vivo / cell‑based binding assays – crosslinking and immunoprecipitation coupled with sequencing (CLIP‑seq) and its derivatives (HITS‑CLIP, PAR‑CLIP, eCLIP, irCLIP), RNA immunoprecipitation (RIP‑seq, RIP‑chip), crosslinking mass spectrometry (XL‑MS), and proximity ligation assays (PLA) for in situ detection.

By output / resolution: Low‑resolution (binding affinity, K_D); Medium‑resolution (protein domain mapping, truncated mutants, RNA fragment mapping); High‑resolution (single‑nucleotide binding site, crosslinking site identification by CLIP‑seq or XL‑MS).

By research / application area: Mechanistic studies of RBPs in splicing, stability, and translation; Drug discovery – small molecules that disrupt protein‑RNA interactions (RIBOTAC, RNA‑targeting compounds); RNA therapeutic development – mapping binding sites of endogenous RBPs on target mRNA to avoid off‑target or saturation effects; Antisense oligonucleotide (ASO) and siRNA validation – testing whether the oligonucleotide binds its intended protein factor; Virus‑host interactions – mapping viral RNA binding sites of host RBPs and viral RBPs (e.g., SARS‑CoV‑2 N protein binding to viral RNA); Phase separation and condensate studies – IDR‑RNA binding contributions; CRISPR‑Cas systems – mapping RNA‑guide binding to Cas proteins.

2. Key Test Items & Measurements We Perform

Our protein‑RNA binding site detection services are organized into three tiers based on resolution and biological context: in vitro biochemical assays, in vivo / cell‑based assays, and integrated structural/computational mapping.

2.1 In Vitro Binding Assays (Biochemical Mapping)

These assays determine binding affinity, stoichiometry, and contact residues using purified proteins and synthetic RNA molecules.

Electrophoretic mobility shift assay (EMSA / gel shift) – A fixed concentration of labeled RNA (³²P, fluorescent, or biotin) is incubated with increasing concentrations of protein. The complex migrates more slowly than free RNA in a native polyacrylamide gel. From the band intensities, the equilibrium dissociation constant (K_D) is calculated. We offer both quantitative K_D determination (nM to μM range) and qualitative binding detection. For competition EMSA, unlabeled RNA competitor is added to assess specificity.

Surface plasmon resonance (SPR – Biacore) – RNA (or protein) is immobilized on a sensor chip, and the binding partner is flowed over the surface. Real‑time sensorgrams yield association rate (kₐ), dissociation rate (k_d), and K_D (pM to mM range). SPR requires no labeling and is ideal for measuring fast on‑off rates and for epitope mapping on RNA.

Biolayer interferometry (BLI – Octet) – Similar to SPR but without microfluidics; higher throughput. We use biotinylated RNA captured on streptavidin biosensors, then dip into protein solution. Provides kₐ, k_d, K_D. Suitable for screening multiple protein mutants or RNA variants.

Microscale thermophoresis (MST) – A fluorescently labeled RNA (or protein) is titrated against unlabeled binding partner. Thermophoretic movement within a temperature gradient changes upon binding. K_D is derived from the dose‑response curve (nM to mM). Low sample consumption, works in complex buffers (serum, cell lysate).

Isothermal titration calorimetry (ITC) – The gold standard for thermodynamics (ΔH, ΔS, ΔG, stoichiometry, K_D). Protein in the cell is titrated with RNA (or vice versa). ITC directly measures heat released or absorbed upon binding. No labeling required. Best for strong binders (K_D ≤ 10 μM).

Fluorescence anisotropy (FP) – Fluorescein‑labeled RNA is excited with polarized light; the anisotropy increases upon protein binding (slower rotation). Quick, homogeneous, and scalable. K_D from nM to μM.

RNA pull‑down (biotin‑RNA) – Biotinylated RNA is immobilized on streptavidin beads and incubated with purified protein or cell lysate. Bound proteins are eluted and identified by Western blot (for known proteins) or LC‑MS/MS (for unknown interactors). For site mapping, we use truncated or mutated RNA variants to pinpoint the binding region.

Crosslinking and pulldown (XL‑PD) – For weak or transient interactions, we use UV crosslinking (254 nm) to covalently trap the protein‑RNA complex. After pull‑down, the crosslinked RNA‑protein adduct is digested and analyzed by mass spectrometry (XL‑MS) to identify the crosslinked peptides (RNA‑peptide crosslinks). This reveals the exact RNA‑binding surface on the protein.

2.2 Cell‑Based (In Vivo) Binding Mapping

These methods capture endogenous or overexpressed protein‑RNA interactions in living cells, providing physiological relevance.

CLIP‑seq (crosslinking immunoprecipitation sequencing) and its variants – The most powerful method for global, nucleotide‑resolution mapping of protein‑RNA interactions in vivo.

HITS‑CLIP (high‑throughput sequencing of CLIP) – Cells are UV‑crosslinked (254 nm) to covalently trap protein‑RNA complexes. After immunoprecipitation of the RBP, RNA is partially digested, end‑labeled with ³²P, and size‑separated. The crosslinked RNA is excised, reverse‑transcribed, and sequenced. Crosslink sites appear as mutations (deletions, substitutions) or truncations, revealing binding regions at ~5‑10 nt resolution.

PAR‑CLIP (photoactivatable ribonucleoside‑enhanced CLIP) – Incorporation of photoactivatable nucleosides (4‑SU, 6‑SG) into RNA, which upon 365 nm UV crosslinking create sequence‑specific mutations in the cDNA. Provides single‑nucleotide resolution. We perform PAR‑CLIP for RBPs that crosslink poorly under 254 nm (e.g., some RRM domains).

eCLIP (enhanced CLIP) – Uses a modified adapter ligation strategy to reduce background and increase library complexity. Our standard CLIP workflow is eCLIP‑compatible, delivering high‑signal, low‑noise libraries.

irCLIP (infrared CLIP) – Uses fluorescent labels and infrared imaging for precise crosslinked band excision. Suitable for RBPs that produce diffuse crosslinking patterns.

RIP‑seq (RNA immunoprecipitation sequencing) – No crosslinking, so it captures stable, direct, and indirect interactions (mediated by bridging proteins). Immunoprecipitation of the RBP under native conditions, followed by RNA purification and sequencing. Lower resolution (region‑level, >200 nt) but simpler and detects indirect interactions. Useful for identifying RNA classes (mRNA, lncRNA, snoRNA) bound by the RBP.

Crosslinking and analysis of cDNAs (CRAC) – A modified CLIP method with tandem affinity purification, reducing background. Works well for low‑abundance RBPs.

Proximity ligation assay (PLA) – In situ detection of protein‑RNA proximity in fixed cells. Primary antibody against the RBP and a labeled oligonucleotide complementary to the target RNA. When bound within 40 nm, a circular DNA template is ligated and amplified. Visualized as distinct fluorescent spots. Provides subcellular localization of binding sites (e.g., nucleus vs. cytoplasm, stress granules).

2.3 Computational & Integrated Structural Mapping

We complement experimental data with computational predictions and integration of multiple datasets.

RNA motif discovery (from CLIP‑seq data) – Using MEME, HOMER, or PEAKachu, we identify over‑represented sequence motifs in the bound RNA regions. For example, the consensus motif for HuR is UUUUU‑; for Nova, YCAY‑; for SRSF1, AGAAGA‑. We provide motif logos, enrichment p‑values, and motif location distribution (e.g., enrichment in 3‘‑UTR vs. CDS).

Protein‑RNA docking (computational modeling) – For RBPs with known 3D structure, we perform rigid or flexible docking (HADDOCK, HDOCK) using experimentally derived binding data (mutagenesis, crosslinking sites) as restraints. Output includes predicted binding pose, interface residues, and binding energy.

Crosslinking mass spectrometry (XL‑MS) – UV or chemical crosslinking (e.g., EDC, disuccinimidyl glutarate) of protein‑RNA complex, followed by digestion and LC‑MS/MS. Crosslinked peptides (to RNA) or inter‑protein crosslinks are identified. Provides residue‑level mapping of the binding interface. For RNA‑protein crosslinks, we use specialized search engines (RNP‑XL, RIBO‑X).

3. Standard Test Methods We Apply

All methods follow established and published protocols. Our laboratory is ISO/IEC 17025 accredited for relevant molecular biology and mass spectrometry procedures.

3.1 In Vitro Binding Standards

EMSA: Native PAGE (4‑20% gradient), 0.5× TBE; detection by phosphorimaging (³²P) or chemiluminescence (biotin). K_D calculation via Hill equation or quadratic binding.

SPR (Biacore): RNA immobilized via biotin‑streptavidin capture or amine coupling. Running buffer: 10 mM HEPES, 150 mM NaCl, 3 mM EDTA, 0.005% P20, pH 7.4. Data fit to 1:1 binding model.

MST (NanoTemper Monolith): Label RNA with RED‑tris‑NTA (His‑tagged protein) or use fluorescently labeled RNA (Cy5). 16 serial dilutions; K_D from dose‑response curves.

ITC (MicroCal PEAQ‑ITC): 20 μM protein (cell), 200 μM RNA (syringe); 25 injections; data fit to one‑site binding model.

RNA pull‑down (biotin RNA): 3’‑biotinylated RNA (or internally labeled) synthesized by in vitro transcription or chemical synthesis; streptavidin magnetic beads (Dynabeads MyOne C1); elution with biotin or SDS loading buffer; Western blot with specific antibody.

3.2 CLIP‑seq & RIP‑seq Standards

UV crosslinking: 254 nm, 150 mJ/cm² for live cells (or 400 mJ/cm² for frozen tissues). For PAR‑CLIP: 4‑SU (100 μM) added 16 h before crosslinking; 365 nm UV.

Immunoprecipitation: Protein G‑sepharose coupled with validated anti‑RBP antibody (or tag antibody for recombinant RBPs). Washes with high‑salt RIPA buffer.

RNA fragmentation: Controlled partial RNase digestion (RNase I or micrococcal nuclease) to produce fragments of 30‑100 nt.

Library preparation: Adapter ligation (3‘ adapter, 5‘ adapter), reverse transcription, PCR amplification. For PAR‑CLIP, we include a RT‑mutagenesis step to introduce nucleotide substitutions.

Sequencing: Illumina NextSeq 2000 or NovaSeq 6000, 50 bp single‑end or paired‑end.

Data analysis: Adapter trimming (Cutadapt); alignment to reference genome (STAR, bowtie2); peak calling (PEPer, Clipper, Piranha); motif analysis (HOMER, MEME); metagene distribution (RSeQC).