Proximity Ligation Assay (PLA): In Situ Detection of Protein Modifications and Interactions
Overview
Post-translational modifications (PTMs) are chemical changes that occur after a protein is synthesized. They alter protein structure, function, and localization, and they play critical roles in cellular signaling, metabolism, and disease development. The proximity ligation assay (PLA), developed by Navinci in Sweden, provides a highly sensitive and specific method for studying PTMs in situ.
By combining two antibody probes with rolling circle amplification, PLA converts a molecular proximity event into a bright, countable signal. That design makes it valuable for detecting modified protein complexes at their native location in cells and tissues.
Post-Translational Modifications: The Regulatory Layer of the Proteome
Proteins are the primary executors of cellular function. PTMs finely regulate protein activity, localization, stability, and interactions with other molecules by adding or removing specific chemical groups. Common PTMs include the following.
Phosphorylation adds phosphate groups and is typically catalyzed by protein kinases. It can alter protein activity, localization, and interactions with other proteins.
Glycosylation adds sugar groups. It can affect protein stability, solubility, and interactions with other proteins.
Ubiquitination attaches ubiquitin proteins. It can mark proteins for degradation.
Acetylation adds acetyl groups. It can influence protein activity, stability, and interactions with other proteins.
These modifications are central to cellular signaling, metabolism, cell cycle regulation, immune responses, and disease development. Phosphorylation can regulate enzyme activity, glycosylation can affect protein stability and localization, and ubiquitination can mark proteins for degradation. Abnormal PTMs are associated with disease. Excessive phosphorylation of certain oncogenes can lead to uncontrolled cell growth, while excessive ubiquitination of tumor suppressor genes can promote tumor progression.
Schematic PTM discovery timeline for major modifications
How the Proximity Ligation Assay Works
The PLA technology was developed by Navinci in Sweden. Its core principle is signal amplification triggered by proximity effects, first described by Söderberg and colleagues in Nature Methods in 2006.
The workflow uses two pairs of antibodies, each pair conjugated to a unique oligonucleotide sequence. When the antibody pairs bind their target protein complex in close proximity, typically within 40 nm, the attached oligonucleotide sequences also come close together. A DNA ligase then connects the adjacent oligonucleotides, forming a complete, amplifiable DNA circle.
Subsequent rolling circle amplification (RCA) amplifies that DNA circle into a long single-stranded DNA containing many repeating sequence units. Fluorescently labeled complementary oligonucleotide probes hybridize to the amplified strand, and fluorescence microscopy or another detection platform reads the signal. The intensity of the fluorescent signal is proportional to the quantity of the target protein complex. This relationship enables both qualitative and quantitative detection of the complex in its native context.
PLA experimental workflow: antibody binding, ligation, and rolling circle amplification
PLA signal detection principle
| Assay Step | What Happens |
|---|---|
| Antibody binding | Two antibody pairs conjugated to unique oligonucleotides bind the target complex within about 40 nm |
| Ligation | DNA ligase joins adjacent oligonucleotides into an amplifiable DNA circle |
| Amplification | Rolling circle amplification (RCA) generates long single-stranded DNA with repeating units |
| Detection | Fluorescent complementary probes hybridize to the amplified strand and emit a countable signal |
| Quantification | Signal intensity is proportional to the amount of target protein complex |
PLA Technology for PTM Detection: Application Examples
PLA is well suited to modified proteins because the readout depends on two recognition events rather than one. That dual requirement improves specificity when a modification and a protein identity must be confirmed together. If only one antibody binds, no DNA circle forms and no signal appears, which suppresses background from single-antibody binding.
Phosphorylated HER2 detection. Using the Naveni pY HER2 kit with the detection fluorophore TEX615, phosphorylated HER2 can be visualized directly in situ. This lets researchers confirm receptor activation at the single-cell level rather than in bulk lysate. Because HER2 activation is heterogeneous across tumor tissue, a spatial readout adds information that a lysate-based assay cannot provide.
In situ detection of phosphorylated HER2 by PLA
Phosphorylated EGFR detection. Using the Naveni pY EGFR kit with the TEX615 fluorophore, phosphorylated EGFR is detected with the same proximity-based strategy. Such readouts are useful when receptor activation status must be resolved across a heterogeneous cell population. They also allow researchers to correlate activation with cell morphology, position within a tissue, or treatment history.
In situ detection of phosphorylated EGFR by PLA
PLA detection of phosphoproteins with modification-specific antibodies
In addition, ANT BIO PTE. LTD. modification-specific antibodies can be paired with the NaveniFlex kit to detect a range of phosphorylated proteins. This combination extends PLA beyond a single target and supports multiplexed PTM analysis in situ. Because the assay reads native protein complexes, it can capture interactions and modification states that are lost during cell lysis.
Practical Considerations for PLA Experiments
Several factors determine whether a PLA experiment produces a clean, interpretable signal. Antibody quality is the first. Both antibodies in a pair must be specific and must bind simultaneously without steric interference. Validating each antibody by conventional immunofluorescence or western blot before setting up PLA reduces troubleshooting later.
Sample preparation is the second factor. Fixation and permeabilization conditions must preserve the epitope while allowing the oligonucleotide-conjugated probes to reach it. Over-fixation can mask the modification, while under-fixation can cause signal loss and uneven staining.
Controls are the third factor. A negative control that omits one antibody confirms that the signal depends on both probes. A positive control using a well-characterized target sets the expected signal level. Together these controls separate true proximity events from nonspecific background and make PLA results reproducible across laboratories.
Conclusion
The proximity ligation assay adds a spatial dimension to PTM research. By requiring two proximal recognition events and then amplifying the resulting DNA circle, PLA delivers sensitive, specific, and countable signals for modified protein complexes. Paired with validated modification-specific antibodies, it supports phosphorylation and broader PTM studies in situ. This combination is well matched to questions where location, cell-to-cell heterogeneity, and activation status all matter. ANT BIO PTE. LTD. supplies a range of modification-specific antibodies that can be combined with NaveniFlex kits for these applications.
Product Information
All products are supplied for research use only and are not intended for diagnostic or therapeutic procedures in humans or animals.
ANT BIO PTE. LTD. – Empowering Scientific Breakthroughs
ANT BIO PTE. LTD. supplies high-quality reagents and solutions for life science research. Our sub-brands cover the full research workflow: Starter for antibodies and immunological assay kits, UA-Bio for recombinant proteins and drug discovery solutions, and Absin for general reagents and other detection kits. Explore our product portfolio at www.antbioinc.com.
Disclaimer
This article was partially created with the assistance of artificial intelligence. If any content involves copyright or intellectual property issues, please inform us, and we promise to verify and remove it immediately.
