Proximity Ligation Assay (PLA)

In Situ Detection of Protein Interactions and Modifications

The Proximity Ligation Assay (PLA) is a highly sensitive method for detecting protein-protein interactions (PPIs), post-translational modifications (PTMs), and molecular proximity events directly in cells and tissue sections. By combining antibody specificity with DNA-based signal amplification, PLA enables researchers to visualise low-abundance targets with single-molecule sensitivity while preserving spatial context.

Unlike conventional immunofluorescence, PLA does more than show co-localisation. It helps demonstrate whether two targets are sufficiently close to indicate a biologically meaningful interaction, making it a valuable tool in cell biology, cancer research, signalling studies, and translational biomarker development.

At Biorbyt, we support PLA workflows with antibodies suitable for immunofluorescence and related in situ applications, helping researchers move from target selection to robust assay performance.

What is a Proximity Ligation Assay?

A Proximity Ligation Assay is an in situ molecular detection technology designed to identify targets that are in very close proximity, typically within approximately 40 nm. In most applications, PLA is used to detect endogenous protein-protein interactions or to measure a protein together with a specific modification such as phosphorylation, ubiquitination, or acetylation.

The assay relies on two antibodies recognising nearby epitopes. When those antibodies are brought close enough together, attached oligonucleotide probes can be ligated and amplified, generating a bright localised fluorescent signal that appears as a discrete dot under the microscope.

This means PLA offers a key advantage over standard immunostaining: it links specific target recognition with functional molecular proximity, rather than simply showing that two proteins are present in the same cell.

Why use PLA?

PLA is widely used when researchers need to study molecular interactions in their native cellular context. It is particularly useful when interactions are transient, low in abundance, or difficult to capture by bulk methods such as co-immunoprecipitation.

Key advantages of PLA include:

• Detection of endogenous protein interactions without overexpression systems
• High specificity through dual antibody recognition
• Very high sensitivity through rolling circle amplification
• Preservation of spatial and subcellular information
• Compatibility with fixed cells, frozen sections, and FFPE tissues
• Quantifiable puncta-based readout for comparative studies

These properties make PLA particularly relevant in oncology, immunology, neuroscience, signal transduction, and biomarker research.

How does a PLA assay work?

PLA combines antibody binding with DNA ligation and amplification chemistry. In a standard in situ assay, the workflow is typically divided into three main stages:

1. Dual recognition and proximity ligation: two primary antibodies from different host species bind nearby targets. Secondary PLA probes carrying complementary oligonucleotides are then added. If the probes are sufficiently close, they are ligated into a circular DNA template.
2. Rolling circle amplification (RCA): a DNA polymerase amplifies the circular template into a long single-stranded DNA product that remains anchored at the original site.
3. Fluorescent detection: labelled detection probes hybridise to the amplified DNA, producing distinct fluorescent dots that can be imaged and counted.

Each punctum theoretically corresponds to a single proximity event, allowing researchers to compare interaction frequency between cell populations, treatment groups, or tissue regions.

 

Proximity Ligation Assay (PLA) Workflow - Source 

What is PLA used for?

Proximity Ligation Assay is used across a broad range of life science applications where protein proximity, signalling state, or molecular interactions need to be measured directly in situ.

• Detection of protein-protein interactions in signalling pathways
• Analysis of post-translational modifications such as phosphorylation or ubiquitination
• Investigation of receptor-ligand proximity in immune and cancer biology
• Assessment of drug response and pathway modulation
• Biomarker studies in FFPE tissue sections
• Multiplex spatial analysis in translational and preclinical research

Because PLA preserves tissue architecture and subcellular localisation, it is especially valuable when the biological question depends on context rather than on bulk protein abundance alone.

PLA compared with other interaction detection methods

Choosing the right method depends on the biological question, sample type, and whether spatial context is essential.

Method	In situ	Endogenous targets	Spatial information	Typical strength
PLA	Yes	Yes	Yes	High sensitivity for direct proximity events
Conventional immunofluorescence	Yes	Yes	Yes	Expression and co-localisation only
Co-immunoprecipitation	No	Yes	No	Useful for complexes, but loses location
FRET / FLIM	Yes	Often no	Yes	Useful for live-cell studies
BioID / APEX	Partly	Usually no	Limited	Strong for proximity labelling and discovery

In short, PLA is often the method of choice when you need to show that two endogenous targets are in close proximity within fixed cells or tissues, rather than simply being expressed in the same sample.

Primary antibody selection: the most important step

Antibody quality is the single biggest determinant of PLA success. Because the assay amplifies signal so strongly, it can also amplify background caused by poor specificity, aggregation, or cross-reactivity.

For best results:

• Use antibodies already validated for immunofluorescence (IF) or IHC
• Ensure the two primary antibodies come from different host species
• Optimise antibody concentrations first by conventional IF where possible
• Avoid using two antibodies from the same species unless the system is specifically designed for that format
• Prioritise clean, specific staining over very strong signal

In many experiments, starting with antibodies that already perform well in IF is the best way to improve PLA robustness and reduce troubleshooting later in the workflow.

Browse antibodies suitable for PLA-related workflows here: PLA-validated antibodies

Sample preparation considerations

Sample preparation has a major impact on signal quality and should be tailored to the specimen type.

• Adherent cells: commonly fixed with 4% paraformaldehyde and permeabilised with mild detergent
• Frozen sections: suitable for PLA, with fixation and permeabilisation conditions optimised by target
• FFPE sections: highly compatible with PLA, but typically require heat-induced antigen retrieval
• Suspension cells: usually need cytospin or smear preparation before staining

For FFPE samples in particular, antigen retrieval conditions should be optimised for each antibody pair, as the wrong retrieval chemistry can lead to weak signal or excessive background.

Essential controls for reliable PLA data

Controls are critical in PLA because the assay is sensitive enough to amplify artefacts if experimental design is weak.

• No-primary control: assesses non-specific probe background
• Single-primary controls: reveal whether one antibody alone generates false puncta
• Positive control: confirms that the assay chemistry, imaging, and target detection are working as expected

Single-primary controls are especially important. If one antibody alone produces signal, that usually indicates a specificity or aggregation problem that should be resolved before interpreting any interaction data.

PLA troubleshooting guide

Even well-designed PLA experiments can require optimisation. Common issues include weak signal, high background, excessive puncta, or uneven staining.

• No or weak signal: check antibody performance, fixation, retrieval conditions, probe storage, and incubation temperature
• High diffuse background: reduce antibody concentration and improve washing conditions
• Puncta in negative controls: replace poorly performing antibodies or improve blocking
• Clusters that cannot be counted: shorten amplification time and avoid imaging overexposure
• Random signal off cells: prevent sample drying and maintain a humidified chamber during incubations

In practice, most PLA problems trace back to antibody quality, sample preparation, or insufficient controls.

Recent advances in PLA

PLA continues to evolve as a powerful platform for spatial biology. Recent advances include improved multiplexing, automated workflows, and expanded use in translational and clinical research.

• Multiplex PLA now supports the simultaneous detection of multiple interactions in a single section
• Automation is reducing hands-on time and improving consistency between runs
• Clinical applications are emerging in immuno-oncology and tissue biomarker analysis
• PLA is increasingly used alongside digital pathology and image analysis workflows

These developments are helping position PLA as more than a niche interaction assay, expanding it into a broader platform for in situ molecular analysis.

Which PLA format should you choose?

The right PLA format depends on your research question.

• In situ PLA: best when spatial information in cells or tissues is important
• Multiplex fluorescent PLA: useful when comparing several interactions in parallel
• Solution-phase PLA: suitable when localisation is less important and quantitative detection is the priority
• qPCR- or digital PLA-based formats: useful for highly sensitive quantitative workflows

For many researchers, in situ PLA remains the preferred starting point because it combines biological context, sensitivity, and straightforward microscopic readout.

Frequently asked questions

Can PLA be used on FFPE tissues?

Yes. FFPE tissue is one of the most common sample types for PLA, especially in translational and pathology-based studies. Antigen retrieval is usually essential and should be optimised for the antibody pair used.

Can PLA detect post-translational modifications?

Yes. PLA can be designed to detect a protein together with a specific modification, making it highly useful for signalling studies and pathway analysis.

How is PLA quantified?

PLA is commonly quantified by counting fluorescent puncta per cell, per nucleus, or per defined tissue region using image analysis tools such as ImageJ, CellProfiler, or QuPath.

Can PLA be multiplexed?

Yes. Multiplex PLA approaches allow multiple interactions or targets to be measured in one sample, although complexity increases with panel size and assay design.

Is PLA suitable for membrane proteins?

Yes, but gentle permeabilisation and careful antibody choice are important to preserve membrane-associated biology and reduce disruption of target architecture.

Why buy PLA-related reagents from Biorbyt?

At Biorbyt, we support researchers with antibodies and reagents relevant to PLA and other in situ detection workflows. Our focus is on helping scientists identify suitable targets and build robust assay strategies for meaningful biological readouts.

• Antibodies suitable for immunofluorescence and in situ applications
• Broad catalogue coverage for signalling, oncology, immunology, and cell biology targets
• Support for target discovery and assay planning
• Reliable supply and global distribution

Explore our range of PLA-compatible antibodies and reagents to support your next experiment.

Conclusion

The Proximity Ligation Assay is one of the most effective methods for detecting protein interactions and modifications directly in cells and tissues. By combining antibody specificity with DNA amplification, PLA provides a unique balance of sensitivity, specificity, and spatial resolution. Whether you are studying signalling pathways, validating biomarkers, or analysing tissue-based molecular interactions, PLA offers a powerful route from target biology to interpretable in situ data.