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QF-Pro Home › QF-Pro Glossary › Protein-Protein Interaction
Functional Biomarkers — Step 2 of 5
1Functional vs Expression
2PPI
3Colocalization
4PLA
5Protein Activation State
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Colocalization
Fundamental Concept

Protein-Protein Interaction

The physical association between proteins that underlies all cellular function–and the target of FRET-based functional biomarker detection at 1–10 nm resolution.

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Definition
Protein-protein interactions are physical associations between two or more proteins that enable biological function. From receptor-ligand binding (PD-1/PD-L1Loading...) to enzyme-substrate complexes to signaling cascades, PPIs are the mechanistic basis of cellular behavior–and the target of FRETLoading...-based functional biomarkerLoading... detection at 1–10 nm resolution.
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Applications Beyond Oncology
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1-10 nm distance
True binding range
Mechanistic basis
How drugs work
Quantifiable by FRET
Physics-based detection
Therapeutic target
Block or modulate PPIs

PPIs as Therapeutic Targets

Most targeted cancer therapies work by modulating protein-protein interactions. Checkpoint inhibitorsLoading... block PD-1/PD-L1 binding. HER2-targeted therapies disrupt receptor dimerization. Kinase inhibitors prevent enzyme-substrate interactions.

This mechanistic reality creates a biomarker paradox: therapies target interactions, but companion diagnosticsLoading... measure expression. A PD-L1 IHCLoading... score tells us how much PD-L1 is present–not whether it's actually engaging PD-1.

Measuring PPIs directly aligns the biomarker with the mechanism of action. If a drug blocks an interaction, the biomarker should measure that interaction.

Simplified

Why PPIs Matter: Most biological processes involve proteins interacting with other proteins. Checkpoint suppression, receptor signaling, enzyme activation—all are protein-protein interactions.

Drug Development: Many drugs work by blocking specific PPIs (checkpoint inhibitors block PD-1/PD-L1 interaction). Measuring PPIs directly assesses what drugs are targeting.

Detection Resolution Matters

PPIs occur at distances of 1–10 nanometers–the length scale of molecular binding interfaces. This is far below the optical diffraction limit (~200 nm), making conventional microscopy unable to distinguish interacting proteins from those merely colocalized.

Different detection methods have different resolution capabilities:

ColocalizationLoading... (70–100 nm): Proteins in the "same building"
Proximity Ligation AssayLoading... (30–40 nm): Proteins in the "same corridor"
FRET (1–10 nm): Proteins in the "same room, having a meeting"

Only FRET provides resolution sufficient to confirm genuine molecular binding–making it uniquely suited to PPI detection in clinical samples.

Simplified

The Challenge: PPIs occur at nanometer distances. Standard microscopy can't resolve them. Proteins in the "same spot" by imaging might not actually be interacting.

FRET's Solution: FRET only works at 1-10nm—the distance of genuine PPIs. Detection of FRET = confirmation of interaction.

QF-Pro: FRET-Based PPI Detection

Clinically Validated

Core Capability: QF-Pro detects PPIs at 1-10nm resolution where genuine molecular engagement occurs. Validated measurements include:
• Intercellular: PD-1/PD-L1[3], CTLA-4/CD80[4]
• Intracellular: PKB/Akt[1,2] phosphorylation-induced conformational changes

Click citation numbers to view full references in QF-Pro Applications & Clinical EvidenceLoading...

Simplified

Core capability: QF-Pro detects protein-protein interactions at 1-10nm resolution. Validated for intercellular checkpoints (PD-1/PD-L1, CTLA-4/CD80) and intracellular activation (Akt).

Clinical Relevance

  • Checkpoint engagement: PD-1/PD-L1 interaction measured by iFRETLoading... predicts immunotherapy response where expression fails
  • Receptor dimerization: HER2-HER3Loading... dimer formation drives oncogenic signaling even at HER2-low expression
  • Kinase activation: Akt activationLoading... (a PPI-dependent conformational change) predicts outcome where Akt expression does not
  • Mechanism-aligned biomarkers: Measuring the PPI targeted by therapy provides direct functional relevance

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