Protein Complex-Based Proximity Labeling Systems for High-Resolution Mapping of Intracellular Microenvironments

Background

Proximity labeling technologies have become essential tools in cell biology, enabling researchers to map protein-protein and protein-nucleic acid interactions within the complex environment of living cells. Understanding these interactions is crucial for elucidating cellular signaling pathways, chromatin organization, and the mechanisms underlying diseases such as cancer. Traditional biochemical approaches, such as co-immunoprecipitation or yeast two-hybrid assays, often fail to capture transient or weak interactions and lack spatial resolution. Proximity labeling overcomes these limitations by covalently tagging molecules in the immediate vicinity of a protein of interest, allowing for the identification of interaction partners and microenvironmental components through downstream proteomics or sequencing. As the field has advanced, there is a growing need for labeling methods that offer higher spatial and temporal precision, especially for studying dynamic processes and subcellular structures that are inaccessible to conventional techniques.

Current proximity labeling approaches, such as those based on engineered peroxidases (e.g., APEX) or promiscuous biotin ligases (e.g., BioID), present several significant challenges. The reactive intermediates generated by these enzymes often have long diffusion radii, leading to nonspecific labeling and reduced spatial resolution. Additionally, these systems typically label only certain amino acid side chains, which can bias the identification of interactors. The enzymes themselves are relatively large, potentially perturbing the native structure or function of the protein complexes under study. Temporal control is also limited, as labeling is often initiated by the addition of hydrogen peroxide or biotin, which can induce cellular stress or require extended incubation times. These limitations hinder the ability to precisely map microenvironments, especially in densely packed or dynamic assemblies such as chromatin, nucleosomes, or ribosomes, and restrict the utility of existing methods in probing rapid or transient biological events.

Technology

This technology enables high-resolution, proximity-based labeling of intracellular protein microenvironments by employing protein complexes covalently linked to modular transition metal photocatalysts. The system allows precise attachment of the transition metal complex to a protein of interest using versatile strategies such as HaloTag or SNAPTag fusions, site-specific incorporation of unnatural amino acids via click chemistry, or post-translational ligation through split-intein protein trans-splicing. Upon activation by light or chemical stimuli, the transition metal complex generates reactive intermediates with a tightly restricted diffusion radius (2–10 nanometers) and an extremely short half-life (less than 5 nanoseconds). This finely tuned spatial and temporal control enables selective labeling of biomolecules in the immediate vicinity of the protein complex, facilitating detailed mapping of protein-protein and protein-nucleic acid interactions, E3 ligase-substrate dynamics, and the identification of molecular glues within complex cellular assemblies such as chromatin and ribosomes.

What differentiates this technology is its unprecedented combination of modularity, precision, and versatility. Unlike traditional proximity labeling methods that suffer from limited spatial resolution, lack of temporal control, and bulky enzyme tags, this approach leverages the unique properties of transition metal complexes to generate highly localized and short-lived reactive species. The modular chemical design of the transition metal complex allows for extensive customization to suit different biological targets and experimental needs, while the flexible attachment strategies ensure compatibility with a wide range of proteins and cellular contexts. The ability to confine labeling to nanometer-scale microenvironments, coupled with robust proteomics workflows for downstream analysis, provides researchers with a powerful tool to dissect complex intracellular networks, study dynamic protein degradation pathways, and explore the effects of drugs or mutations on cellular interactomes with exceptional fidelity.

Benefits

• Enables high-resolution, spatially and temporally controlled proximity labeling within living cells with nanometer-scale precision.
• Allows precise mapping of protein-protein and protein-nucleic acid interactions, including challenging targets like chromatin and ribosomes.
• Uses modular transition metal complexes that can be chemically customized for diverse biological applications and attachment strategies.
• Supports multiple intracellular photocatalyst attachment methods (HaloTag/SNAPTag, unnatural amino acids, split inteins) for versatile targeting of proteins of interest.
• Generates reactive intermediates with very short diffusion radius and half-life, minimizing off-target labeling and improving specificity.
• Facilitates characterization of protein degradation pathways and identification of E3 ligase-substrate interactions and molecular glues.
• Integrates with proteomics workflows for comprehensive identification and quantification of labeled proteins in complex cellular environments.
• Applicable to studying disease-relevant processes such as cancer signaling, epigenetic regulation, and effects of oncomutations in chromatin.

Applications

• Mapping protein-protein interactions
• Profiling chromatin microenvironments
• Identifying E3 ligase-substrate dynamics
• Screening molecular glue compounds
• Studying protein-nucleic acid interactions

Additional info

These protein complexes utilize a substrate covalently linked to a modular transition metal photocatalyst via HaloTag fusions, click chemistry, or split-intein trans-splicing. Stimuli-activated catalysis generates reactive intermediates with a restricted 2–10 nm diffusion radius and sub-5 nanosecond half-life. This system enables high-resolution mapping of intracellular microenvironments, profiling protein-protein interactions, E3 ligase-substrate dynamics, and molecular glue mechanisms in chromatin and ribosomes.