B-291 Proximity Labeling at Sites of Protein-Protein Interactions Using a Switchable DNA Catalyst
Authors: Ogorek A , Zhou X , Martell J 
Published: September 2023

Abstract

Background
Extracellular protein-protein interactions (PPIs) play a crucial role in cell-cell adhesion, communication, and signaling. The dysregulation of PPIs in cancer caused by differential protein expression can drive oncogenic factors. Traditional methods to study PPIs disfavor weak or transient interacting protein partners, leaving them undiscovered. Proximity labeling (PL) is an emerging tool in which a catalyst is attached to a protein of interest (POI) and creates reactive species that tag nearby endogenous proteins. Tagged proteins are subsequently isolated and identified as potential interacting partners with the POI. If PL could be activated at a more specific subcellular location, such as the site of a PPI, more detailed information could be unveiled concerning the interactome of cancer-relevant PPIs. Recent advances in PL involve the use of a synthetic catalyst that can be localized to a POI via an antibody or ligand and avoids the need for PL enzyme-POI fusion proteins to be constructed, allowing for analysis of primary tissue samples.

Methods
To create a PL catalyst with pseudo-allosteric regulation, we created a switchable DNA catalyst. The DNA catalyst is composed of synthetic photocatalyst and quencher moieties covalently linked to DNA oligomers such that when hybridized together the photocatalyst is quenched and inactive. The DNA scaffold affords conformational control that is utilized to change the distance between photocatalyst and quencher, thus regulating photocatalyst activity. Split triggers composed of DNA oligos that bind to catalyst-DNA and simultaneously disrupt quencher-DNA hybridization were used to create an AND logic gate where both split triggers need to be in proximity to activate the PL catalyst. The split triggers were localized to POIs via connection to aptamers, short pieces of synthetic DNA that bind targets. Using the c-Met homodimer as a model system, we first labeled c-Met expressing HEK293T cells with aptamer-split trigger DNA oligomers as well as the quenched photocatalyst. After the catalyst was activated at the site of the PPI, we performed proximity labeling using a biotin-phenol substrate in the presence of an oxidizing agent and 450 nm light. Fluorescent streptavidin was used to visualize sites of biotinylation and detected using fluorescence microscopy.

Results
We observed extracellular biotin signal only of c-Met transfected cells as evidenced by the expression of a co-transfection plasmid encoding a fluorescent nuclear protein. When one of the split triggers had the aptamer motif removed, meaning it couldn’t be localized to the cell surface, no labeling was observed which shows that both split probes must be localized to the cell surface to activate labeling. Additionally, controls including omission of light, photocatalyst, or biotin phenol showed no labeling. Using a free-floating version of the photocatalyst to label cells, we confirmed that lysates contained biotinylated proteins via western blot.

Conclusion
We localized two split triggers to interacting proteins to activate a PL catalyst and performed proximity labeling at the site of the PPI. This technique demonstrates non-genetic split proximity labeling to enable downstream analysis of cancer cell proteomes in an ultra-specific manner and expands the toolkit for spatiotemporally controlled proximity labeling.