Mapping Interactions between Glycans and Glycan-Binding Proteins by Live Cell Proximity Tagging.

Interactions between glycans and glycan-binding proteins (GBPs) consist of weak, noncovalent, and transient binding events, making them difficult to study in live cells void of a static, isolated system. Furthermore, the glycans are often presented as protein glycoconjugates, but there are limited efforts to identify these proteins. Proximity labeling permits covalent tagging of the glycoprotein interactors to query GBP in live cells. Coupled with high-resolution mass spectrometry, it facilitates determination of the proteins bearing the interacting glycans. In this method, fusion protein constructs of a GBP of interest with a peroxidase enzyme allows for in situ spatiotemporal radical-mediated tagging of interacting glycoproteins in living cells that can be enriched for identification. Using this method, the capture and study of glycan-GBP interactions no longer relies on weak, transient interactions, and results in robust capture and identification of the interactome of a GBP while preserving the native cellular environment. This protocol focuses on (1) expression and characterization of a recombinant fusion protein consisting of a peroxidase and the GBP galectin-3, (2) corresponding in situ labeling and visualization of interactors, (3) and proteomic workflow and analysis of captured proteins for robust identification using mass spectrometry. © 2021 Wiley Periodicals LLC. Basic Protocol 1: Expression, purification, and characterization of recombinant fusion protein Alternate Protocol 1: Manual Ni-NTA purification of recombinant fusion protein Basic Protocol 2: In situ proximity labeling and evaluation by fluorescence microscopy Alternate Protocol 2: Western blot analysis of in situ proximity labeling Basic Protocol 3: Proximity labeling of cells for quantitative MS-based proteomics with tandem mass tags.