Galectin-3 is a glycan-binding protein (GBP) that binds β-galactoside glycan structures to orchestrate a variety of important biological events, including the activation of hepatic stellate cells and regulation of immune responses. While the requisite glycan epitopes needed to bind galectin-3 have long been elucidated, the cellular glycoproteins that bear these glycan signatures remain unknown. Given the importance of the three-dimensional (3D) arrangement of glycans in dictating GBP interactions, strategies that allow the identification of GBP receptors in live cells, where the native glycan presentation and glycoprotein expression are preserved, have significant advantages over static and artificial systems. Here we describe the integration of a proximity labeling method and quantitative mass spectrometry to map the glycan and glycoprotein interactors for galectin-3 in live human hepatic stellate cells and peripheral blood mononuclear cells. Understanding the identity of the glycoproteins and defining the structures of the glycans will empower efforts to design and develop selective therapeutics to mitigate galectin-3–mediated biological events.

Galectin-3 is a glycan-binding protein (GBP) that binds b-galactoside glycan structures to orchestrate a variety of important biological events, including the activation of hepatic stellate cells to cause hepatic fibrosis. While the requisite glycan epitopes needed to bind galectin-3 have long been elucidated, the cellular glycoproteins that bear these glycan signatures remain unknown. Given the importance of the threedimensional arrangement of glycans in dictating GBP interactions, strategies that allow the identification of GBP receptors in live cells, where the native glycan presentation and glycoprotein expression are preserved, possess significant advantages over static and artificial systems. Here, we describe the integration of a proximity labeling method and quantitative mass spectrometry to map the glycan and glycoprotein interactors for galectin-3 in live hepatic stellate cells. Understanding the identity of the glycoproteins and defining the structures of the glycans required for galectin-3 mediated hepatic stellate cell activation will empower efforts to design and develop selective therapeutics to mitigate hepatic fibrosis. SignificanceBecause of the weak interactions between individual glycan-binding proteins (GBP), such as galectin-3, and glycans, strategies that allow the direct interrogation of these interactions in living cells remain limited.Thus, the glycan and glycoprotein ligands that are physiologically relevant for galectin-3 binding are insufficiently described. Here, we used a proximity labeling approach that catalytically tags interactors for galectin-3 and identified its pertinent glycan and glycoprotein counter-receptors in live hepatic stellate cells.This study demonstrates that proximity labeling is a powerful tool for mapping GBP complexes in living cells, and when coupled with chemical inhibitors, it can discriminate between protein-protein and proteinglycan interactions. Graphical Abstract

Myogenic differentiation, the irreversible developmental process where precursor myoblast muscle stem cells become contractile myotubes, is heavily regulated by glycosylation and glycan–protein interactions at the cell surface and the extracellular matrix. The glycan-binding protein galectin-1 has been found to be a potent activator of myogenic differentiation. While it is being explored as a potential therapeutic for muscle repair, a precise understanding of its glycoprotein interactors is lacking. These gaps are due in part to the difficulties of capturing glycan–protein interactions in live cells. Here, we demonstrate the use of a proximity tagging strategy coupled with quantitative mass-spectrometry-based proteomics to capture, enrich, and identify the glycan-mediated glycoprotein interactors of galectin-1 in cultured live mouse myoblasts. Our interactome dataset can serve as a resource to aid the determination of mechanisms through which galectin-1 promotes myogenic differentiation. Moreover, it can also facilitate the determination of the physiological glycoprotein counter-receptors of galectin-1. Indeed, we identify several known and novel glycan-mediated ligands of galectin-1 as well as validate that galectin-1 binds the native CD44 glycoprotein in a glycan-mediated manner.

Cell surface proteins (CSPs) are vital molecular mediators for cells and their extracellular environment. Thus, understanding which CSPs are displayed on cells, especially in different cell states, remains an important endeavor in cell biology. Here, we describe the integration of cell surface engineering with radical-mediated protein biotinylation to profile CSPs. This method relies on the prefunctionalization of cells with cholesterol lipid groups, followed by sortase-catalyzed conjugation with an APEX2 ascorbate peroxidase enzyme. In the presence of biotin-phenol and H 2 O 2 , APEX2 catalyzes the formation of highly reactive biotinyl radicals that covalently tag electron-rich residues within CSPs for subsequent streptavidin-based enrichment and analysis by quantitative mass spectrometry. While APEX2 is traditionally used to capture proximitybased interactomes, we envisioned using it in a "baitless" manner on cell surfaces to capture CSPs. We evaluate this strategy in light of another CSP labeling method that relies on the presence of cell surface sialic acid. Using the APEX2 strategy, we describe the CSPs found in three mammalian cell lines and compare CSPs in adherent versus three-dimensional pancreatic adenocarcinoma cells.