Mapping the Glycogen Synthase 1 Interactome in Brown Adipocytes

Brown adipose tissue (BAT) is distinguished by its ability to dissipate energy through thermogenesis, a process with considerable therapeutic potential for obesity, type 2 diabetes, and related metabolic disorders. Although glycogen represents a relatively minor energy reserve in adipocytes compared with liver and muscle, emerging evidence shows that glycogen turnover increases sharply during metabolic transitions and is required for full thermogenic activation in brown adipocytes. The rate-limiting enzyme glycogen synthase 1 (GYS1) catalyzes glycogen chain elongation, and adipose-specific Gys1 deletion impairs adaptive thermogenesis, highlighting the importance of GYS1 in coordinating glucose and lipid metabolism; however, the mechanisms regulating GYS1 in BAT remain poorly defined. To address this gap, we used an unbiased proximity-labeling strategy in brown adipocytes by fusing GYS1 to the biotin ligase TurboID and identifying biotinylated proteins through mass spectrometry, revealing 425 putative interactors. These included canonical glycogen-handling proteins such as glycogenin and glycogen phosphorylase, validating the approach, while gene ontology analysis uncovered broader enrichment in cytoskeletal remodeling, vesicle trafficking, and transcriptional regulation, suggesting previously unrecognized roles for GYS1 beyond glycogen synthesis. Notably, we identified several novel candidate interactors, including CAST, PDAP1, and CCDC102A. Together, these findings define the most comprehensive GYS1 interactome reported to date and provide new insights into the molecular mechanisms linking glycogen metabolism to thermogenic function in BAT.