Plasticity in the glucagon interactome reveals novel proteins that regulate glucagon secretion in αTC1-6 cells

Glucagon is stored within secretory granules of pancreatic alpha cells until a stimulus, such as a change in microenvironmental conditions, triggers its release. We hypothesized that the secretory response of the alpha cell to various stimuli could be determined by plasticity in the network of proteins that interact with glucagon within alpha cell secretory granules. To answer this question, we isolated secretory granules from alpha TC1-6 cells and identified glucagon-interacting proteins by affinity purification coupled with liquid chromatography/tandem mass spectrometry. Proteomic analyses revealed a network of cytoplasmic and histone proteins. Specifically, the interaction between glucagon and histone H4 and the ER stress protein GRP78 was confirmed through co-immunoprecipitation of secretory granule lysates, and co-localization within secretory granules using high-resolution confocal microscopy. The composition of these networks was altered at different glucose levels (25 mM vs 5.5 mM) and in response to the paracrine inhibitors of glucagon secretion, GABA and insulin. Finally, siRNA-mediated silencing of a subset of nonhistone proteins revealed novel proteins that may regulate glucagon secretion. We have therefore described a novel and dynamic glucagon interactome within alpha cell secretory granules, and suggest that plasticity in the interactome governs the alpha cell secretory response to paracrine and nutritional stimuli.