EBAG9 Adds a New Layer of Control on Large Dense-Core Vesicle Exocytosis via Interaction with Snapin

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Abstract
Regulated exocytosis is subject to several modulatory steps that include phosphorylation events and transient protein–protein interactions. The estrogen receptor-binding fragment-associated gene9 (EBAG9) gene product was recently identified as a modulator of tumor-associated O-linked glycan expression in nonneuronal cells; however, this molecule is expressed physiologically in essentially all mammalian tissues. Particular interest has developed toward this molecule because in some human tumor entities high expression levels correlated with clinical prognosis. To gain insight into the cellular function of EBAG9, we scored for interaction partners by using the yeast two-hybrid system. Here, we demonstrate that EBAG9 interacts with Snapin, which is likely to be a modulator of Synaptotagmin-associated regulated exocytosis. Strengthening of this interaction inhibited regulated secretion of neuropeptide Y from PC12 cells, whereas evoked neurotransmitter release from hippocampal neurons remained unaltered. Mechanistically, EBAG9 decreased phosphorylation of Snapin; subsequently, association of Snapin with synaptosome-associated protein of 25 kDa (SNAP25) and SNAP23 was diminished. We suggest that the occurrence of SNAP23, Snapin, and EBAG9 also in nonneuronal cells might extend the modulatory role of EBAG9 to a broad range of secretory cells. The conjunction between EBAG9 and Snapin adds an additional layer of control on exocytosis processes; in addition, mechanistic evidence is provided that inhibition of phosphorylation has a regulatory function in exocytosis.