Abstract
Depolarization‐induced vesicle exocytosis is a complex mechanism involving a number of proteins. In this process, synaptotagmins work as members of the Ca^2+^‐sensing system that triggers the fusion of the synaptic vesicle with the plasma membrane. Synaptotagmin IV (SytIV), an immediate‐early gene induced by depolarization in PC12 pheochromocytoma cells and in the hippocampus, has been suggested to work as a negative regulator of neurotransmitter release. Unlike other synaptotagmins, SytIV has an evolutionarily conserved substitution of an aspartate to a serine in the Ca^2+^ coordination site of its C2A domain, preventing SytIV from binding anionic lipids in a Ca^2+^‐dependent fashion. We used the secretion of human growth hormone (hGH) as a reporter system with which to examine the effects of overexpressing SytIV and other depolarization‐induced immediate‐early genes (the protein kinases KID‐1, SIK, and PIM‐1 and the transcription factors rTLE3 and Nurr1) on depolarization‐induced vesicle exocytosis in PC12 cells. SytIV overexpression resulted in decreased depolarization‐induced hGH release. However, conversion of the unique serine in SytIV to an aspartate eliminated this inhibitory activity. In addition, rTLE3 overexpression produced only a modest increase in spontaneous vesicle exocytosis, whereas KID‐1, SIK, PIM‐1, and Nurr1 overexpression had no effect on depolarization‐induced exocytosis. © 2004 Wiley‐Liss, Inc.