PKC h activity maintains normal quantal size in chromaffin cells

Studies in mast cells, neuroendocrine cells, and neurons demonstrate that calcium-dependent protein kinase C (PKC) activity plays an important role in the regulation of exocytosis (Burgoyne et al. 2001). Protein kinase Cs encompass a family of at least 12 serine/threonine kinases involved in a multitude of cellular signaling cascades. They are divided into three groups based on their activation requirements. The conventional PKC isoforms a, bI, bII and c, are Ca2+-dependent and activated by either diacylglycerol (DAG) or phosphatidylserine (PS). The novel PKC isoforms d, e, l, g, and h, are activated by DAG or PS but do not require Ca2+. Finally, the atypical PKC isoforms, f, s, and k, are activated by PS but not DAG or Ca2+ (Dempsey et al. 2000; Way et al. 2000). Protein kinase C isoforms phosphorylate multiple proteins implicated in secretory transmission, including SNAP25 (Nagy et al. 2002; Shoji-Kasai et al. 2002), Ca2+ channel