Regulation of the exocytotic machinery by cAMP-dependent protein kinase: implications for presynaptic plasticity

For over a decade, the enhancement of regulated exocytosis by cAMP-dependent protein kinase (PKA) has remained unexplained at the molecular level. The fact that this phenomenon has been observed in such a wide variety of secretory cell types, from pancreatic beta-cells to neurons, suggests that it is an important and fundamental mechanism. Extensive analysis of the phosphorylation of exocytotic proteins has yielded few substrates of PKA in vitro, and fewer still have had physiological effects attributed to their phosphorylation. Here we review two proteins that do fulfil these criteria: the synaptic vesicle proteins cysteine string protein (CSP) and Snapin. Phosphorylation of these proteins by PKA produces changes in their respective protein-protein interactions, and has been attributed to modulation of the vesicle priming (Snapin) and vesicle fusion (CSP) stages of exocytosis. We also discuss how the function of CSP and Snapin phosphorylation might fit into an interesting aspect of the PKA-dependent enhancement of exocytosis: presynaptic plasticity in the brain.