Phosphorylation of Syntaxin-1a by casein kinase 2a (CK2a) regulates presynaptic vesicle exocytosis from the reserve pool

The t-SNARE protein Syntaxin-1a (Stx-1a) is abundantly expressed at presynaptic terminals where it plays a critical role in the exocytosis of neurotransmitter-containing synaptic vesicles. Stx-1a is phosphorylated by Casein kinase 2α (CK2α) at Ser14, which has been proposed to regulate the interaction of Stx-1a and Munc-18 to control of synaptic vesicle priming. However, the role of CK2α in synaptic vesicle dynamics remains unclear. Here, we show that CK2α over-expression reduces evoked synaptic vesicle release. Furthermore, shRNA-mediated knockdown of CK2α in primary hippocampal neurons strongly enhanced vesicle exocytosis from the reserve pool (RP), with no effect on the readily releasable pool (RRP) of primed vesicles. In neurons in which endogenous Stx-1a was knocked down and replaced with a CK2α phosphorylation deficient mutant, Stx-1a(D17A), vesicle exocytosis was also increased. These results reveal a previously unsuspected role of CK2α phosphorylation in specifically regulating the reserve synaptic vesicle pool, without changing the kinetics of release from the RRP.