Identification of syntaxin-1A sites of phosphorylation by casein kinase I and casein kinase II

Casein kinases I (CKI) are serine/threonine protein kinases widely expressed in a range of eukaryotes including yeast, mammals and plants. They have been shown to play a role in diverse physiological events including membrane trafficking. CKIalpha is associated with synaptic vesicles and phosphorylates some synaptic vesicle associated proteins including SV2. In this report, we show that syntaxin-1A is phosphorylated in vitro by CKI on Thr21. Casein kinase II (CKII) has been shown previously to phosphorylate syntaxin-1A in vitro and we have identified Ser14 as the CKII phosphorylation site, which is known to be phosphorylated in vivo. As syntaxin-1A plays a key role in the regulation of neurotransmitter release by forming part of the SNARE (soluble N-ethylmaleimide-sensitive factor attachment protein receptor) complex, we propose that CKI may play a role in synaptic vesicle exocytosis.