Abstract

Syntaxin 1A/HPC-1 is a key component of the exocytotic molecular machinery, namely, the solubleN-ethylmaleimide-sensitive factor attachment protein (SNAP) receptor mechanism. Although >10 syntaxin-binding proteins have been identified, they cannot completely explain the regulation of exocytosis. Thus, novel proteins may interact with syntaxin. Because exocytosis requires both Ca²⁺ and ATP, we searched for Ca²⁺/ATP-dependent syntaxin-binding proteins from the rat brain and discovered Ca²⁺/calmodulin-activated protein kinase II (CaMKII)-α. At Ca²⁺ concentrations of >10⁻⁶m, only autophosphorylated CaMKII bound to syntaxin. Bound CaMKII was released from syntaxin by EGTA or by phosphatase, indicating that the binding is reversible. CaMKII bound to the linker domain of syntaxin, unlike any other known syntaxin-binding proteins. CaMKII–syntaxin complexes were also detected in synaptosomes by immunoprecipitation, and when reconstitutedin vitro, they recruited larger amounts of synaptotagmin and SNAP-25 than syntaxin alone. The microinjected CaMKII-binding domain of syntaxin specifically affected exocytosis in chromaffin cells and in neurons. These results indicate that the Ca²⁺/ATP-dependent binding of CaMKII to syntaxin is an important process in the regulation of exocytosis.