PHOSPHORYLATION OF SYNAPSIN-I AND MARCKS IN NERVE-TERMINALS IS MEDIATED BY CA2+ ENTRY VIA AN AGA-GI SENSITIVE CA2+ CHANNEL WHICH IS COUPLED TO GLUTAMATE EXOCYTOSIS

Ca2+ entry is a prerequisite for both exocytosis and the phosphorylation of synapsin I and MARCKS proteins in mammalian cerebrocortical synaptosomes. The novel spider toxin Aga-GI completely blocks KCl-evoked glutamate exocytosis but only partially inhibits KCl-evoked cytoplasmic Ca2+ elevations, thus revealing at least two pathways for KCl-induced Ca2+ entry. Aga-GI completely attenuates KCl-induced phosphorylation of synapsin I and MARCKS proteins. We therefore conclude that both exocytosis and the phosphorylation of synapsin I and MARCKS proteins are specifically coupled to Ca2+ entry via a subset of voltage dependent Ca2+ channels at the nerve terminal which are sensitive to Aga-GI.