Abstract

Presynaptic inhibitory G protein-coupled receptors (GPCRs) can decrease neurotransmission by inducing interaction of Gβγ with the soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) complex. We have shown that this action of Gβγ requires the carboxyl terminus of the 25-kDa synaptosome-associated protein (SNAP25) and is downstream of the well known inhibition of Ca²⁺ entry through voltage-gated calcium channels. We propose a mechanism in which Gβγ and synaptotagmin compete for binding to the SNARE complex. Here, we characterized the Gβγ interaction sites on syntaxin1A and SNAP25 and demonstrated an overlap of the Gβγ- and synaptotagmin I -binding regions on each member of the SNARE complex. Synaptotagmin competes in a Ca²⁺-sensitive manner with binding of Gβγ to SNAP25, syntaxin1A, and the assembled SNARE complex. We predict, based on these findings, that at high intracellular Ca²⁺ concentrations, Ca²⁺-synaptotagmin I can displace Gβγ binding and the Gβγ-dependent inhibition of exocytosis can be blocked. We tested this hypothesis in giant synapses of the lamprey spinal cord, where 5-HT works via Gβγ to inhibit neurotransmission (Blackmer et al., 2001). We showed that increased presynaptic Ca²⁺ suppresses the 5-HT- and Gβγ-dependent inhibition of exocytosis. We suggest that this effect may be due to Ca²⁺-dependent competition between Gβγ and synaptotagmin I for SNARE binding. This type of dynamic regulation may represent a novel mechanism for modifying transmitter release in a graded manner based on the history of action potentials that increase intracellular Ca²⁺ concentrations and of inhibitory signals through G_i-coupled GPCRs.