Gi/o-coupled G protein-coupled receptors can exert an inhibitory effect on vesicle release through several G protein-driven mechanisms, more than one of which may be concurrently present in individual presynaptic terminals. The synaptosomal-associated protein of 25 kDa (SNAP25) is a key downstream effector of G?? subunits. It has previously been shown that proteolytic cleavage of SNAP25 by botulinum toxin A reduces the ability of G?? to compete with the calcium sensor synaptotagmin 1 (Syt1) for binding to SNAP25 in a calcium-dependent manner. These truncated SNAP25 proteins sustain a low level of exocytosis but are unable to support serotonin-mediated inhibition of exocytosis in lamprey spinal neurons. Here, we generate a SNAP25 extreme C-terminal mutant that is deficient in its ability to bind G?? while retaining normal calcium-dependent Syt1 binding to soluble N-ethylmaleimide attachment protein receptor (SNARE) and vesicle release. The SNAP25?3 mutant, in which residue G204 is replaced by a stop codon, features a partial reduction in G?1?2 binding in vitro as well as a partial reduction in the ability of the lamprey 5-hydroxytryptamine1b-type serotonin receptor to reduce excitatory postsynaptic current amplitudes, an effect previously shown to be mediated through the interaction of G?? with SNAP25. Syt1 calcium-dependent binding to SNAP25?3 was reduced by a small extent compared with the wild type. We conclude that the extreme C terminus of SNAP25 is a critical region for the G??-SNARE interaction.