Gβγ directly modulates vesicle fusion by competing with synaptotagmin for binding to neuronal SNARE proteins embedded in membranes.

Abstract

Gi/o-coupled GPCRs can inhibit neurotransmitter release at synapses via multiple mechanisms. In addition to Gβγ-mediated modulation of voltage-gated calcium channels(VGCC), inhibition can also be mediated through the direct interaction of Gβγ subunits with the soluble N-ethylmaleimide attachment protein receptor (SNARE) complex of the vesicle fusion apparatus. Binding studies with soluble SNARE complexes have shown that Gβγ binds to both ternary SNARE complexes, t-SNARE heterodimers, and monomeric SNAREs, competing with synaptotagmin(syt)1 for binding sites on t-SNARE. However, in secretory cells, Gβγ, SNAREs, and synaptotagmin interact in the lipid environment of a vesicle at the plasma membrane. To approximate this environment, we show that fluorescently-labeled Gβγ interacts specifically with lipid-embedded t-SNAREs consisting of full-length syntaxin 1 and SNAP-25B at the membrane, as measured by fluorescence polarization. Fluorescently-labeled syt 1 undergoes competition with Gβγ for SNARE binding sites in lipid environments. Mutant Gβγ subunits that were previously shown to be more efficacious at inhibiting Ca2+-triggered exocytotic release than wild-type Gβγ were also shown to bind SNAREs at higher affinity than wild-type in a lipid environment. These mutant Gβγ subunits were unable to inhibit VGCC currents. Specific peptides, corresponding to regions on Gβ and Gγ shown to be important for the interaction, disrupt the interaction in a concentration-dependent manner. In in vitro fusion assays utilizing full-length t- and v-SNAREs embedded in liposomes, Gβγ inhibited Ca2+/synaptotagmin-dependent fusion. Together, these studies demonstrate the importance of these regions for the Gβγ-SNARE interaction, and show that the target of Gβγ, downstream of VGCC, is the membrane-embedded SNARE complex.