Gβγ reduces the number and quantal size of exocytotic events in neurosecretory chromaffin cells

Abstract

G-protein βγ subunits (Gβγ) inhibit regulated exocytosis by several mechanisms including modulation of voltage gated Ca2+ channels (VGCC). We have shown that Gβγ also binds to the soluble N-ethylmaleimide sensitive factor attachment protein receptor (SNARE) complex. Gβγ interacts with individual SNARE proteins and the ternary SNARE complex in vitro. Furthermore, Gβγ competes with Ca2+-dependent synaptotagmin binding to the SNARE complex. We postulate that Gβγ inhibits exocytosis by parallel mechanisms that target both Ca2+ influx (VGCC) and the exocytotic fusion event. To investigate this possibility, we explored Gβγ function in bovine adrenal chromaffin cells. GFP-fused Gβ was expressed with Gγ in chromaffin cells. Confocal microscopy confirmed plasma membrane targeting, and functional expression was confirmed electrophysiologically using prepulse facilitation (reversal of tonic VGCC inhibition). We used Ca2+ ionophores to bypass VGCC and directly stimulate secretion. Exocytotic events were detected using carbon fiber amperometry. The total number of exocytotic fusion events (amperometric spikes) was significantly reduced (~ 42%) in Gβγ expressing cells compared to GFP expressing control cells. In addition, the mean charge and peak amplitude of the amperometric spikes were significantly reduced in Gβγ-expressing cells. These data support the hypothesis that Gβγ can inhibit secretion downstream from VGCC. We propose that Gβγ attenuate exocytosis by parallel mechanisms, both inhibiting VGCC, and reducing the number and quantal size of exocytotic events by interacting with the SNARE complex. Supported by NIH EY10291