DIRECT CONTROL OF EXOCYTOSIS BY RECEPTOR-MEDIATED ACTIVATION OF THE HETEROTRIMERIC GTPASES G(I) AND G(O) OR BY THE EXPRESSION OF THEIR ACTIVE G-ALPHA SUBUNITS

The exocytotic release of potent hormones is a tightly controlled process. Its direct regulation without the involvement of second messengers would ensure rapid signal processing. In streptolysin O-permeabilized insulin-secreting cells, a preparation allowing dialysis of cytosolic macromolecules, activation of alpha(2)-adrenergic receptors caused pertussis toxin-sensitive inhibition of calcium-induced exocytosis. This inhibition was mimicked very efficiently by the use of specific receptor-mimetic peptides, indicating the involvement of G(i) and, to a lesser extent, of G(o). The regulation was exerted beyond the ATP-dependent step of exocytosis. In addition, low nanomolar amounts of pre-activated G(i)/G(o) directly inhibited exocytosis. As transient overexpression of constitutively active mutants of G alpha(i)1, G alpha(i)2, G alpha(i)3 and G alpha(o)2 but not of G alpha(o)1 reproduced this regulation, the G alpha subunit alone is sufficient to induce inhibition. These results define exocytosis as an effector for heterotrimeric G-proteins and delineate the properties of the transduction pathway.