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 proc ess. Its direct regulation without the involvement of second messenger s would ensure rapid signal processing. In streptolysin O-permeabilize d insulin-secreting cells, a preparation allowing dialysis of cytosoli c macromolecules, activation of alpha(2)-adrenergic receptors caused p ertussis toxin-sensitive inhibition of calcium-induced exocytosis. Thi s inhibition was mimicked very efficiently by the use of specific rece ptor-mimetic peptides, indicating the involvement of G(i) and, to a le sser extent, of G(o). The regulation was exerted beyond the ATP-depend ent step of exocytosis. In addition, low nanomolar amounts of pre-acti vated G(i)/G(o) directly inhibited exocytosis. As transient overexpres sion of constitutively active mutants of G alpha(i)1, G alpha(i)2, G a lpha(i)3 and G alpha(o)2 but not of G alpha(o)1 reproduced this regula tion, the G alpha subunit alone is sufficient to induce inhibition. Th ese results define exocytosis as an effector for heterotrimeric G-prot eins and delineate the properties of the transduction pathway.