A putative heterotrimeric G protein inhibits the fusion of COPI-coated vesicles. Segregation of heterotrimeric G proteins from COPI-coated vesicles.

Heterotrimeric G proteins have been implicated in the regulation of intracellular protein transport, but their mechanism of action remains unclear. In vivo, secretion of chromogranin B, tagged with the green fluorescent protein, was inhibited by the addition of a general activator of trimeric G proteins (AlF4-) to stably transfected Vero cells and resulted in an accumulation of the tagged protein in the Golgi apparatus. In an in vitro assay that reconstitutes intra-Golgi protein transport, we find that a membrane-bound and AlF4--sensitive factor is involved in the fusion reaction.