IN many mammalian cells brefeldin A interferes with mechanisms that keep the Golgi apparatus separate from the endoplasmic reticulum1-8. The earliest effect of brefeldin A is release of the coat protein β-COP from the Golgi9-11. This release is blocked by pretreatment with GTP-γS or AIF-4 (ref. 12). The AIF- 4 ion activates heterotrimeric G proteins13 but not proteins of the rassuperfamily14, suggesting that a heterotrimeric G protein might control membrane transfer from the endoplasmic reticulum to the Golgi. We report here that mastoparan, a peptide that activates heterotrimeric G proteins15,16, promotes binding ofβ-COP to Golgi membranes in vitro and antagonizes the effect of brefeldin A on β-COP in perforated cells and on isolated Golgi membranes. This inhibition is greatly diminished if cells are pretreated with pertussis toxin before perforation., Thus, a heterotrimeric G protein of the Gi/GO subfamily regulates association of coat components with Golgi membranes.
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