Budding from Golgi membranes requires the coatomer complex of non-clathrin coat proteins

Abstract

Do the coats on vesicles budded from the Golgi apparatus actually cause the budding, or do they simply coat buds (Fig. 1)? One view (the membrane-mediated budding hypothesis¹) is that budding is an intrinsic property of Golgi membranes not requiring extrinsic coat proteins. Assembly of coats from dispersed subunits is superimposed upon the intrinsic budding process and is proposed to convert the tips of tubules into vesicles. The alternative view (the coat-mediated budding hypothesis¹) is that coat formation provides the essential driving force for budding. The membrane-mediated budding hypothesis was inspired by the microtubule-dependent extension of apparently uncoated, 90-nm-diameter membrane tubules from the Golgi apparatus² and other organelles^3–5 in vivo after treatment with brefeldin A, a drug that inhibits the assembly of coat proteins onto Golgi membranes^6–9. This hypothesis predicts that tubules will be extended when coat proteins are unavailable to convert tubule-derived membrane into vesicles. Here we use a cell-free system in which coated vesicles are formed from Golgi cisternae to show that, on the contrary, when budding diminishes as a result of immunodepletion of coat protein pools, tubules are not formed at the expense of vesicles. We conclude that coat proteins are required for budding from Golgi membranes.