GOLGI MEMBRANE DYNAMICS IMAGED BY FREEZE-ETCH ELECTRON-MICROSCOPY - VIEWS OF DIFFERENT MEMBRANE COATINGS INVOLVED IN TUBULATION VERSUS VESICULATION

We used high resolution three-dimensional electron microscopy to visua lize Golgi cisternal structure and analyze morphological transitions i nduced by various in vitro incubations. Our images show that Golgi cis ternae have two distinct surface coatings with different distributions and apparent functions. The first type, probably a coatomer coat, con sists of tightly packed approximately 10 nm surface particles. These a re localized exclusively to sites of membrane budding and are as intim ately involved in bud formation as clathrin or caveolin coats. When th is coating is exaggerated by GTPgammaS, the periphery of all cisternae is partitioned into coated vesicles that remain attached at their sit es of formation. A second, much finer coating is evenly distributed ov er the periphery of cisternae, including tubules enclosing the fenestr ae. It appears to stabilize the membrane curvature associated with tub ules and edges. These different coatings must be considered in further attempts to unravel Golgi membrane trafficking mechanisms.