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 visualize Golgi cisternal structure and analyze morphological transitions induced by various in vitro incubations. Our images show that Golgi cisternae have two distinct surface coatings with different distributions and apparent functions. The first type, probably a coatomer coat, consists of tightly packed approximately 10 nm surface particles. These are localized exclusively to sites of membrane budding and are as intimately involved in bud formation as clathrin or caveolin coats. When this coating is exaggerated by GTP gamma S, the periphery of all cisternae is partitioned into coated vesicles that remain attached at their sites of formation. A second, much finer coating is evenly distributed over the periphery of cisternae, including tubules enclosing the fenestrae. It appears to stabilize the membrane curvature associated with tubules and edges. These different coatings must be considered in further attempts to unravel Golgi membrane trafficking mechanisms.