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The cover shows an electron micrograph of the Golgi apparatus in a BHK cell labeled with antibodies against the cargo protein, VSVG, from the ts045 mutant strain of vesicular stomatitis virus. The picture shows VSVG exiting the Golgi within a pleimorphic transport intermediate, termed Golgi-to-plasma membrane carrier (GPC). When observed in living cells, GPCs have been shown to move out of the Golgi along microtubule tracks and to deliver cargo proteins to the cell surface (Hirschberg et al., (1998), J. Cell Biol., 143, 1485-1503; Toomre et al., (1999), J. Cell Sci., 112, 21-33). They exhibit variable tubular-reticular-saccular shapes and sizes (from 0.3 to 10.0 or more micron in length; Hirschberg et al., (1998), J. Cell Biol., 143, 1485-1503; Toomre et al., (1999), J. Cell Sci., 112, 21-33; Polishchuk et al., (2000), J. Cell Biol., 148, 45-58) and are therefore very different from the small "traditional" transport vesicles with diameters of 60-90 nm. A crucial question is how such elaborate structures form and emerge from the Golgi apparatus.

Two main schemes have been proposed to explain the formation of GPCs. In one scheme, they form by direct protrusion and subsequent fission of complex tubular-reticular membranes at the trans-Golgi network (TGN). In a second scheme, they form by budding and homotypic fusion of small vesicles at the TGN, similar to the proposed formation of pre-Golgi transport carriers at endoplasmic reticulum export sites (Bannykh and Balch, (1997), Trends Cell Biology, 8, 21-25). The study by Polishchuk et al. (in this issue, pages 4470-4485) supports the first scheme by providing evidence from correlative-EM experiments that GPCs protrude out of specialized regions of the TGN and then undergo en bloc cleavage.

-Roman Polishchuk and Alberto Luini