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Cover  Our understanding of membrane trafficking through the Golgi complex has been evolving over the past two decades. Cargo proteins were once thought to move exclusively through the Golgi via 50- to 60-nm vesicles, dissociating from a donor cisterna and fusing with the next. More recently, it has been recognized that, for cargo that polymerizes into large aggregates (e.g., casein micelles, algae scales, and procollagen), alternative pathways for cargo transport through the Golgi complex must be considered. Such alternative pathways include cisternal progression and direct transport of cargo by tubule interconnections between stacks (Mironov et al., J. Cell Biol. 138, 481Ð484). The images on the cover show a Golgi complex in human fibroblasts that is transporting procollagen molecules, which are too large to fit into small intra-Golgi transport vesicles. Procollagen-I folds inside the endoplasmic reticulum into rigid 300-nm long rod-like triple helices, which, after transport into the Golgi complex, assemble into large electron-dense aggregates. Such aggregates traverse the Golgi stack in about 20 min without ever leaving the lumen of Golgi cisternae in small carrier vesicles or in large dissociative carriers (also called "mega-vesicles" [Volchuk et al., Cell, 102, 335Ð348]). This was demonstrated by electronic microscopy (Bonfanti et al., Cell, 95, 993Ð1003), using specific reversible inhibitors of procollagen folding to synchronize the intracellular traffic of procollagen aggregates. The cover images show the distribution of procollagen labeled with horseradish peroxidase in the Golgi after such synchronization. The Golgi complex was examined by electron microscopy after ultrafast cryofixation, which eliminates ultrastructural artifacts typical of other fixation methods. Note in the 3-D reconstruction made by Alvar Trucco that cisternal distensions containing procollagen aggregates (red) are always connected with cisternae (yellow). This implies that procollagen traverses the Golgi stack without leaving cisternal lumen.—Roman Polishchuck
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