Technological breakthroughs of the last decade have advanced our understanding of vesicular trafficking from a largely descriptive approach to a molecular science. An impressive conservation of the molecular transport machinery across phyla has been revealed (30). Although the current consensus is that intracellular protein transport is mediated by transport vesicles, evidence suggestive of nonvesicular transport mechanisms continues to accumulate. As a consequence, two formerly rejected models, namely transport by cisternal progression (13) and traffic via membranous tubules (24, 37), are attracting renewed interest. Here, we provide updated versions of these models, review both new and old evidence relevant to their application to transport pathways, and discuss the implications as well as the difficulties posed by these alternative mechanisms.
The model of transport by cisternal progression has been largely based on morphological observations of scale-covered green algae. The best characterized case is...
