Vesicle docking in regulated exocytosis

In electron micrographs, many secretory and synaptic vesicles are found 'docked' at the target membrane, but it is unclear why and how. It is generally assumed that docking is a necessary first step in the secretory pathway before vesicles can acquire fusion competence (through 'priming'), but recent studies challenge this. New biophysical methods have become available to detect how vesicles are tethered at the target membrane, and genetic manipulations have implicated many genes in tethering, docking and priming. However, these studies have not yet led to consistent working models for these steps. In this study, we review recent attempts to characterize these early steps and the cellular factors to orchestrate them. We discuss whether assays for docking, tethering and priming report on the same phenomena and whether all vesicles necessarily follow the same linear docking-priming-fusion pathway. We conclude that most evidence to date is consistent with such a linear pathway assuming several refinements that imply that some vesicles can be nonfunctionally docked ('dead-end' docking) or, conversely, that the linear pathway can be greatly accelerated (crash fusion).