Arrivals and departures at the plasma membrane: direct and indirect transport routes

Studies carried out during the last 2 decades have dramatically increased our knowledge of the pathways and mechanisms of intracellular membrane traffic, most recently due to the developments in light microscopy and in vivo imaging of fluorescent fusion proteins. These studies have also revealed that certain molecules do not behave according to the classical transportation rules first documented in cell biology textbooks in the 1980s and 1990s. Initially, unconventional mechanisms of secretion that do not involve passage of cargo through the stacked Golgi cisternae were thought to confer on cells the ability to discard excess amounts of protein products. With time, however, more physiological mechanisms and roles have been proposed for an increasing number of secretory processes that bypass the Golgi apparatus.