Visualization of secretory cargo transport within the Golgi apparatus.

To describe trafficking of secretory cargo within the Golgi apparatus, the cisternal maturation model predicts that Golgi cisternae change their properties from cis to trans while cargo remains in the cisternae. Cisternal change has been demonstrated in living yeast Saccharomyces cerevisiae; however, the behavior of cargo has yet to be examined directly. In this study, we conducted simultaneous three-color and four-dimensional visualization of secretory transmembrane cargo together with early and late Golgi resident proteins. We show that cargo stays in a Golgi cisterna during maturation from cis-Golgi to trans-Golgi and further to the trans-Golgi network (TGN), which involves dynamic mixing and segregation of two zones of the earlier and later Golgi resident proteins. The location of cargo changes from the early to the late zone within the cisterna during the progression of maturation. In addition, cargo shows an interesting behavior during the maturation to the TGN. After most cargo has reached the TGN zone, a small amount of cargo frequently reappears in the earlier zone.