Fig. 1.

Sketch of the Golgi apparatus as a polarized stack of connected cisternae exchanging material. (A) Proteins synthesized in the ER go through the ER-Golgi intermediate compartment (ERGIC) before entering the Golgi through its cis face. After biochemical maturation and sorting, they exit the Golgi through the trans face to join the trans-Golgi network (TGN). (B) Relevant transport processes, including cisternal progression (translation), diffusion through connecting membrane tubules, vesicular transport, and exit. (C) Spatiotemporal evolution of an initially narrow protein distribution (as produced by a pulse of secretion from the ER); pure convection produces a uniform translation of the peak (dashed line), diffusion broadens the peak, and exit exponentially decreases the protein content.