a, In the cisternal maturation mechanism, vesicles derived from the endoplasmic reticulum fuse to form the cis cisternae of the Golgi (green). This cisterna then transforms into the medial cisterna (outlined in red) by a maturation process, possibly via an intermediate (yellow). Finally, the medial cisterna matures into the trans face of the Golgi (blue), to be consumed during secretory traffic to the cell surface. Sorting proteins for different destinations is accomplished by dividing cargo into different regions of the trans compartment of the Golgi before it fragments to generate vesicles for different destinations. COPI vesicles would retrieve and recycle resident Golgi proteins at each step. b, In the vesicle-shuttle mechanism, the cis, medial and trans cisternae of the Golgi remain in place, without maturing, and COPI vesicles deliver cargo between cisternae. At the trans compartment, vesicles containing different secretory cargoes bud off to deliver their contents to distinct destinations.