Figure 5.

Stable compartments as cisternal progenitors. Secretory cargoes travel from the ER to the intermediate compartment and cis-Golgi in dissociative carriers. Golgi compartments are stable and biochemically distinct, but can segregate into domains by a Rab conversion process. A domain in a cis-cisterna undergoes Rab conversion and acquires medial character, resulting in a transient “homotypic” fusion with a medial cisterna in an adjacent stack. Small and large secretory cargoes can move forward through the resulting connection. Similar domain fusion events in later compartments enable secretory cargoes to traverse the entire stack.