Transport Across the Golgi

The cisternal maturation model proposes that Golgi cisternae are dynamic structures that mature and move through the stack as new vesicles arrive from the ER. The vesicular transport model describes the Golgi as a static structure where COPI coated vesicles bud from one cisterna and fuse with the next, shuttling cargo forward. Both models may operate simultaneously in cells.

Vesicles from the ER fuse to form a vesicular tubular cluster that matures into a cis-cisterna. As new cisternae form behind it, the older cisterna progressively matures into a medial cisterna and then trans-cisterna. The enzyme composition of each cisterna changes as it moves through the stack, with COPI-coated vesicles recycling enzymes backward to maintain proper function.

Collagen molecules are too large to fit inside classical vesicles used for transport between cisternae. This observation contradicted the vesicular transport model's assumption that all cargo moves through small, membrane-bound vesicles. The finding prompted scientists to propose the cisternal maturation model as an alternative mechanism for moving large proteins through the Golgi.

COPI-coated vesicles bud from cisternae and transport cargo forward to the next cisterna in the vesicular transport model. They also recycle resident enzymes backward to newly formed cisternae in the cisternal maturation model. This bidirectional movement ensures proper enzyme distribution and cargo progression through the Golgi stack.

Current evidence suggests that Golgi transport involves a combination of both mechanisms. A stable core of long-lasting cisternae may exist in the center, while some cargo moves rapidly through vesicular transport and other cargo moves slowly through cisternal maturation. This hybrid approach explains how cells handle both small and large proteins efficiently.

Proteins move from the cis-cisterna entry face to the trans-cisterna exit face in a specific sequence. Each cisterna contains unique enzymes that modify substrates as cargo passes through. The order of cisternae and their distinct enzyme compositions ensure proteins receive modifications in the correct sequence for proper function.

As cisternae mature and move through the Golgi stack, their enzyme composition changes to reflect their position and function. COPI-coated vesicles bud from mature cisternae carrying enzymes backward to newly formed cisternae where those enzymes are needed. This recycling mechanism ensures each cisterna maintains appropriate enzymes for its maturation stage.