17.17
Transport Across the Golgi
While it is unclear how molecules move between adjacent Golgi cisternae, it is apparent that the molecules move from cis- cisterna, the entry face, to…
Two widely-used hypotheses describing the transport of proteins across Golgi cisternae are the cisternal maturation model and the vesicular transport model.
The cisternal maturation model proposes that the Golgi cisternae are dynamic structures where the vesicular tubular cluster or VTC arrives from the ER to become cis-cisterna.
As more VTCs get added, the older cisterna with its cargo moves through the Golgi stack to become medial-cisterna and eventually, trans-cisterna.
The enzyme composition of the cisterna changes as it moves through the stack and matures.
As coated vesicles bud off the trans-cisternae, this network gets replaced by maturing cisternae just behind it.
The vesicle transport model proposes that the Golgi cisternae are stable structures with unchanging enzyme composition.
A cargo-carrying vesicle from the ER fuses with the cis Golgi.
A COPI vesicle buds off from one cisterna and fuses with the next shuttling the cargo through the Golgi stack to the trans face.
From here, the cargo is released in a vesicle targeted to its final destination.
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Q1: What are the two main models explaining protein 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.
Q2: How does the cisternal maturation model explain Golgi cisterna progression?
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.
Q3: Why did the collagen rod experiment challenge the vesicular transport model?
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.
Q4: What role do COPI-coated vesicles play in Golgi transport?
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.
Q5: How do the two Golgi transport models work together in cells?
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.
Q6: What determines the order of protein modifications in the Golgi?
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.
Q7: How does enzyme composition change as cisternae mature in the Golgi?
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.