17.10
Vesicular Tubular Clusters
After budding out from the ER membrane, some COPII vesicles lose their coat and fuse with one another to form larger vesicles and interconnected tubul…
COPII vesicles bud from specialized regions in the ER membrane that lack bound ribosomes. These regions are called ER exit sites and are dispersed throughout the ER membrane.
After budding, some transport vesicles shed their coat and fuse with one another to form larger vesicles. Fusion between identical membrane types is called homotypic fusion.
In contrast, heterotypic fusion occurs when non-identical membranes fuse, such as a Golgi-derived vesicle fusing into the ER membrane.
In homotypic fusion, N-ethylmaleimide sensitive factor or NSF proteins force t-SNAREs and v-SNAREs apart from each other. Complementary SNAREs on the adjacent vesicle interact, and the identical membranes fuse.
Such fusion between COPII vesicles forms an extended membranous structure called a vesicular tubular cluster or VTC.
This occurs in a region between the ER and Golgi, called the ER-Golgi intermediate compartment or ERGIC.
The VTC moves along microtubule tracks, carrying ER proteins that are not fully folded and functional, cargo receptors, and SNAREs.
As it moves, new COPI vesicles spontaneously bud off from the tubular clusters. The mechanism of switching the coat protein is yet unknown.
These new transport vesicles carry escaped cargo involved in the ER budding and vesicle fusion back to the ER in a process called the ER retrieval pathway.
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Q1: What are vesicular tubular clusters and where do they form?
Vesicular tubular clusters (VTCs) are larger membranous structures formed when COPII vesicles shed their coat and fuse with one another through homotypic fusion. They form in the ER-Golgi intermediate compartment (ERGIC), a region between the ER and Golgi apparatus. VTCs serve as mobile cargo transport systems that carry ER proteins and cargo receptors toward the Golgi.
Q2: How do COPII vesicles fuse to form vesicular tubular clusters?
COPII vesicles fuse through homotypic fusion, where identical membranes combine. NSF proteins force t-SNAREs and v-SNAREs apart, allowing complementary SNAREs on adjacent vesicles to interact. This interaction triggers membrane fusion between the vesicles, creating the extended membranous structure characteristic of VTCs.
Q3: What cargo do vesicular tubular clusters transport?
VTCs transport ER proteins that are not yet fully folded and functional, cargo receptors, and SNAREs. These components move along microtubule tracks toward the Golgi apparatus. The VTC gradually changes composition as it moves, eventually maturing into the cis Golgi network upon arrival at the Golgi.
Q4: How do vesicular tubular clusters move through the cell?
VTCs move along microtubule tracks with the help of motor proteins such as kinesin and dynein. These motor proteins facilitate transport of the VTC from the ER-Golgi interface toward the Golgi apparatus. As the VTC moves, COPI vesicles spontaneously bud off to retrieve escaped cargo through the ER retrieval pathway.
Q5: What is the difference between homotypic and heterotypic fusion?
Homotypic fusion occurs when identical membrane types fuse together, as when COPII vesicles combine to form VTCs. Heterotypic fusion involves non-identical membranes fusing, such as a Golgi-derived vesicle fusing into the ER membrane. Both fusion types require SNARE proteins and NSF to facilitate membrane merging.
Q6: What happens to COPI vesicles that bud from vesicular tubular clusters?
COPI vesicles spontaneously bud off from VTCs as they move along microtubule tracks. These transport vesicles carry escaped ER cargo proteins that need to be returned to the ER. This recycling process is called the ER retrieval pathway and helps maintain proper protein distribution between the ER and Golgi.
Q7: How do vesicular tubular clusters relate to Golgi maturation?
According to the cisternal maturation model, VTCs arriving from the ER become the cis Golgi network. As they continue moving and changing composition, they mature into medial cisterna and eventually trans-cisterna. This progressive transformation represents a key mechanism of protein transport across the Golgi apparatus.