Cisternal Maturation Model, Allen and Balch 1999 Science copy

Cisternal Maturation Model, Allen and Balch 1999 Science copy

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REVIEW: CELL BIOLOGY Protein Sorting by Directed Maturation of Golgi Compartments Bernard B. Allan and William E. Balch* How does the Golgi stack mediate transport of cargo from the endoplasmic reticulum (ER) to the cell surface? A possibility is that cargo-containing vesicles derived from the ER form early Golgi compartments that then mature by retrieval of processing enzymes from later Golgi compartments. Maturation continues at terminal Golgi compartments by retrieval of transport components from the endocytic pathway to promote sorting of cargo to multiple cellular destinations. Hence, retrograde move- ment may integrate exocytic and endocytic pathways in eukaryotic cells and coordi- nate membrane flow and cargo transport through the Golgi stack. T he Golgi exists as a stack of function- ally polarized cisternae in mammalian cells (Fig. 1). Cargo exported from the ER by vesicle carriers first appears in pre- Golgi intermediates that move on microtu- bule tracks to the cis Golgi region. As cargo moves through the stack, it is modified by Golgi-associated processing enzymes. Final- ly, the trans side of the Golgi stack serves as a key sorting station, directing cargo to mul- tiple intracellular and extracellular destina- tions. The challenge to understanding Golgi function has been relating the morphological organization of the stack to the function of vesicle carriers, whose role in cargo move- ment has been controversial. We highlight results that suggest that vesicles recycle pro- cessing enzymes and post-Golgi sorting de- terminants to promote progressive maturation of ER-derived intermediates to form func- tional Golgi compartments. We illustrate how this process, referred to as directed matura- tion, uses the primary activity of recycling to integrate membrane flow through the exo- cytic pathway to achieve normal growth in development. The Transport Machinery Movement of cargo between exocytic and endocytic compartments requires transiently coated vesicle carriers ( 1 ). The term “vesicle” encompasses any membrane-enclosed struc- ture into which cargo is segregated by a mem- brane fission event. Fission establishes a new boundary to distinguish one compartment from the next. Biosynthetic cargo exiting the ER includes newly synthesized proteins and lipids that are moved to distinct cellular and extracellular destinations. Other cargo incor- porated into vesicles includes proteins that are continuously recycled between compart- ments. These components encompass the transport machinery involved in cargo selec- tion, vesicle formation, and targeting and fu- sion of vesicles. A fundamental principle of membrane traffic is that vesicle formation is initiated by the selection and concentration of cargo. This occurs through interactions between sorting determinants on the cargo and cytosolic coat components that direct cargo to the forming vesicle ( 2 ). Soluble cargo (cargo found in the lumen of the compartment) will necessarily require sorting receptors to couple the protein to the cytosolic coat machinery. A variety of
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Cisternal Maturation Model, Allen and Balch 1999 Science copy

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