Protein Transport Between Organelles Flashcards
Define ERGIC.
Endoplasmic reticulum Golgi Intermediate compartment. After secretory proteins bud from the ER membrane, these transport vesicles fuse with one another to form larger vesicles and interconnected tubules in the region between the ER and Golgi. This region is called the ERGIC.
Defines VTCs.
VTCs are the vesicular-tubular carriers that form in the ERGIC.
Define CGN and describe its role in the Golgi.
The CGN is an interconnected network of tubules known as the cis Golgi network. It functions primarily as a sorting station that distinguishes between proteins to be shipped back to the ER and those to proceed to the next Golgi station.
Define TGN and describe its role in the Golgi.
The distinct network of tubules and vesicles on the trans-most face of the organelle contains a distinct network of tubules and vesicles called the trans Golgi network. The TGN is a sorting station where proteins are segregated into different types of vesicles heading either to the plasma membrane or to various intracellular destinations.
What happens as newly synthesized soluble and membrane glycoproteins pass through the cis and medial cisternae of the Golgi stack?
Most of the mannose residues are removed from the core oligosaccharides and other sugars are transferred sequentially by various glycosyltransferases.
How does glycosylation within the Golgi differ from glycosylation within the ER?
Glycosylation in the ER produces a single core oligosaccharide. Glycosylation in the Golgi is more varied, producing diverse carbohydrate domains. N-linked oligosaccharides are usually synthesized in the ER and O-linked oligosaccharides in the Golgi.
Describe the cisternal maturation model of the Golgi.
This model says that Golgi cisternae are transient structures. Cisternae form at the cis face by fusion of membranous carriers from the ER and ERGIC and each cisterna physically moves from the cis to the trans end of the stack, changing in composition as it progresses.
Describe the vesicular transport model of the Golgi.
This model says that the cisternae of the Golgi remain in place as stable compartments. Cargo (secretory, lysosomal, and membrane proteins) is shuttled through the Golgi stack, from the CGN to the TGN in vesicles that bud from one membrane compartment and fuse with a neighboring compartment farther along the stack.
What are the two functions of protein coats on coated vesicles?
- They act as a mechanical device that causes the membrane to curve and form a budding vesicle.
- They provide a mechanism for selecting the components to be carried by the vesicle. These components might be cargo proteins or the machinery required to target the vesicle to the correct acceptor membrane, for example.
How does the structure of a vesicle coat help it perform its function?
The vesicle coat is composed of two distinct protein layers: an outer cage or scaffolding that forms the framework for the coat and an inner layer of adaptors that binds both to the outer surface of the lipid bilayer and the membrane’s cargo. The adaptors are able to select specific cargo molecules by virtue of their specific affinity for the cytosolic “tails” of integral proteins that reside in the donor membrane.
What is the role of COPII-coated vesicles?
They move materials from the ER forward to the ERGIC and Golgi.
What is the role of COPI-coated vesicles?
They move materials in a retrograde direction 1) from the ERGIC and Golgi stack backward toward the ER and 2) from trans Golgi cisternae backward to cis Golgi cisternae.
What is the role of clathrin-coated vesicles?
They move materials from the TGN to endosomes, lysosomes, and plant vacuoles. They also move materials from the plasma membrane to cytoplasmic compartments along the endocytic pathway. They have also been implicated in trafficking from endosomes and lysosomes.
How are proteins maintained in an organelle (kept distinct)?
- Retention of resident molecules that are excluded from transport vesicles. Retention may be based primarily on the physical properties of the protein. For example, soluble proteins that are part of large complexes or membrane proteins with short transmembrane domains are not likely to enter a transport vesicle
- Retrieval of “escaped” molecules back to the compartment in which they normally reside.
How are “escaped” molecules in the ERGIC returned to the ER?
Specific receptors capture the molecules and return them to the ER in COPI-coated vesicles. Soluble resident proteins of the ER lumen typically possess the retrieval signal (KDEL).
What happens to lysosomal proteins once they are synthesized in the ER?
They are carried to the Golgi complex, and once in the cisternae, soluble lysosomal enzymes are specifically recognized by enzymes that catalyze the two-step addition of a phosphate group to certain mannose sugars of the N-linked carbohydrate chain.
Describe the structure of clathrin-coated vesicles.
The coats of the vesicles contain:
1. An outer honeycomb-like lattice composed of the protein clathrin, which forms a structural scaffold.
2. An inner shell composed of protein adaptors, which covers the surface of the vesicle membrane that faces the cytosol.
What role do SNAREs play in vesicle fusion?
SNAREs play an important role in bringing the membranes of the vesicle and target compartment into close contact with one another as a result of the interaction between the cytosolic regions of integral proteins of the two membranes.
