Golgi and Vesicular Transport (Lec. 13) Flashcards
Relate the structure of the Golgi apparatus to its function. (12.2)
The Golgi is a stack of sacs called cisternae, which are not continuous and don’t contain ribosomes. Cisternae are usually swollen at the margins, where vesicles arise and fuse. The Golgi has polarity (the sides of it are different from each other. The cis face receives from the ERGIC, the medial cisternae are in the middle of the stack, the trans face faces away from the ER towards the cell surface, and the Trans-Golgi network receives from the trans face.
Describe the types of protein glycosylation that take place in the Golgi.
Secreted and plasma membrane proteins have glucose residues that are removed, but lysosomal proteins do not have their residues removed. Addition of single sugar residues can also happen in the Golgi.
Summarize the role of the Golgi in synthesis of membrane lipids.
The Golgi functions in the synthesis of sphingomyelin and glycolipids.
Explain the role of coat proteins
Assembly of coat proteins drives the budding of vesicles containing selected cargo proteins from the donor membrane. The coats are removed at the target membrane, allowing the membranes to fuse.
Describe the mechanism by which vesicles fuse with the correct target membranes.
The initial interaction between vesicles and target membranes is mediated by tethering factors and Rab (small GTP-binding) proteins. Rab proteins in the active GTP state bind to the tethering factors, providing the initial bridge. This is followed by the formation of complexes between specific pairs of transmembrane proteins called SNAREs on the vesicle and target membrane.
What is the Golgi complex?
An assembly line for modifying and packaging lumenal and transmembrane proteins
What are the three functions of the Golgi complex?
- Protein modification (medial and trans compartments): removing, adding, and/or modifying the carbohydrate and lipid compartments; covalently coupling protein and lipid components attached to membrane/secretory proteins; some proteins polymerize in the Golgi.
- Lipid metabolism (medial and trans compartments): converting ceramide (a base sphingolipid) into sphingomyelin or a glycolipid.
- Sorting proteins (trans Golgi network): for delivery to their final cellular destination
Describe the modification of glycoproteins in the Golgi.
Polysaccharides were added to proteins in the ER, and in the Golgi they will be modified to result in a variety of glycoproteins serving specific functions (depend on protein structure and the enzymes available in each cell type).
Give an example of glycoproteins made in the Golgi
Proteoglycans
What are the two different mechanisms of organization of the Golgi?
- Cisternal maturational model: the entire stacks will move from cis to trans, maturing their proteins as they move.
- Stable cisternae model: the stacks stay put, but send proteins from one stack to the next as the proteins mature.
What is the function of vesicles? How are they formed?
They are “pinched” off of cell membranes (plasma membrane, ER, Golgi, endosome, lysosome, but NOT mitochondria or chloroplasts), and they move cargo and membrane-bound proteins between compartments within the cell.
Why is vesicular transport important?
vesicles must recognize and fuse only with the appropriate membrane; selectivity is key to maintaining the functional organization of the cell
What are the three different types of coated vesicles?
- COP II-coated vesicles carry proteins from the ER to the ERGIC and to the Golgi apparatus.
- COP I-coated vesicles bud from the ERGIC or Golgi and carry their cargo back, returning proteins to earlier compartments.
- Clathrin-coated vesicles transport in both directions between the trans Golgi network, endosomes, lysosomes, and the plasma membrane.
What happens to the coats on the way to their destination?
Coats are removed in the cytosol before the vesicle reaches its target. The coats are recycled. The vesicles fuse with the target membrane and empty their cargo.
What is clathrin?
Has a tri-skeleton shape base unit that induces curvature of the plasma membrane when it polymerizes. It creates a small sphere, and when dynamin pinches it off the original membrane, it unbinds from the vesicle.
What does dynamin do?
When the vesicle coming out of a membrane is spherical and ready to bud, dynamin comes in and pinches it off the stalk that holds it to the original membrane.
What regulates the formation of coated vesicles?
ARF is a small GTP-binding molecule (active when bound to GTP, inactive when bound to GDP). When active, ARF recruits adaptor proteins that mediate vesicle assembly by interacting with cargo and coat proteins.
What are the two steps of vesicle fusion?
- The vesicle must recognize the correct target membrane.
- Vesicle and target membrane must fuse, delivering the contents to the target organelle.
What are the two stages of recognition in vesicle docking?
- loose V-SNARE, T-SNARE, Rab, and tether interaction.
- Rab activation of SNAREs and tight V/T SNARE interaction. Rab helps V and T snares associate with their specific counterpart when they reach the appropriate destination. The SNARE proteins form a natural coil that twists and pulls the membranes tight until the lipids intermix.
How do the different coatings on vesicles signal for destinations?
If the protein signals for COP I coating, the vesicle will be sent back to the ER. If the protein signals for COP II coating, the vesicle will typically stay within the Golgi network. If the protein signals for clathrin coating, the vesicle will go to the cell membrane, lysosomes, or endosomes.
What are the two different pathways of vesicle function?
- Constitutive pathway: default pathway in most cells; if a protein has a signal sequence it enters the ER and will eventually be secreted; vesicles will bud off the Golgi, travel to the membrane, and fuse.
- Regulated pathway: responding to a specific regulated signal (often involving Ca2+ changes); the secretory vesicles fuse with the plasma membrane and release their contents; mechanism of neurotransmitter release.
