Golgi Apparatur Flashcards
Define the Golgi apparatus and describe its structure.
Golgi apparatus: cis-face with cis-Golgi network -> cis- medial- and trans- cisternae (Golgi stack) -> trans-face with trans-Golgi network all interconnected with microtubules. Microtubules rearrange based on their polarization pattern. Cis-Golgi interconnects with ER.
1 Golgi stacks= 4-6 cisternae, more than 1 cisterna per category (e. g. cis=2 cisternae)
Proteins and lipids enter the cis-Golgi network in vesicular tubular clusters arriving from the ER and exit from the trans Golgi network bound for the cell surface or another compartment. During their passage through the Golgi apparatus, transported molecules undergo an ordered series of covalent modifications.
What protein modifications follow the ER N-linked oligosacharide attachment and trimming?
Further modifications and additions occur in the Golgi apparatus, depending on the protein. The outcome is that two broad classes of N-linked oligosaccharides.
- Complex oligosaccharides are generated by a combination of trimming the original N linked oligosaccharide added in the ER and the addition of further sugars
- High mannose oligosaccharides have no new sugars added to them in the Golgi apparatus. They contain just two N acetylglucosamines and many mannose residues , often approaching the number originally present in the lipid linked oligosaccharide precursor added in the ER
Define complex oligosaccharides and high mannose oligosaccharides attached in the Golgi.
They share a common core region derived from
the original N linked oligosaccharide added in the ER =
2 N-acetylglucoseamine and 3 mannoses
Each complex oligosaccharide consists of a core region together with a terminal region that contains a variable number of copies of a special trisaccharide unit (N-acetylglucosamine galactose sialic acid) linked to the core mannoses.
High mannose oligosaccharides are not trimmed back all the way to the core region and contain additional mannose residues.
Describe the proces of glycosilation in the ER and Golgi.
Processing begins in the ER with the removal of the glucoses from the oligosaccharide initially transferred to the protein. Then a mannosidase in the ER membrane removes a specific mannose.
The remaining steps occur in the Golgi stack, where Golgi mannosidase I first removes three more mannoses and N-acetylglucosamine transferase I then adds an N-acetylglucosamine , which enables mannosidase II to remove two additional mannoses. At this stage, the bond between the two N acetylglucosamines in the core becomes resistant to attack by a highly specific endoglycosidase (Endo Since all later structures in the pathway are also Endo H resistant, treatment with this enzyme is widely used to distinguish complex from high mannose oligosaccharides. Finally, additional N-acetylglucosamines, galactoses, and sialic acids are added. These final steps in the synthesis of a complex oligosaccharide occur in the cisternal compartments of the Golgi apparatus.
What is the purpouse of glycosilation?
- aids folding
- aids transport
- greater resistance to proteases
- cell-cell adhesion processes, oligosacharides are recognized by the transmembrane lectins (selectins)
- signaling through the cell surface signaling receptor (Notch, development)
Do prokaryotes and yeast have N-linked glycosilation?
prokaryots - no
yeast - yes
What is the function of each Golgi apparatus segment and how do the proteins more downstream?
- ER
- CGN - sorting, phosphorylation of oligosacharides on lysosomal proteins
- cis-cisternae - Man removal
- medial-cisternae - Man removal and ClcNac addition
- trans-cisternae - Gal addition and NANA addition
- TGN - sorting sulfation of tyrosines and carbohydrates
- Lysosomes, PM, secretory vesicles
The lumen of the trans compartment is thought to be continuous with the trans Golgi network, where proteins are segregated into different transport packages and dispatched to their final destination.
It is likely that each processing enzyme is not completely restricted to a particular cisterna but that its distribution is graded across the stack such that
-early acting enzymes are present mostly in the cis Golgi
cisternae
-later acting enzymes are mostly in the trans Golgi cisternae
Each oligosaccharide processing enzyme can accept a glycoprotein as a substrate only after it has been properly processed by the preceding enzyme.
What are the Two possible models explaining the organization of the Golgi apparatus and the transport of proteins from one cisterna to the next?
1) Vesicular transport model: Golgi apparatus is static and forward moving transport vesicles in a cis-trans direction, carried by transport vesicles, retrograde flow retrieves escaped ER and Golgi proteins and returns them to preceding compartments.
Although both types of vesicles are likely to be COP I coated, the coats may contain different adaptor proteins to confer selectivity on the packaging of cargo molecules.
2) Cisternal maturation model: , each Golgi cisterna matures as it migrates outwards through a stack, at each stage, the Golgi resident proteins that are carried forward in a cisterna are moved backward to an earlier compartment in COP I
coated vesicles.
What proteins organize the Golgi apparatus structure?
Microtubule cytoskeleton and cytoplasmic Golgi matrix proteins.
How is the Golgi apparatus redistributed upon cell division?
When the cell prepares to divide , mitotic protein kinases phosphorylate the Golgi matrix proteins , causing the Golgi apparatus to fragment and disperse throughout the cytosol. During disassembly, Golgi enzymes are returned in vesicles to the ER, while other Golgi fragments are distributed to the two daughter cells. There, the matrix proteins are dephosphorylated, leading to the reassembly of the Golgi apparatus. The Golgi matrix proteins can assemble into appropriately localized stacks near the centrosome even when Golgi membrane proteins are experimentally prevented from leaving the ER . This observation
suggests that the matrix proteins are largely responsible for both the structure and location of the Golgi apparatus.
