Lecture 10: ER/ Golgi Complex and vesicle transport Flashcards
Oligosaccharide
Less than 15 monosaccharide
Glycosylation in the rough ER (4)
- Majority of proteins produced at the RER become glycosylated into glycoproteins
- Carbohydrate groups have roles as binding sites
- Aid in proper folding and stabilization
- Sorting/directing proteins to different cellular compartments
N-linked glycosylation (2)
What it is+where it is initiated
- Likage to asparagine (asn)
- Is initiated in the RER
O-linked Glycosylation
- Linkage to serine or threonine
- Occurs in the Golgi complex
N-linked glycosylation in the RER steps (7):
- First 7 sugars are transferred one at a time to a lipid: dolichol pyrophosphate, embedded in the ER membrane
- Initially assembly is on the cytosolic side
- Sugars are added by glycosyltranferases
- Dolichol and attached oligosaccharide is flipped across the membrane
- Remaining sugars are attached to doliochol on the cytosolic side: flipped across the membrane and attached to the growing oligosaccharide chain
- Completed oligosaccharide is transfered to an asparagine residue of the polypeptide being translated
- Transfer by the enzyme oligosaccharyltransferase to an Asn within the sequence: x–x-Asn-X-ser/thr
After N-linked glycosylation in the rough ER is the protein for the Golgi?
No, it needs to be checked for misfolded proteins
Quality control for misfolded proteins steps (7):
- Glucosidase 1 and 2 removes 2 glucoses
- Glycoprotein with one glucose is recognized by calnexin (chaperone protein in the ER)
- Calnexin removes another glucose
- Incompletely folded proteins are recognized by UGGT (a conformation sensing enzyme): detects exposed hydrophobic residues. Adds glucose molecule.
- rechecks by calnexin if UGGT adds glucose
- Properly folded proteins exit to Golgi
- Improperly folded proteins are degraded in a proteosome in the cytosol
Exiting the ER steps:
- Membrane vesicles with enclosed cargo bud from the ER
- Transport vesicles fuse with one another to form larger vesicles called vesicular tubular carriers (VTCs) in a region called the ERGIC
Golgi complex is composed of cisternae arranged in a stack. Distinct compartments arranged from —– (closest to the ER) to the —— (exit, furthest from the ER)
- cisface
- transface
Trans Golgi network
- network of tubules and vesicles
- Sorting station where proteins are segregated into different types of vesicles (heading to the plasma membrane or others)
Cis Golgi Network (CGN) (2)
- Interconnected network of tubules
- Sorting station that distinguishes between proteins that need to be returned to the ER and those that should proceed through the Golgi
Protein modification with the Golgi complex (4)
- Newly synthesized proteins leaving the ER are sequentially modified (ex: modification of N-linked carbohydrate chains)
- Order that sugars are incooporated depends on the location of specific glycosyltransferase in the membrane of the Golgi complex
- Glycosylation in the Golgi complex can be quite varied
- O-linked carbohydrates are entirely assembled within the Golgi
Vesicular transport model (3)
- Golgi Cisternae are stable compartments
- Vesicles carrying cargo bud from one compartment and fuse with the next
- Evidence: Lots of vesicles bud from the edges of Golgi cisternae and Golgi cisternae have differnt enzymes
Cisternal Maturation Model (2)
-Cisternae form at the cis face and nove towards the trans face “maturing” as they move
- As a new cis cisterna is formed it traverses the Golgi stack, changing as it matures by accumulating medial, then trans enzymes through vesicles that move from later to earlier cisternae (retrograde traffic).
- Evidence: Drugs blocking vesicle formation at the ER leads to the golgi complex disappearing
Anterograde
From cis to trans
