Lecture 3 - Exam 4: The Endoplasmic Reticulum & Golgi - Protein Sorting - Exam 4 Flashcards
Is the Golgi apparatus a nonpolar organelle?
NO! It is a polarized organelle
What are the main roles of the Golgi apparatus?
- It is a factory in which proteins received from the ER are further processed and sorted for transport to their eventual destination: secretion, endosomes, lysosomes
- Synthesis of glycolipids and sphingomyelin
What is the structure of the Golgi appartus?
It is composed of flattened membrane enclosed sacs (cisternae) and associated vesicles.
What does the Golgi apparatus serve as in the plants?
As the site in which complex polysaccharides of the cell wall are synthesized.
What are the four (functionally) compartments of the Golgi apparatus?
The cis, medial, and trans compartments and the trans-Golgi network.
Describe the cis face. Describe the medical and trans compartments.
Cis: Proteins coming from the ER enter the cis face.
Medial & Trans: Modifications occur in the medial and trans compartments
Trans: Sorting distribution center: secretion, endosomes, lysosomes.
Discuss the transport through the golgi.
This is an area of controversy. There are two different models:
The stable cisternae model: proteins are carried between cisternae in transport vesicles. Active enzymes in each cisterna (sac)
The cisternal maturation model: proteins are carried within the cisternae, which gradually mature and progressively move through the Golgi in the cis to trans direction.
- In both models transport vesicles also carry Golgi resident proteins back to earlier compartment for re-use.
Who is Camilo Golgi? What did he do?
Golgi performed his “black reaction” using a salt solution which he stained the nerve cells of the cerebellum of an owl. The first visualization of neurons ever!!!
-He noticed thread-like networks inside the cells and named them apparato reticolar interno (internal reticular apparatus)
Describe Protein Processing: N-Linked Oligosaccharides in the Golgi. Generic example.
There is orderly progression as the protein moves through the Golgi.
The Glycoproteins have 11 sugar residues as they leave the ER.
-To start with, GlcNac (2) & mannose (9) in the glycoprotein in the cis Golgi.
- In the first processing step, mannose residues are removed (6), GlcNAc is added (1) in the medial Golgi
- Then fucose (1), Galactose (3), GlcNAc (1) are added in the trans and trans-Golgi network
- Sialic acid is added (3) in the trans and trans-Golgi network
Discuss a specific example of Protein Processing in the Golgi for proteins directed to the Lysosome.
First step: GlcNAc phosphate is transferred to the mannose residues from UDP-GlcNAc. This phosphorylation is key for the receptor.
-Reaction catalyzed by glycosyltransferases (enzymes).
Second step: the GlcNAc group is removed, leaving a mannose-6-phosphate. After this addition, there is no further modifications.
Mannose-6-phosphate recognized by mannose-6-phosphate receptor in the trans-Golgi network and then shuttled to the lysosome.
What is O-linked glycosylation?
Carbohydrates added to side chains of serine and threonine. Done sequentially by addition of single sugar residues.
What is an example of a glycosaminoglycan?
The GAG chain
What does the processing of proteoglycans involve?
Involves addition of 100 or more carbohydrate chains to a polypeptide that is further modified by addition of sulfate groups.
What are proteoglycans?
Secreted proteins necessary for the ECM.
Discuss sphingomyelin.
It is found in animal cell membranes, especially in the membranous myelin sheath that surrounds some nerve cell axons. It has a role as an insulator of nerve fibers.
The degradation of sphingomyelin can produce ceramide which is involved in the apoptotic signaling pathway.
Sphingomyelin hydrolysis and ceramide production signals apoptosis (early in the process).
Structurally, it differs from other lipids because it has serine.
Describe glycolipids.
Lipids with a carbohydrate attached by a glycosidic (covalent) bond. Their role is to maintain the stability of the cell membrane and to facilitate cellular recognition.
They are found on the surface of many eukaryotic cell membranes, where they extend from the phospholipid bilayer into the extracellular environment (blood types). Can extend from a red blood cell.
What are the four main human blood types and how are they determined?
A, B, AB, O. They are determined by the oligosaccharide attached to the specific glycolipid on the surface of RBCs, which acts as an antigen.
Where are glycerol phospholipids, cholesterol, and ceramides (a type of waxy lipid) synthesized at>
In the ER!
Where are glycolipids and sphingomyelin synthesized, and what are they synthesized from?
They are synthesized from ceramide in the Golgi.
How is sphingomyelin synthesized?
By transfer of a phosphorylcholine to ceramide (which are both imported from the ER).
How are glycolipids synthesized?
Where do glycolipids end up?
By the addition of carbohydrates to ceramide (hydroxyl group).
End up on the exterior half of the plasma membrane.
The glycolipids end up on the exterior half of the plasma membrane, so where are they synthesized? The inside or outside of the Golgi membrane?
The first glycogen added to it happens on the outside, and then it is flipped to the inside of the Golgi membrane.
What happens in the trans-Golgi network?
Molecules are sorted and packaged into transport vesicles.
Export from the Golgi Complex:
Transport from the Golgi to the cell surface can occur by three routes. What are they?
- Direct transport to the plasma membrane.
- Recycling endosomes : use of endosomes as an intermediate compartment.
- Regulated secretory pathways: granules store their contents until signals direct their fusion with the plasma membrane (e.g. hormones, digestive enzymes, etc.)
(there’s also a lysosome pathway but will be discussed later)
