ER, Trafficking, Co-Translational Import (Lecture 19) Flashcards
Chloroplasts are the sites for what?
What does photophosphorylation create?
Why is apoptosis important?
What is abnormal apoptosis associated with?
What is involved in dismantling the cell?
What mediates exocytosis and endocytosis? via what?
What can be visualized by using GFP?
Chloroplasts are the sites for photosynthesis
Photophosphorylation creates ATP and NADPH.
Apoptosis is an important, regulated process that eliminates redundant or damaged cells.
Aberrant Apoptosis is associated with cancer (not enough) and neurodegenerative disease (too much)
Caspases are proteases that activate proteins that are involved in dismantling the cell.
The endomembrane system mediates exocytosis and endocytosis via vesicular transport
Protein traffic patterns can be visualized by using GFP.
Vesicular Transport (Trafficking)
What is the transport of material between compartments?
How are transport vesicles utilized?
- transport of material between compartments
- organelle → PM (& vice versa)
- organelle → organelle
- utilizes transport vesicles (~ 50-75 nm)
- small, spherical, membrane-enclosed organelles that bud off donor compartment and fuse with an acceptor compartment
Vesicular Transport / Trafficking
What are the 4 ways to traffic vesicles to a compartment?
➡ Trafficking vesicles to a compartment
- Movement of vesicles — uses cytoskeleton and motor proteins.
-
Tethering vesicle to target compartment
* via proteins called Rabs and tethering proteins. -
Docking of the vesicle to the target compartment
* uses proteins called SNAREs. - Fusion of vesicle and target membrane.
What is an example of vesicular transport?
✴ organelle → PM
= exocytosis
e. g. secretion of mucin (constitutive)
e. g. secretion of neurotransmitters (regulated)
✴ PM → organelle
= endocytosis
e.g. recycling of plasma membrane proteins
Exocytosis and Endocytosis
What are examples of exocytosis?
Examples of exocytosis:
Organelle → Plasma membrane
Secretion of neurotransmitter
(e.g. Acetylcholine)
Exocytosis and Endocytosis
What are examples of endocytosis?
Examples of endocytosis:
Plasma membrane → organelle
Endoplasmic reticulum
What is the smooth ER?
What is the rough ER?
The smooth ER lacks ribosomes and is the primary site of lipid synthesis
The rough ER is associated with ribosomes. Many proteins, including those that are destined for secretion, are synthesized by ribosomes associated with the rough ER
What are the functions of the smooth ER (SER)?
Functions of smooth ER (SER
- Lipid synthesis
-
Production of steroid hormones like glucocorticoids, androgens, and estrogens
- endocrine cells
-
Detoxification
- e.g. liver cells contain enzymes that modify foreign compounds (oxygenases, cytochrome P450)
-
Sequestration (storage) of Ca2+
- (in muscle cells, it is called the Sarcoplasmic Reticulum)
What are the functions of Rough ER (RER)?
- Synthesis of membrane phospholipids.
-
Glycosylation of proteins:
- Addition of carbohydrate chains to specific proteins.
- Protein folding — quality control:
- Involve the activity of molecular chaperones, specific types of protein that assist in the folding process.
- Protein synthesis, modification, and transport:
- Proteins targeted to ER.
- Proteins targeted to other endomembrane compartments (soluble and transmembrane).
- Proteins targeted the plasma membrane (secreted and transmembrane).
Brief review of protein synthesis
Label the diagram
Briefly explain the review of Protein Synthesis
In the cytoplasm, ribosomes synthesize polypeptides from mRNA
(= translation)
All protein translation begins on what?
The translation is completed in 1 of 2 ways, what is the 1st way?
All protein translation begins on free ribosomes (i.e. in the cytoplasm)
The translation is completed in 1 of 2 ways: -
- by free or ER-bound ribosomes
1) Translation completed on free ribosomes - cytosolic proteins
- peripheral membrane proteins
- proteins targeted to nucleus, mitochondria, peroxisomes, chloroplasts
except for the few proteins made in mitochondria & chloroplasts
All protein translation begins on what?
The translation is completed in 1 of 2 ways, what is the 2nd way?
All protein translation begins on free ribosomes (i.e. in the cytoplasm)
2) Translation completed by ribosomes associated with ER membrane (rough ER)
✤ secreted proteins
✤ integral membrane proteins
✤ soluble proteins associated with the inside (lumen) of the endomembrane system; e.g. proteins that function within the ER, Golgi, lysosomes
How do ribosomes on RER surface get there?
Ribosomes are targeted to the ER membrane by a ‘signal sequence’
- protein contains ‘signal sequence’
- located at its amino-terminus (N-terminus)
- contains several consecutive hydrophobic amino acids
- ‘signal sequence’ directs synthesis to ER
- protein moves through a channel into ER
= Cotranslational import
Cotranslational protein import
Where does translation stop and resume?
Cotranslational protein import
What is step 1 and what is it after?
After translation of signal sequence…
1) Signal Recognition Particle (SRP) binds to signal sequence – translation stops
Cotranslational protein import
What is step 2?
Cotranslational protein import
2) Targeting of translation complex to ER [SRP/ribosome + mRNA/new polypeptide]
- SRP binds to SRP receptor
Cotranslational protein import
What is step 3?
Cotranslational protein import
3) SRP is released and the ribosome binds the translocon
- protein synthesis resumes
Cotranslational protein import
What is step 4?
Cotranslational protein import
4) Polypeptide enters the ER (through the translocon) as it is translated
- signal sequence cleaved off
- chaperone folds protein
What happens to a protein once it is in the ER?
Once in the ER, a protein is part of the biosynthetic/secretory endomembrane system and may ultimately become part of a compartment, or be secreted.
e.g. ER Golgi lysosome extracellular space
