Transmembrane proteins, Golgi, COPI, II (Lecture 20) Flashcards
What do transmembrane vesicles move?
What is the endoplasmic reticulum?
Differentiate between rough and smooth ER
What takes place at the RER
What determines where protein ends up?
What can defective protein targeting cause?
Transport Vesicles move soluble proteins and membrane proteins into other compartments or out of the cell.
The Endoplasmic Reticulum (ER) is a compartment of flattened sacs and tubules.
Rough (RER) has ribosomes on the cytoplasmic membrane side; Smooth (SER) has no associated ribosomes.
Cotranslational Import takes place at the RER.
Intrinsic information in protein sequences determines where proteins end up.
Defective protein targeting causes many diseases
Abnormal Protein Trafficking and Mislocalization in Diseases
What is Cystic fibrosis (CF)?
- Cystic fibrosis (CF) is caused by a mutation in the gene cystic fibrosis transmembrane conductance regulator (CFTR). The most common mutation, ΔF508, is a deletion of three nucleotides that results in the loss of the amino acid phenylalanine (F) at the 508th position on the protein.
- This mutation accounts for two-thirds (66–70%) of CF cases worldwide.
Cotranslational protein import
How do soluble proteins get into the endomembrane system?
- The signal-recognition particle (SRP) binds to a signal sequence in the amino-terminal end of the growing polypeptide and halts translation.
- The SRP binds to the SRP receptor on the ER membrane
- The SRP receptor brings the ribosomes to the transmembrane channel; the SRP dissociates; protein synthesis resumes; and the growing polypeptide chain is threaded through the channel
- The protein ends up in the lumen of the ER, where it may remain, be transported to the lumen of another organelle, or be secreted out of the cell
What happens to the protein once 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, plasma membrane
Synthesis of Integral Membrane proteins
How are transmembrane proteins are integrated into a membrane?
(except for those in mitochondria and chloroplast membranes)
- Proteins with signal-anchor sequences are threaded through a channel in the ER membrane until the signal-anchor sequence is encountered
- The ER channel releases the protein into the membrane
- When translation completed, the protein remains in the membrane
Synthesis of Integral Membrane proteins
how transmembrane proteins are integrated into a membrane
(except for those in mitochondria and chloroplast membranes)
*Another different image to describe it*
The polypeptide chain is being built inside the lumen. Then a stop-transfer anchor sequence occurs in the open translocon. The polypeptide continues to be built but instead of in the lumen it is being built the other way in the cytosol because of the stop-transfer anchor sequence.
How do proteins get targeted to mitochondria and chloroplasts?
Which intrinsic sequences within the protein direct to the correct compartment or membrane of the organelle?
- N-terminal sequences* direct proteins to their respective organelles
- Other intrinsic sequences within the protein direct it to the correct compartment or membrane of the organelle
Mitochondria
- OMM
- IMM
- Intermembrane space
- Matrix
Chloroplast
- OCM
- ICM
- Thylakoid membrane
- Stroma
- Thylakoid Lumen
*also called ‘signal sequences’
* Remember: These proteins are synthesized completely in the cytoplasm first
Transport from ER to Golgi complex
What is the Golgi apparatus?
The Golgi apparatus receives proteins and lipids from the ER and sorts them to other organelles, the plasma membrane, or the cell exterior.
Transport from ER to Golgi complex
Explain how the materials move?
Material moves from ER to Golgi and then to other compartments and the plasma membrane in a proximal to distal direction
Transport from ER to Golgi complex
What are the two types of Golgi networks?
- trans-Golgi network (TGN)
- cis-Golgi network (CGN)
Golgi complex
What is its structure?
Structure
- smooth, flattened, disk-like cisternae (~ 0.5 - 1 micron in diameter)
- ~ 8 (or fewer) cisternae/stack (range from a few to several 1000 stacks per cell)
- curved like a shallow bowl
- shows polarity:
- ‣ cis - medial - trans cisternae
- ‣ cisternae are biochemically unique
- membrane supported by protein “skeleton” (actin, spectrin)
- scaffold linked to motor proteins that direct movement of vesicles into and out of the Golgi
Golgi Complex
Why the structural differences?
‣ CGN acts as a “sorting station” i.e., sorts whether proteins should continue on to the next Golgi stack or be shipped back to the ER
‣ TGN sorts protein into different types of vesicles
- Vesicles go to the Plasma membrane or other intracellular destinations (e.g. lysosomes)
What happens to proteins as they transverse the Golgi?
- Proteins are modified ‘step-wise’ as they traverse the Golgi.
- Different cisternae of the Golgi contain different enzymes that modify proteins.
- The differential staining of the Golgi cisternae reflects their biochemical differences
What are the functions of the Golgi complex?
- ‘processing factory’ of the cell
- synthesis of complex polysaccharides
- modification of proteins and lipids
- glycosylation (glycoproteins & glycolipids)
- proteolytic modification
- Transport and sorting of proteins
Endocytic Pathways of Protein Sorting
Where are fully processed proteins exported to?
Fully processed proteins are exported to the trans-Golgi network (TGN) and then sorted and delivered to their final destinations.
Endocytic Pathways of Protein Sorting
The transfer of vesicles from ER to Golgi, and between the Golgi sub-compartments, is achieved by coat proteins
Coat proteins have 2 functions what are they?
- Help form the vesicle.
- Help select ‘cargo’ (i.e. material in vesicle lumen or inserted in the membrane).
Vesicular Transport + coat proteins
Electron micrographs reveal dense protein coat (COPI and COPII) on vesicles
How do COPI and COPII proteins carry out functions (1) and (2)?
COPI and COPII are protein complexes that assemble on the cytosolic surface of donor compartment membranes at sites where budding takes place.
COPI-coated vesicles move in retrograde direction
COPII-coated vesicles move in anterograde direction
COPI and COPII proteins assemble on the cytosolic surface of donor compartment membranes at sites where budding takes place.
What is the movement between Golgi and ER with respect to COPII and COPI?
COP proteins and other coat proteins interact with what?
Golgi → ER (COPI = retrograde movement)
ER → Golgi (COPII = anterograde movement)
COP proteins and other coat proteins interact with cytosolic parts of transmembrane cargo receptors
What directs vesicles to other parts of the cell?
i.e, - trans Golgi to endosomes
- trans Golgi to lysosomes
- PM to endosomes
Clathrin-coated vesicles move from TGN to other vesicles (e.g. lysosomes, endosomes, plant vacuoles)
Clathrin also helps form endocytic vesicles; i.e. that from at the Plasma membrane (i.e., PM to endosomes, lysosomes)
Plasma Membrane → Organelle
What are examples of vesicular transport?
ER → Golgi → Organelle → Plasma Membrane
= exocytosis
Plasma membrane → organelle
= endocytosis
What are Lysosomes?
digestive organelles
- 25nm-1μm
- internal pH of 4.6
- H+ - ATPase
- hydrolytic enzymes
- acid hydrolases
- lysosomal membrane
- glycosylated proteins
- protective lining next to lumen
