The Endoplasmic Reticulum And The Secretory Pathway Flashcards
Explain the structure of the ER
- single membrane compartment (a continuous network of tubular and flat vesicular structures in the cytoplasm)
- space inside is connected with the space between the two membrane surfaces of the nuclear membrane (filled with a fluid- not cytoplasm)
- two parts with different functions
RER facts?
- has ribosomes
- found near the nucleus
- originated from nuclear membranes
- mainly composed of cisternae
- involved with the synthesis, folding and transport of proteins
- well developed in protein forming and secretory cells
SER facts?
- found closer to the cell membrane
- originates from rough ER by giving off ribosomes
- mainly composed of tubules
- synthesis and transport of lipids
- mainly present in lipid forming cells
Functions of the RER
- protein synthesis
- glycosylation
- Folding and assembly of multi protein complexes
Functions of SER
- lipid synthesis (cholesterol, phospholipids)
- Ca2+ sequestration
- detoxification by cytochromes P450 enzymes
What is Ca2+ sequestration?
The binding or confining of calcium ions so they are separated from other components of a biological system
Where are newly synthesised proteins targeted to?
ER, nucleus, mitochondria or peroxisomes
What are signal sequences?
Amino acids at the N-terminal end of the protein that are recognised by enzyme systems within the cell that transport the protein to the correct destination
What do the free ribosomes in the cytosol do?
Send proteins to the nucleus, peroxisomes, mitochondria or chloroplasts
What do the membrane bound proteins on the RER do?
Send proteins to plasma membrane, endoscopes, secretory vesicles or lysosomes
What are the four steps of signal sequences directing proteins to the ER signal recognition particle? (SRP)
- recognition of the signal peptide by the ribosomes
- SRP protein bonds to the ribosome - nascent chain complex (ribosome and aa chain that is growing)
- the complex binds to SRP receptor which is attached to the ER membrane. GTP must be present.
- signal peptide is transferred from the SRP complex to the translocation channel
What are the steps for protein modification in the RER?
- Proteolysis (signal peptide)
- disulphides bond formation
- glycosylation
- deglycosylation
What does glycosylation/ deglycosylation mean?
Sugars being added/ taken from proteins as they enter the ER
What is important about the sugar groups that come from glycosylation?
They’re important for the recognition of proteins- especially if they’re going to be secreted
What happens if a protein fails a quality check?
They are not exported from the ER and are degraded by ubiquitination and protease ERAD (endoplasmic reticulum associated protein degradation)
Golgi apparatus structure
- Single membrane compartment consisting of 4 to 8 stacked layers of thin, flat, enclosed residues (cisternae) lying near one side of the nucleus.
- three networks: cis, medial and trans
Golgi apparatus functions
Protein modification, lipid synthesis and protein and lipid sorting
How do things deposit their cargo into the golgi complex?
Budding- movement- fusion
What groups does the golgi sort the proteins and lipids into?
- secretory granules
- plasma membrane
- basolateral vs apical membrane
- endosomes
- lysosomes
How do proteins get from one cisternae to another?
Using vesicles
How is the destination of a protein determined?
In the ER when the protein binds to a specific receptor. Various characteristics of the cargo protein are recognised
Where do vesicles transport proteins from/to?
- Golgi apparatus to lysosomes, plasma membrane or exterior
- plasma membrane to lysosomes
- endosomes to the plasma membrane
What is bud formation facilitated by?
The binding of different coat proteins (COPs)
What happens once the transport vesicle is formed and released?
The coat proteins are removed and the V-SNARE is revealed
