Endoplasmic Reticulum and Secretory Pathway

Question 1

What is the structure of endoplasmic reticulum?

Answer 1

• single membrane compartment - network of tubular and flat vesicular structures in the cytoplasm
• space inside is connected with the space between the 2 membrane surfaces of nuclear envelope.
• 2 parts with different functions - rough er and smooth er

Question 2

What is the function of the rough ER?

Answer 2

• protein synthesis, glycosylation, folding and assembly and multi protein complexes

Question 3

What are the functions of the smooth ER?

Answer 3

• Lipid synthesis (cholesterol, phospholipids)
• Ca2+ sequestration
• Detoxification by cytochrome P450 enzymes.

Question 4

Rough ER structure?

Answer 4

• Has ribosomes
• found near the nucleus
• originates from the nuclear membrane
• mainly composed of cisternae
• synthesis, folding and transport of proteins
• well developed in protein forming and secretory cells.

Question 5

Smooth ER structure?

Answer 5

• does not have ribosomes
• found closer to the cell membrane
• originates from the rough ER by giving off the ribosomes
• mainly composed of tubules
• synthesis and transport of lipids
• mainly present in lipid forming cells.

Question 6

Function of ER in protein synthesis?

Answer 6

• Newly synthesised proteins would be targeted to the ER, nucleus, mitochondria or peroxisomes.
• signal sequences ( small sequences of amino acids) at the n- terminus of amino acid - which are recognised by enzymes within the cell that transport the protein to the correct destination of the cell.

Question 7

What happens to the proteins that have signal sequences?

Answer 7

they are going to target ER which will be transported to the membrane bound ribosomes to the ribosomes that aren’t bound to the membrane of the ER.

The rest of the proteins will be directed to the free ribosome in the cytosol.

Question 8

What are signal recognition particles? (SRP)

Answer 8

They are very abundant in the cytoplasm
They recognise and target specific proteins to ER. they consist of 4 different aspects

1) Recognition of the signal peptide via the ribosomes - those proteins are going to be transported to the rough ER and have a signal peptide in the n terminal end. Peptide of the first part of the protein that is translated.
2) Ribosomes will recognise and understand that they need to be translocated and bind to the rough ER. The SRP protein binds to the complex which is formed by the ribosomes)
3) complex binds to the SRP. GTP needs to be present for energy.
4) signal peptide is transferred and SRP to translocation channel. SRP complex via the SR receptor is going to translocate and move around until it finds a translocation channel. GTP hydrolysis occurs and the complex is dissociated. Protein is going to be placed inside the translocation channel. it will need a ribosome for translation.

Question 9

What happens to the proteins which fail any quality check in the ribosomes?

Answer 9

They will not be exported from the ER and they will be degraded by ubiquitation and proteasome.

Question 10

What are the functions of the golgi apparatus?

Answer 10

• transports vesicles from the er- carrying their cargo to the golgi complex.
• single membrane compartment - consisting of 4-8 stacked layers of thin, flat enclosed vesicles lying near one side of the nucleus.
• 3 networks: cis (first cisternae structure, closer to the nucleus), medial and trans compartments (final structure, closer to the cell membrane).
• It is responsible for packing and exporting the secretory granules.

Question 11

Which lipids are synthesised in the ER?

Answer 11

Cholesterol and Phospholipids

Question 12

Which lipids are synthesised in the Golgi Apparatus?

Answer 12

Sphingomyelin and glucosylceramide.

Question 13

What do vesicle transport proteins do?

Answer 13

• from the golgi apparatus to the lysosomes, the plasma membranes or the exterior
• from the plasma membrane to lysosomes
• from endosomes to the plasma membrane
  1. destination is determined in the ER when the protein/cargo binds to a specific receptor.
  2. Various characteristics of the cargo protein are recognized e.g. aa sequence or added CHO.
  3. bud formation is facilitated by binding of different coat proteins
  Once the transport vesicle is formed and released coat proteins are removed revealing the v-SNARE (integral protein).
  4. v-SNARE binds to t-SNARE in the target membrane, the transport vesicles to the target membrane and the cargo is delivered .