endoplasmic reticulum

Question 1

describe the structure of the ER

Answer 1

• Single-membrane compartment -continuous network of tubular and flat vesicular structures in the cytoplasm (“little net”).
• space inside the tubules and vesicles are filled with an endoplasmic matrix, different to the fluid outside the ER in the cytosol
• Space inside is connected with the space between the two membrane surfaces of the nuclear membrane- continuous with the nuclear envelope.

Question 2

what are the ER functions

Answer 2

• Protein synthesis, glycosylation, folding and assembly and forming multi-protein complexes. RER
• Lipid synthesis (cholesterol, phospholipids). -SER
• Ca2+ sequestration. – regulating concentration of Ca key for function of tissues, eg contraction of muscle cells -SER
• Detoxification by cytochrome P450 enzymes. – SER : drugs, waste generated in metabolic processes are detoxified by cytochrome p450 and expelled from the body. This enzyme catalyse reactions that can make drugs and waste be soluble, which facilates their expulsion from the body. This is a key role in the liver.

Question 3

which enzyme is responsible for dextoxification?

Answer 3

P450

Question 4

compare the rer and ser

Answer 4

rer - has ribosomes
ser - has no ribosomes

rer- found near the nucleus
ser- found closer to the cell membrane

rer- originates from nucleus membranes
ser- originates from the rer by giving off the ribosomes

rer - mainly composed of cisternae
ser - mainly xomposed of tubules

rer - synthesis, folding and transport of proteins
ser-synthesis and transport of lipids

rer-well developed in protein forming and secretory cells.
ser-mainly present in lipid forming cells.

Question 5

amino acids at the N-terminal end of the protein are called?

Answer 5

signal sequences

Question 6

newly synthesised target proteins are targeted to the ?

Answer 6

er, nucleus, mitochondria or peroxisomes

Question 7

how are proteins targeted to their correct destinations?

Answer 7

Signal sequences are aminoacids 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. Therefore aa determine the final destination.

• those proteins with signal sequences are going to target the ER will be transported to the membrane bound ribosome of the ER, the rest of the proteins targeting other organelles will be directed to the free ribosomes present in the cytosol.
• so those signal sequences are initially nucleotides that encode for amino acids that are going to be located at the N-terminal end of the protein, and then those amino acids will determine the final destination

Question 8

what are the final destinations of proteins synthesised on the free ribosomes in the cytosol?

Answer 8

nucleus, peroxisomes, mitochondria and chlorphlasts

Question 9

what are the final destinations of proteins synthesised on membrane bound ribosomes attached to the er ?

Answer 9

plasma membrane, secretory vesicles, endosomes – and later lysosomes.

Question 10

overline the steps of protein synthesis

Answer 10

ribosomes - rer- golgi

Question 11

what is srp

Answer 11

Signal recognition particle. It’s a complex of rna and protein and is very abundant in the cytoplasm.

Question 12

outline the steps in the SRP cycle.

Answer 12

1) The recognition of the signal peptide by the ribosomes. The proteins that are going to be targeted to the RER, have a single peptide in the n terminal end. This peptide is the first part of the protein that is translated.

2) The srp protein binds to the ribosome nascent chain complex – collection of molecules that constitute a ribosome attached to the polypeptide chain that it is synthesising. This causes translation to pause, allowing time for the protein to be targeted to the ER via the SRP.
3) The complex binds to the SRP receptor which is attached to the endoplasmic reticulum membrane. Gdp needs to be present for this to happen.
4) The signal peptide is transferred from the srp complex to the sec61 translocation chanel. The srp complex formed by the srp, the sr receptor, the ribosome, and the nascent chain, Is going to translocate until it finds a translocation channel.
5) The hydrolysis of GTP causes the complex to dissociate. The protein is already in the translocation chanel and therefore wont need the SRP protein or the srp receptor. It will still need the ribosome as translation is no longer paused and translation continues through the translocation chanel.
6) Srp can be recycled.

Question 13

explain how insulin is modified in the rer

Answer 13

Proteolysis :
- Initially the insulin is synthesised as preproinsulin.
- It then undergoes proteolysis which is the removal of the singal sequence forming proinsulin
- The proinsulin that no longer carry the signal sequence, is then cleaved into 3 peptides, a,b,c
- The different chains have different amino acids.
Disulfide bond formation :
- Between a and b chain.
- Removal of c chain.
- This produces biologically active insulin- 51 AA – mature insulin

gylcosyation and deglycosyation : it is usually finalised in the golgi

protein folding and assembly :

• TERTIRAY , QUATERNARY STURCUTRES
• Determines function
• After this proteins can be directed to the golgi, and secreted to the different destinations.

Question 14

what is ERAD

Answer 14

Endoplasmic-reticulum-associated protein degradation.

Question 15

proteins that fail quality checks will not be exported and are degraded by ?

Answer 15

ubiquitination and proteasome.

Question 16

proteins that are misfolded are translocated outside the er through which process

Answer 16

protein retrotranslocation

Question 17

transport vesicles from the er carry their cargo to?

Answer 17

golgi complex

Question 18

what is the golgi apparatus ?

Answer 18

• Single-membrane compartment consisting of 4 to 8 stacked layers of thin, flat, enclosed vesicles (cisternae) lying near one side of the nucleus.

Question 19

what are the functions of the golgi ?

Answer 19

protein modifications (glycosidases, glycotransferases, o-linked glycosylation, sulfatases, proteases) lipid synthesis ( sphingomylein, glycosylceramide), and protein and lipid sorting to : secretory granules, plasma granules, basolateral versus apical granules, endosomes, lysosomes

Question 20

outline the steps in the fusion of vesicles to the golgi

Answer 20

budding-movement-fusion

Question 21

describe the pathway of vesicles

Answer 21

• Destination is determined in the ER when the protein/cargo bind to a specific receptor. The vesicle budding occurs due to a selective incorporation of the cargo into the following vesicles, while resident proteins remain in the donor compartments. The vesicles are subsequently targeted to a specific acceptor compartment into which they unload their cargo upon membrane fusion
• Various characteristics of the cargo protein are recognised e.g. aa sequence or added CHO.
• bud formation is facilitated by binding of different coat proteins ( e.g COPs)
• once transport vesicle is formed and released, coat proteins are removed revealing v-SNARE (integral protein.)
• SNARE proteins – molecular motifs that drive the fusion of two membranes.
• v-SNARE binds to t-SNARE (target) in the target membrane, the transport vesicle fuses to the target membrane and the cargo delivered.

Question 22

what are SNARE proteins

Answer 22

molecular motifs that drive the fusion of two membranes.

Question 23

which protein facilitates bud formation

Answer 23

coat proteins

Question 24

what are the 3 networks of the golgi

Answer 24

• Three networks: cis (first cisternae structure, closer to nucleus), medial and trans compartments (final structure, closer to cell membrane).

Question 25

which protein is involved in transport and the recognition of the final destination on the membrane?

Answer 25

snare protein