L20- Post translational modifications

Question 1

what happens after translation of the polypeptide chain by the ribosome during translation

Answer 1

it folds to form a 3D activated structure

Question 2

some proteins may need

Answer 2

additional processing after translation

Question 3

what sorts of additional processing may proteins require after translation

Answer 3

• Proteolytic cleavage - Chemical modification

Question 4

proteolytic cleavage

Answer 4

breaking peptide bonds to remove part of protein o e.g. N and C terminals

Question 5

chemical modification

Answer 5

addition of functional groups to amino acid residues o E.g. glycosylation

Question 6

what is protein targeting

Answer 6

how proteins know where to go into their cell e.g. proteins have intrinsic signals that governs heir transport and localisation in the cell e..g SRP

Question 7

what is required for protein sorting

Answer 7

1. A signal intrinsic to the protein - e.g. Signal sequence in polypeptide of secretory proteins 2. A receptor that recognises the signal and which directs it to he correct membrane 3. A translocation machinery 4. Energy for translocation (GDP)

Question 8

two locations where translation (protein synthesis) can occur

Answer 8

• on free ribosomes - by ribosomes on the RER

Question 9

proteins translated by free ribosomes are destined

Answer 9

for the cytosol or post translational import into organelles e.g. - nucleus - mitochondria - chloroplast - peroxisome

Question 10

proteins translated by ribosomes found on the RER

Answer 10

proteins destined for the membrane or secretory (Golgi) pathway via translational. insertion

Question 11

what sort of proteins are targeted for secretion

Answer 11

• extracellular proteins (collagen) - membrane protein (channels etc) - vesicular proteins (lysosomes)

Question 12

an example of a cell that has a high secretory rate

Answer 12

pancreatic acing cells

Question 13

pancreatic acinar cells are the

Answer 13

make up the exocrine tissue of the pancreas - stuffed full of secretory granules - e.g. pancreatic amylase

Question 14

how are pancreatic acinar cells designed to carry out their function?

Answer 14

• lots of RER —> high rate of secretory protein synthesis

Question 15

secretory pathway of a cell

Answer 15

1) the endoplasmic reticulum,
2) Golgi apparatus and the
3) vesicles that travel in between them as well as the cell membrane and lysosomes.

It’s named ‘secretory’ for being the pathway by which the cell secretes proteins into the extracellular environment.

Question 16

Types of secretion from cells

Answer 16

1. Constitutive
2. Regulated

Question 17

constitutive secretion

Answer 17

happens all the time to maintains basal levels e.g. albumin

Question 18

regulated secretion

Answer 18

Controlled

• can be upregulated or downregulated by calcium
  e. g. endocrine cells- secreting hormones
  e. g. exocrine cells- secreting digestive
  e. g. Neurocrine cells- secreting NT

Question 19

what part of a protein targets it for the ER

Answer 19

signal sequence at the N-terminus of secretory proteins e.g. Preproalbumin has a signal sequence which targets it to certain areas of the cell

Question 20

N terminus

Answer 20

at the beginning of the protein (will be on outside of the cell)

Question 21

when secretory proteins, destined for the ER are being translated by free ribosomes, what binds to the signal sequence (first part of the mRNA translated) to target it to the ER

Answer 21

the signal recognition particle

Question 22

the SRP is able to

Answer 22

bind to the SRP receptor on the ER - causing protein synthesis of mRNA to occur within the RER and not in the cytosol

Question 23

preproalbumin as an example- the N terminal

Answer 23

N -terminal aa sequence has a central region rich in hydrophobic residues (red part)

• 5-30 aa in length - able to form alpha helix

Question 24

what is the ‘pre’ part of the preproalbumin

Answer 24

defines the signal sequence which is removed during processing

Question 25

when we lose signal sequence it becomes

Answer 25

pro albumin

Question 26

Synthesis of secretory proteins and their translocation across the ER membrane: Process

Answer 26

1. Free ribosome starts to translate the mRNA
2. first part of the mRNA sequence (N-terminus) is translated and signal sequence produced
3. as soon as signal sequence appears it can be bound by the SRP
4. when the SRP bidns ot the ribosome and the signal seqeunce, it halts protein synthesis- translational arrest
5. SRP protein binds to SRP receptor- binding the ribosome to the translocon (GDP dependent-energy)
   
   1. causes translocon to open to allow the translated protein to enter the ER
6. Signal seqence get cleaved by signal peptidase befor ethe res tof the protein is translated
7. mRNA translated into a protein in the ER lumen
8. polypeptide chain folds to form a protein
9. once trasnlation is finished, the ribosome will detach fromt he ER and go off to repeat the process

Question 27

Function of the ER

Answer 27

1. Glycosylation (N-linked)
2. Formation of S-S bonds (disulphide bonds)
3. Proper folding of proteins

There are more but these are the ones to concentrate on

Question 28

N- linked glycosylation involves

Answer 28

sugars being added onto an asparagine side chain

Question 29

what does N-linked eman

Answer 29

sugars added to amino group (asparagine)

Question 30

why is glycosylation of proteins important

Answer 30

• Correct protein folding
• Protein stability
• Facilitates interactions with other molecules- e.g. recognition of self proteins

Question 31

deficincies in N-linked glycosylation leads to

Answer 31

a severe inherited human disease e.g. congenital disorders of glcosylation (CDG)

Question 32

Proper folding of proteins is facilaited by

Answer 32

chaperone proteins

Question 33

chaperone proteins

Answer 33

ER chaperone proteins attempt to correct problem

• Retain unfolded proteins in ER
• Act as sensors to monitor extent of misfolding
• Mediate increased transcription of protein mis-folding
• Mediate reduction in translation

Question 34

which enzyme catalyses the formation of disulphide bonds during protein folding

Answer 34

protein disulphide isomerise (PDI)

Question 35

where does formation of disulphide bonds occur

Answer 35

the ER lumen

Question 36

formation of disulphide bonds includes

Answer 36

• disulphide bond between 2 cystein residues (SH)
• correctly folded protein will have the correct pairsings of S-S bonds
• PDI ensure correct disulphide bond forms and therefore correct protein folding

Question 37

what happens if there are fodling prolems - linekd to formation of disulphide bonds

Answer 37

• Trapped in wrong compartments
• Trapped in mis-folded conformation
• Protein contains mutation resulting in mis-folding
• Protein may be incorrectly associated with other sub-unit

Question 38

What happened if mis-folding cannot be corrected?

Answer 38

1. Protein may be returned to the cytosol for degradation
2. Protein may accumulate to toxic levels in the ER resulting in disease- may arise due to single mutation

Question 39

what modifies proteins are they leave the ER

Answer 39

golgi- a continuation of the RER