Lecture 26

Question 1

Which blood group is the universal acceptor

Answer 1

AB

Question 2

Give an example of where different cells can process the same peptide into different hormones

Answer 2

Cleavage of opiomelanocortin can give rise to a whole range of different proteins which varies from cell to cell. Opiomelanocortin can be cleaved in neurons to give, B-MSH, B-endorphin, ?-MSH and ?-lipoprotein. In contrast, opiomelanocortin can be cleaved in the pituitary to give corticotropin (ACTH) and B-lipotropin

Question 3

What are the three reasons why glycosylation is important

Answer 3

Increase protein stability in the harsh extracellular environment, enables cell-cell recognition as well as cross species separation

Question 4

What is characteristic of the signal sequence that directs a protein to the mitochondria

Answer 4

Proteins destined for the mitochondria alternate between positive and hydrophobic amino acid residues

Question 5

Trimming and growth of carbohydrate chains proceeds step-by-step in individual Golgi cisternae, T or F

Answer 5

T

Question 6

Where does initial carbohydrate addition to proteins start

Answer 6

ER

Question 7

Explain what is meant by SRPs and their receptors acting as molecular matchmakers

Answer 7

SRPs and SRP receptors function as molecular matchmakers as they connect the ribosomes synthesising proteins containing the endoplasmic reticulum signal sequence to available endoplasmic reticulum translocation channels

Question 8

Describe the differences in the terminal oligosaccharide in people with the A, B and O antigens

Answer 8

People with the A blood type antigen contain an acetylated terminal galactose (N-acetylgalactosamine. People with the B blood type antigen possess a non-acetylated terminal galactose. AB patients will possess red blood cells with both the acetylated and the non-acetylated terminal galactose whereas O blood types will have neither.

Question 9

Explain how proteins insert into the ER membrane once the SRP has bound to its receptor

Answer 9

The signal sequence in the protein opens the translocation channel. The signal sequence remains bound to the channel while the rest of the protein chain is threaded through the membrane as a large loop before it is then cleaved off and degraded. A protein plug then closes the translocation channel in the endoplasmic reticulum membrane

Question 10

Give an example of protein trimming in the process of maturation

Answer 10

Protein trimming occurs prior to the secretion of inulin. The signal peptide is removed from preproinsulin which produces the still inactive proinsulin molecule. Proinsulin is then moved to the Golgi and passes through it. Then, in the final stage, several amino acids in the middle of the of protein (known as chain C) are removed. The cleavage of proinsulin produces mature insulin and the C-terminal peptide. Chain A and chain B are held together by disulphide bridges

Question 11

What are signal patches

Answer 11

Signal patches are specific 3D arrangements of amino acids that are used to target proteins

Question 12

Whereas membrane-bound and secreted proteins are made in the RER, where are nuclear and cytosolic proteins made

Answer 12

Nuclear proteins are made on the outer nuclear membrane whereas cytosolic proteins are made in the cytosol

Question 13

What can be said about the affinity of start and stop transfer sequences in ER membrane proteins

Answer 13

They are hydrophobic

Question 14

What is meant by the process of conformational maturation that occurs after signal sequence removal by signal peptidases

Answer 14

Conformational maturation is the process whereby the maturing proteins adopts a thermodynamically stable shape

Question 15

Describe how transmembrane proteins insert into the ER membrane and how this differs from ER luminal proteins

Answer 15

Proteins that are meant to insert into the ER membrane contain stop transfer sequences that halt the transfer process into the endoplasmic reticulum lumen. Stop transfer sequences are released laterally into the lipid bilayer and form a membrane spanning segment anchoring the protein to the membrane

Question 16

What is the initial purpose of carbohydrate chains added to proteins

Answer 16

Quality control tags

Question 17

How does insertion into the ER membrane differ for multipass transmembrane proteins

Answer 17

Mulitpass membrane proteins have internal signal sequences consisting of several stop and start pairs which help to stitch the protein into the membrane. Combinations of Start-transfer and Stop-transfer signals determine the orientation of multipass membrane proteins in the membrane

Question 18

Explain how proteins are transported and insert into the ER membrane

Answer 18

Signal sequences direct the protein to the endoplasmic reticulum through the action of signal recognition particles (SRPs). SRPs bind to the signal sequence in a newly synthesised protein as it emerges from the ribosome. Protein synthesis then slows down until the SRP-ribosome complex binds to the SRP receptor on the endoplasmic reticulum membrane. The SRP is then released passing the ribosome to the protein translocation tunnel in the endoplasmic reticulum membrane.

Question 19

Give an example of where defective protein glycosylation causes disease

Answer 19

Myelination of neurons requires complex gangliosides enriched in sialic acid. Deficiency in sialic acid causes severe psychomotor delay evident by the end of the first year of life

Question 20

Which blood group is the universal donor

Answer 20

O

Question 21

What attribute of protein glycosylation is responsible for the limitations in xenotransplantation and anima donors

Answer 21

Humans contain B-galactose attached to proteins whereas animals use ?-galactose

Question 22

Ribosomes associate with the membrane of the target compartment via a direct interaction, T or F

Answer 22

F - Electron microscopy has shown that ribosomes associate with the membrane via protein translocators present in the membrane

Question 23

How does the structure of the various blood type antigens account for the antibodies that those patients will possess

Answer 23

A antigen patients will possess antibodies for the B antigen and vice versa. O group blood people will have antibodies for both A and B antigens whereas if you are AB you would have neither.

Question 24

What is the role of signal recognition particles in membrane and secreted protein synthesis

Answer 24

SRPs direct the delivery of the ribosome and protein to the membrane

Question 25

What happens once proteins reach the Golgi

Answer 25

In the Golgi there is a final addition of sugars and then the proteins are sorted and concentrated into transport vesicles

Question 26

Which amino acid residue is often used to add initial carbohydrate modifications

Answer 26

Asparagine

Question 27

What type of bonding often occurs during protein maturation and acts to solidify protein shape

Answer 27

Disulphide bridges

Question 28

By what other key method other than transmembrane domains can proteins become anchored into the ER membrane

Answer 28

Swapping the signal peptide for a lipid anchor in the ER can take place to continue membrane association. Lipid pairs can be added to the protein by different enzymes. These anchors are usually glycosylphosphatidylinositol anchors (GPI)

Question 29

Describe an experimental technique used to separate vesicles from the SER and RER

Answer 29

Light and heavy vesicles can be separated by differential centrifugation. Firstly the cell is homogenised in a blender to form a homogenate containing various vesicles depending on whether they were derived from the SER or RER. This homogenate is then added to a tube containing a sucrose gradient. The tube is then centrifuged at high speed to separate the vesicles. Smooth ER microsomes have a low density so stop sedimenting and float at a low sucrose concentration. Rough microsomes have a high density so stop sedimenting and float at a high sucrose concentration. A hole can then be punched in the bottom of the tube and the various fractions can be separated and extracted drop by drop

Question 30

Signal recognition particles guide signal peptide sequences to the target compartment where the ribosome can then assemble, T or F

Answer 30

F - Signal recognition particles guide ribosomes following binding to the signal peptide sequence

Question 31

What enzymes remove signal sequences once proteins have been sorted

Answer 31

Signal peptidases

Question 32

Following protein synthesis which compartment do the proteins move into were lipids are made and the transport vesicles are formed

Answer 32

Smooth endoplasmic reticulum

Question 33

Which sequence is contained within ER-resident enzymes to direct their return to the ER and which amino acids are involved

Answer 33

KDEL sequence: lysine-aspartate-glutamate-leucine

Question 34

Give an example of a disease caused by improper/defective protein cleavage

Answer 34

A genetic type of Type 1 diabetes is caused by a mutation that causes the misfolding of proinsulin in the ER. This means that proteases cannot cleave off C-peptide from Pro-insulin in secretory vesicles. This in-turn leads to the secretion of a dysfunctional pro-insulin into bloodstream as well as triggering an immune response to kill off the pancreatic cells producing the dysfunctional insulin

Question 35

Why is it that each glycosylation step requires separate Golgi compartments

Answer 35

This keeps specific glycosylation enzymes away from each other

Question 36

As well as being cleave out often signal sequences contains internal structures that remain part of the protein, T or F

Answer 36

T

Question 37

What structures recognise signal sequences that guide proteins towards their target destinations

Answer 37

Sorting receptors which recognise SRPs

Question 38

Where in the protein sequence are the signal sequences often found

Answer 38

At the N-terminus

Question 39

The blood group of an individual is determined by the structure of the oligosaccharides attached to which proteins and phospholipids in the red blood cell membrane

Answer 39

Sphingomyelin and Band 3 protein

Question 40

What is characteristic of the signal sequence that directs a protein to the endoplasmic reticulum

Answer 40

Proteins going to the endoplasmic reticulum have a signal sequence of 5-10 hydrophobic amino acids