Protein Sorting and Modification

Question 1

what are the functions of post-translational modifications?

Answer 1

controls protein activation, guides protein transport, alters protein-protein or protein-DNA interactions, targets proteins for degradation

Question 2

what is proteolytic cleavage and what are examples?

Answer 2

enzyme mediated removal or AA residues

ex. removal of start met
ex. removal of signal sequence
ex. removal of certain aa sequence to form an active protein

Question 3

what is a zymogen?

Answer 3

inactive enzyme needing to be processed to become active

Question 4

ex of zymogen?

Answer 4

hormones - many need to be cleaved from large protein to smaller active peptides
ex. proinsulin – insulin + Cpeptide (serum levels of C peptide evaluate amt of insulin produced)

Question 5

what is phosphorylation?

Answer 5

removal or addition of phosphate group to either serine, threonine or tyrosine as an on/off switch

Question 6

what are protein kinases?

Answer 6

enzymes to add phosphate grou[p

Question 7

what are phosphatases?

Answer 7

enzyme to remove phosphate group

Question 8

what is glycosylation?

Answer 8

addition of oligosaccharides to protein via glycosidic bond to the hydroxyl or amine group of AA side chain

Question 9

where are O-linked glycosylated proteins added?

Answer 9

golgi

Question 10

where are N-linked glycosylated proteins added?

Answer 10

ER

Question 11

examples of acetylation?

Answer 11

lysines acetylated on histones (acetylated = relaxed, deacetylated = repressed)

Question 12

what is myristoylation

Answer 12

myristoyl is added to N terminus at glycine residue resulting in protein association with intracellular vesicles

Question 13

what is palmitoylation and prenylation?

Answer 13

addition of a lipid on cysteine side chain, the palmitoyl groups (C16) anchor proteins to cell surface

Question 14

what other lipids are found on cell surface proteins used in regulation/cell signaling?

Answer 14

farnesyl (C15) and geranylgerany (C20)

Question 15

what is hydroxylation?

Answer 15

addition of hydroxyl group to AA

Question 16

clinical example of hydroxylation?

Answer 16

proline and lysine residues hydroxylated on alpha chains of collagen. mutation in the gene encoding the lysyl hydroxylase leads to form of ehlers danlos syndrome?

Question 17

pathology of ehlers danlos syndrome?

Answer 17

mutations in gene encoding lysyl hydroxylase (less elasticity)

Question 18

what is carboxylation and why is it important?

Answer 18

carboxyl groups added to glutamine residues by vitamin K dependent carboxylation reaction. is important modification for blood clotting factors

Question 19

what is ubiquitination

Answer 19

addition of small polypeptide ubiquitin to a protein via a lysine residue to target proteins for degradation in the proteasome

Question 20

what proteins/enzymes are synthesized in the Er and modified and sorted through the golgi

Answer 20

secretory proteins, resident ER proteins, lysosomal enzymes

Question 21

signal sequences are important for transport where?

Answer 21

import into ER, retention in lumen of ER, import into mitochondria, import into nucleus, import into peroxisomes

Question 22

what proteins are transported into the ER?

Answer 22

transmembrane proteins, proteins found in the lumen or the ER or other membrane bound organelles, secreted proteins

Question 23

what is a signal sequence?

Answer 23

sequences that specify which organelle protein is targeted to

Question 24

steps of recruitment of ribosome to the ER (for translation of resident ER/secreted proteins)

Answer 24

1. ribosome begins translation mRNA, comes across ER signal sequence of hydrophobic/nonpolar AA via the signal-recognitition particle (SRP) which stalls translation
2. the signal recognition particle is recognized by SRP-receptor on the cytosol side of ER membrane and the SRP-ribosome/RNA to the surface of the ER at the Sec61 complex
3. translation continues through the translocator Sec61. this makes a “rough ER”
4. protein snakes into translocator via signal sequence, as polypeptide is made, is being pushed down into ER. peptidase cleaves signal protein which allows mature protein to enter ER lumen and be properly folded by chaperone proteins

Question 25

what is the difference between making a soluble resident ER/secreted protein and a transmembrane ER protein?

Answer 25

a hydrophobic stop transfer sequence tells translocator to stop (translating through that pore) and translation starts again on the outside of the ER

multiple hydrophobic regions result in more than 1 transmembrane region

Question 26

what if protein is mutated, chaperones are not working properly in the ER?

Answer 26

misfolded proteins bind to receptors in ER to activate transcription of more chaperone genes to properly fold the protein. if the process doesn’t resolve, cell can initiated apoptosis and die

Question 27

pathology of alpha-1-antitrypsin deficiency

Answer 27

mutation in SERPINA1 gene that encodes alpha-1-antitrypsin, a protein that inhibits elastase, and is made in the liver. causes protein to be misfolded in ER and activates UPR.
accumulation of unfolded proteins causes liver and lung damage.

Question 28

what does alpha-1-antitrrypsin do?

Answer 28

inhibits elastase, which breaks down elastin in the lung

Question 29

how are proteins transported into the nucleus and which are?

Answer 29

through the nuclear pore complex (nuclear porins) via facilitated transport requiring GTP hydrolysis. these require nuclear localization signals and chaperones (nuclear import receptors bind to NLS)
-important for moving from cytoplasm to nucleus

Question 30

what proteins are transported into the nucleus?

Answer 30

histones, transcription factors, polymerases

Question 31

how are proteins brought into the mitochondira?

Answer 31

outer membrane recognizes signal sequence, translocator called TOM complex (translocator on the outer membrane) and TIM complex (translocator on the inner membrane) (if needs to go to lumen of mitochondria)
this is powered by ATP and H+ gradient