16. Ubiquitin proteosome

Question 1

what 3 enzyme complexes are required for ubiquitnylation

Answer 1

E1
E2
E3

Question 2

what is the role of E1

Answer 2

ubiquitin activating enzyme

Question 3

what is the role of E2

Answer 3

conjugating enzyme

Question 4

what is the role of E3

Answer 4

ubiquitin ligase

Question 5

how does Ub become conjugated to E1

Answer 5

in an ATP dependent reaction

Question 6

E1 then transfers Ub to E2, how is this attached

Answer 6

covalently

Question 7

what does E2 and E3 then do with Ub

Answer 7

ubiquitinylated E2 binds to E3, the E2/E3 complex transfers Ub to the substrate

Question 8

the process of ubiquitinylation forms a ternary complex, what are the 3 subunits

Answer 8

E2
E2
and the substrate

Question 9

once E2 transfers Ub to the substrate, what forms

Answer 9

long chains of ubiquitin

Question 10

what recognises these chains of ubiquitin

Answer 10

the proteosome

Question 11

what happens once the proteosome recognises ubiquitin chains attached to the substrate

Answer 11

they are catalysed in the catalytic site and AAs are recycled

Question 12

what two factors regulate cell cycle transitions

Answer 12

cyclin-CDKs

proteosomes

Question 13

what maintains low CDK activity

Answer 13

G1 ubiquitin ligase

Question 14

what degrades Cdt1 to prevent re-replication

Answer 14

S-phase ubiquitin ligase

Question 15

what is S-phase ubiquitin ligase another word for

Answer 15

SCF

Question 16

what does SCF stand for

Answer 16

SKP-cullin-F-box

Question 17

what does APC stand for

Answer 17

anaphase promoting complex

Question 18

what regulated G1 phase

Answer 18

APC-Cdh1 = maintains low CDK activity

Question 19

an increase in CDK activity promotes what

Answer 19

activation of SCF

Question 20

how does SCF become activated

Answer 20

degradation of CDK inhibitor protein

destruction of Cdh1 promotes cyclin A accumulation (s-phase)

Question 21

how is CDH1 degraded

Answer 21

it is polyubiquitinylated by SCF

Question 22

what happens once CDH1 is degraded

Answer 22

the cell can now exit G1

Question 23

what is the difference between Sic1 and p27

Answer 23

Sic1 is found in S. cerevisiae

p27 is found in humans

Question 24

why is CDK activity low in G1 phase

Answer 24

to allow DNA licensing

- CDK inhibitor proteins ensure low CDK in G1

Question 25

name an inhibitor of CDK

Answer 25

p27

Question 26

why must p27 be degraded

Answer 26

it inhibits CDKs, so must be degraded before S phase where high CDK activity is required

Question 27

how is p27 degraded

Answer 27

it is phosphorylated and then targeted for degradation by SCF

SCF ubiquitinylates

Question 28

summarise the role APC-cDH1 during G1 phase

Answer 28

• APC-cDH1 activity ensures CDK inhibitors are high

- APC-cDH1 maintains SCF at low levels during G1

Question 29

summarise the role of the proteasome at the G1/S transition

Answer 29

p27 and cdt1 are phosphorylates and ubiquitnylated to increase CDK activity

increased CDK activity promotes entry into S-phase

= expression of S-phase cyclins

Question 30

name 2 s-phase cyclins

Answer 30

cyclin A and cyclin B

Question 31

what restricts MPF activity to the correct stages of the cell cycle

Answer 31

Wee1 and Cdc25

Question 32

what triggers mitosis

Answer 32

MPF

Question 33

what does MPF do

Answer 33

ensures chromosome adhesion

forms cohesin complex loops around sister chromatids

aids formation of mitotic spindle

Question 34

what does APC stand for

Answer 34

anaphase promoting complex

Question 35

what does APC do once activated

Answer 35

polyubiquitnylates securing = proteosomal degradation

this activates separase which cleaves apart the sister chromatids

Question 36

what does APC do during anaphase

Answer 36

degrades cyclin B via the proteosome = prevents MPF activity until the next cycle