16. Ubiquitin proteosome Flashcards

1
Q

what 3 enzyme complexes are required for ubiquitnylation

A

E1
E2
E3

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2
Q

what is the role of E1

A

ubiquitin activating enzyme

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3
Q

what is the role of E2

A

conjugating enzyme

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4
Q

what is the role of E3

A

ubiquitin ligase

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5
Q

how does Ub become conjugated to E1

A

in an ATP dependent reaction

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6
Q

E1 then transfers Ub to E2, how is this attached

A

covalently

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7
Q

what does E2 and E3 then do with Ub

A

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

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8
Q

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

A

E2
E2
and the substrate

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9
Q

once E2 transfers Ub to the substrate, what forms

A

long chains of ubiquitin

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10
Q

what recognises these chains of ubiquitin

A

the proteosome

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11
Q

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

A

they are catalysed in the catalytic site and AAs are recycled

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12
Q

what two factors regulate cell cycle transitions

A

cyclin-CDKs

proteosomes

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13
Q

what maintains low CDK activity

A

G1 ubiquitin ligase

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14
Q

what degrades Cdt1 to prevent re-replication

A

S-phase ubiquitin ligase

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15
Q

what is S-phase ubiquitin ligase another word for

A

SCF

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16
Q

what does SCF stand for

A

SKP-cullin-F-box

17
Q

what does APC stand for

A

anaphase promoting complex

18
Q

what regulated G1 phase

A

APC-Cdh1 = maintains low CDK activity

19
Q

an increase in CDK activity promotes what

A

activation of SCF

20
Q

how does SCF become activated

A

degradation of CDK inhibitor protein

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

21
Q

how is CDH1 degraded

A

it is polyubiquitinylated by SCF

22
Q

what happens once CDH1 is degraded

A

the cell can now exit G1

23
Q

what is the difference between Sic1 and p27

A

Sic1 is found in S. cerevisiae

p27 is found in humans

24
Q

why is CDK activity low in G1 phase

A

to allow DNA licensing

- CDK inhibitor proteins ensure low CDK in G1

25
name an inhibitor of CDK
p27
26
why must p27 be degraded
it inhibits CDKs, so must be degraded before S phase where high CDK activity is required
27
how is p27 degraded
it is phosphorylated and then targeted for degradation by SCF SCF ubiquitinylates
28
summarise the role APC-cDH1 during G1 phase
- APC-cDH1 activity ensures CDK inhibitors are high | - APC-cDH1 maintains SCF at low levels during G1
29
summarise the role of the proteasome at the G1/S transition
p27 and cdt1 are phosphorylates and ubiquitnylated to increase CDK activity increased CDK activity promotes entry into S-phase = expression of S-phase cyclins
30
name 2 s-phase cyclins
cyclin A and cyclin B
31
what restricts MPF activity to the correct stages of the cell cycle
Wee1 and Cdc25
32
what triggers mitosis
MPF
33
what does MPF do
ensures chromosome adhesion forms cohesin complex loops around sister chromatids aids formation of mitotic spindle
34
what does APC stand for
anaphase promoting complex
35
what does APC do once activated
polyubiquitnylates securing = proteosomal degradation this activates separase which cleaves apart the sister chromatids
36
what does APC do during anaphase
degrades cyclin B via the proteosome = prevents MPF activity until the next cycle