16. Ubiquitin proteosome Flashcards
what 3 enzyme complexes are required for ubiquitnylation
E1
E2
E3
what is the role of E1
ubiquitin activating enzyme
what is the role of E2
conjugating enzyme
what is the role of E3
ubiquitin ligase
how does Ub become conjugated to E1
in an ATP dependent reaction
E1 then transfers Ub to E2, how is this attached
covalently
what does E2 and E3 then do with Ub
ubiquitinylated E2 binds to E3, the E2/E3 complex transfers Ub to the substrate
the process of ubiquitinylation forms a ternary complex, what are the 3 subunits
E2
E2
and the substrate
once E2 transfers Ub to the substrate, what forms
long chains of ubiquitin
what recognises these chains of ubiquitin
the proteosome
what happens once the proteosome recognises ubiquitin chains attached to the substrate
they are catalysed in the catalytic site and AAs are recycled
what two factors regulate cell cycle transitions
cyclin-CDKs
proteosomes
what maintains low CDK activity
G1 ubiquitin ligase
what degrades Cdt1 to prevent re-replication
S-phase ubiquitin ligase
what is S-phase ubiquitin ligase another word for
SCF
what does SCF stand for
SKP-cullin-F-box
what does APC stand for
anaphase promoting complex
what regulated G1 phase
APC-Cdh1 = maintains low CDK activity
an increase in CDK activity promotes what
activation of SCF
how does SCF become activated
degradation of CDK inhibitor protein
destruction of Cdh1 promotes cyclin A accumulation (s-phase)
how is CDH1 degraded
it is polyubiquitinylated by SCF
what happens once CDH1 is degraded
the cell can now exit G1
what is the difference between Sic1 and p27
Sic1 is found in S. cerevisiae
p27 is found in humans
why is CDK activity low in G1 phase
to allow DNA licensing
- CDK inhibitor proteins ensure low CDK in G1
name an inhibitor of CDK
p27
why must p27 be degraded
it inhibits CDKs, so must be degraded before S phase where high CDK activity is required
how is p27 degraded
it is phosphorylated and then targeted for degradation by SCF
SCF ubiquitinylates
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
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
name 2 s-phase cyclins
cyclin A and cyclin B
what restricts MPF activity to the correct stages of the cell cycle
Wee1 and Cdc25
what triggers mitosis
MPF
what does MPF do
ensures chromosome adhesion
forms cohesin complex loops around sister chromatids
aids formation of mitotic spindle
what does APC stand for
anaphase promoting complex
what does APC do once activated
polyubiquitnylates securing = proteosomal degradation
this activates separase which cleaves apart the sister chromatids
what does APC do during anaphase
degrades cyclin B via the proteosome = prevents MPF activity until the next cycle
