18.2 Regulators of Cell Cycle Progression Flashcards
what were the 3 experiments that led to the discovery of maturation promotion factor (MPF)
- frog oocyte
- yeast genetics
- sea urchin embryos
summarize the findings of the frog oocyte experiment
frog oocytes could be induced to enter M phase by microinjection of cytoplasm from oocytes that have already been hormonally stimulated → a cytoplasmic factor present in hormonally stimulated oocytes was enough to trigger progression from G2 → M
in the frog oocyte experiment, what was the factor that was found to induce progression from G2 to M phase
maturation promotion factor (MPF)
what led to the conclusion that MPF is a general regulator of transition from G2 to M
MPF was found to be present in somatic cells, where it induces entry in to M phase
summarize the findings of the yeast genetics experiment
temperature-sensitive cdc28 mutants were found to be arrested at the START regulatory point; cdc28 protein was required to progress past START
cdc genes (cdc28 in S. cerevisiae, cdc2 in S. pombe) were found to encode (), which is a conserved cell cycle regulator in all eukaryotes
a protein kinase called Cdk1
summarize the findings of the sea urchin embryo experiments
Hunt and colleagues identified 2 proteins (cyclin A and cyclin B) that accumulate throughout sea urchin embryo interphase but are rapidly degraded at the end of each mitosis → suggests that cyclins have a role in inducing mitosis
further findings of sea urchin embryo experiment
- cyclin A was analogous to MPF
- MPF was composed of a regulatory cyclin B subunit and a catalytic Cdk1 subunit
further findings of the sea urchin embryo experiment found that () was analogous to MPF
cyclin A
further findings of the sea urchin embryo experiment found that MPF was composed of (1) and (2)
- regulatory cyclin B subunit
- catalytic Cdk1 subunit
in association with Cdk1, drive G2 → M transition
mitotic B-type cyclins
in assocation with Cdk1, control passage through START
G1 cyclins (Clns)
other () - association with Cdk1 is required for progression through S phase
B-type cyclins
cell cycles of higher eukaryotes are also controlled by multiple Cdk1-related protein kinases, known as ()
Cdks (cyclin-dependent kinases)
activity of Cdks is regulated by binding of inhibitory proteins called ()
Cdk inhibitors (CKIs)
in mammalian cells, there are 2 CKI families: ()
Ink4 and Cip/Kip
provide a link between growth factor signaling and cell cycle progression
D-type cyclins
explain how D-type cyclins link growth factor signaling and cell cycle progression
- growth factors can induce the synthesis of cyclin D through the Ras/Raf/MEK/ERK pathway
- D cyclins are rapidly degraded by APC/C ubiquitin ligase → in the absence of growth factors, D cyclins have low concentrations in the cell
- mutations in this mechanism can lead to cancer
The () protein is a substrate protein of Cdk4,6/cyclin D comlpex
Rb
The Rb protein is a substrate protein of ()
Cdk4,6/cyclin D complex
the Rb protein is a prototype ()
tumor suppressor gene
inactivation of () gene leads to tumor development
tumor suppressor
proteins encoded by tumor suppressor genes act as breaks that slow down cell cycle progression via ()
regulation of transcription
control of Rb by () couples the regulation of gene expression to the availability of growth factors in G1
Cdk4,6/cycD phosphorylation
Rb activity is regulated by changes in phosphorylation → Rb is phosphorylated by Cdk4,6/cycD complexes as cells pass through the ()
G1 restriction point
underphosphorylated Rb (present in G0 or early G1) binds to members of the () → regulates expression of genes involved in cell cycle progression
E2F family of transcription factors
explain how the E2F family of transcription factors regulates expression of genes involved in cell cycle progression
- cyclin E is expressed by E2F transcription factors
- E2F binds to target sequence regardless of whether Rb is present or not
Rb acts as a repressor → Rb/E2F complex suppresses transcription of ()
E2F-regulated genes
how does Rb phosphorylation lead to activation of E2F-regulated genes
Rb phosphorylation leads to dissociation of Rb/E2F complexes; Rb no longer acts as a suppressor
progression through G1 restriction point and entry into S phase is mediated by activation of ()
Cdk2/cyclin E complexes
high levels of (1) and (2) drive progression through S and G2
- cyclins
- Cdk2 activity
Cdk2/cycE activity is inhibited in G0 or early G1 by the CKI () (part of the Cip/Kip family)
p27
Cdk2 inhibition by p27 is relieved by several mechanisms as cells progress through G1:
- growth factor signaling reduces both transcription and translation of p27
- Cdk2 activation brings about the complete degradation of p27 by phosphorylating it → p27 becomes a target for ubiquitylation
- positive autoregulation further activates Cdk2/cycE, which also phosphorylates and inactivates APC/C ubiquitin ligase → prevents cycE degradation
Cdk2/cycE complexes initiate S phase by activating DNA synthesis at ()
replication origins
it is important that once a segment of DNA has been replicated in the S phase, control mechanisms must ()
prevent the reinitiation of DNA replication until the cell cycle has been completed
DNA replication is initiated by the activity of ()
MCM helicase proteins
MCM helicase binds to () found at replication origins during G1
origin recognition complex (ORC) proteins
MCM/ORC complex remains inactive as a () throughout G1 until it is activated by the action of Cdk2/cycE (as the cell enters the S phase)
pre-replication complex
how does Cdk2/cycE activate the MCM/ORC complex
Cdk2/cycE phophorylates activating proteins that are recruited to the MCM/ORC complex
APC/C inhibition (by Cdk2/cycE activation) leads to activation of () (protein kinase), which directly phosphorylates MCM
DDK
() during S, G2, and M phases prevents MCM proteins from re-associating with replication origins
high activity of Cdks
cell cycle arrest at DNA damage checkpoints is mediated by () (protein kinases) → recognize damaged DNAs
ATM and ATR
ATR is activated by ()
single-stranded or unreplicated DNA
ATM is activated by ()
double-strand breaks
ATR and ATM phosphorylate and activate (), respectively
Chk1 and Chk2 (checkpoint kinases)
Chk1/Chk2 activations both inhibit/induce degradation of () via phosphorylation
Cdc25
Chk1/Chk2 activations both inhibit/induce degradation of Cdc25 via ()
phosphorylation
Cdc25 inhibition leads to inhibition of (1) and (2)
- Cdk2 (cell cycle arrest in G1 and S)
- Cdk1 (cell cycle arrest in G2)
in mammalian cells, arrest is also mediated by the (), which is phosphorylated by both ATM and Chk2
p53 transcription factor protein
increased p53 levels lead to induction of CKI (), which inhibits complexes of Cdk2 with cycE or cycA
p21
