Proteine und Kapitel 19

Those signalling Pathways monitor a cells nutrient status, size, attachment status and the density of surrounding cells

Cyclin B accumulates during G2-phase

By binding CDK1 it drives the cell into mitosis

G1: cyclin Ds

-bind to CDK4 and 6

-show no strong fluctuations

G1/S: cyclin E

-bind to CDK2

-accumulate during late G1, peak in S phase and decline during S phase

-trigger S phase transition

S: cyclin E and A

-bind to CDK2

-remain high throughout S phase

-responsible for DNA synthesis

M: cyclin A and B

-bind CDK1

-promote entry and progression trough mitosis

-snythesized during S phase and G2 but activity is held back until DNA synthesis is completed

Phosphorylation

-threonine in a critical activation loop is required for correct activity

-mediated by CAK

inhibitory Phosphorlyation

-tyrosine and threonine in a G-loop of CDKs

-Wee1 brings phosphorylation

-Cdc25 de-phosphorlyation

CKI

-CDK inhibitors

-E3-ligase

-substrates are phosphorylated to target them for degradation

-controls G1/S transition by targeting G1 and G1/S cyclins for degradation

-controls G2/M transition by targeting Wee1 and CDK inhibitory proteins for degradation

-APC/C itself is phosphorylated

-active at the onset of mitosis and remains through the next G1 phase

-targets both S phase and mitotic cyclins for degradation

-controls onset of chromosome segregation and promotes mitotic exit

-during G1 inhibits mitotic and S phase CDKs

-during anaphase transition APC/C is bound to Cdc20, targeting cyclin A and B

-during anaphase APC/C is bound to Cdc20, ubiquitinilating proteins that inhibit chromosome segregation

-during late anaphase, telophase and G1 APC/C bound to Cdh1 targets different proteins for degradation

These phosphoserine-/threonine-binding proteins bind to CDKs or it’s substrates and works either by targeting the bound proteins to specific subcellular locations, altering their three-dimensional confromation, changing their activity, or targeting them for specific modifications such as additional phosphorylation or ubiquitin-mediated destruction.

• These phospho-binding modules are necessary to form the positive and negative feedback loops that control progression through the cell cycle and for surveillance mechanisms that stop the cell cycle in response to catastrophic events.

-transcriptional repressor that gets exportet out of the nucleus, when certan nutrient levels are reached, allowing transcription of G1/S phase cyclin genes and others for DNA replication

-forward loop because from these genes it also gets phosphorylated

-50 of Whi5 has exited the nucleus => irreversably commited to division

-during G1 E2Fs are inactive through binding to Rb

-active G1 cyclin CDKs activate E2Fs through phosphorylation of Rb, freeing E2F

-E2F can activate genes involved with DNA synthesis and G1/S phase cyclin (cyclin E) and S phase cyclin (cyclin A)

-Cyclin E forms a complex with G1/S phase CDK (CDK2) which further phosphorylates Rb

=> positive forward loop

-Cdh1 with APC/C ubiquitinilate S phase and mitotic cyclins

-Cdh1 is phsphorylated by G1/S phase cyclin CDK complexes, disassociating from APC/C

=> rise of S phase cyclins

later mititic CDKs take over in inactivating Cdh1 until they get inactivated and Cdc14 gets activated and removes the inhibitory phosphorylation of Cdh1

There are five basic steps of genome replication:

1. recognizing the replication origins

2. assembling replication initiation factors that then load a replicative helicase in order to form a so-called prereplication complex (pre-RCs)

3. recruiting critical helicase cofactors

4. activation of the helicase to unwind the to DNA strands

5. recruitment of the replicative DNA polymerase machinery

Putting all together the key concept emerging is that the increase in CDK and DDK activity during S phase enforces the strict temporal separation of Pre-RC formation, which only happens during G1, from firing of the origins to start DNA replication, which only happens during S phase.

helicases are loaded in G1, when CDK activity is low. Phosphorylation of MCM helicase activators by G1/S phase CDKs and S phase CDKs activates the helicases and promotes polymerase loading. Phosphorylation of the MCM helicase-loading factors Cdc6 and Cdt1 prevents reloading of MCM helicases until the cell passes through mitosis, thereby ensuring that replication from each origin occurs only once during each cell cycle

Abrupt activation of mitotic CDKs at the G2/M transition is the consequence of rapid inactivation of Wee1 together with activation of Cdc25. Central to this are two positive feedback loops built from CDKs, polo-like kinase 1 (Plk1) and phosphoserine/threonine-binding domains, in which mitotic CDKs and Plk1 simultaneously activate Cdc25 and inactivate Wee1.