Lecture 17: Cell Cycle: Part 2

Cyclin-Cdk Complexes that Drive the Cell Cycle

• M-Cdk: Promotes entry into M phase

• G1/S-Cdk: Promotes entry into the cell cycle

• S-Cdk: Promotes entry into S phase

• G1/S-Cdks and S-Cdks act sequentially to control entry into S-phase

  • G1/S-Cdks, which are required for the G1/S transition, activate S-Cdks

  • S-Cdks trigger DNA replication

Levels of M-Cyclin: Embryonic vs. Non-embryonic Cells

• The cell cycle of the early embryonic divisions is faster

• This is due to division without growth (which is why cells get smaller)

• As a result, in early embryo divisions, M-cyclin levels rise immediatey upon the start of interphase

• This leads to mostly alternation between M and S

Control of Entry into the Cell Cycle (During G1)

• Upon exit from mitosis, levels of all cyclins are low

• Cells stay in G1, where they grow and perform their normal cellular functions

• Decision of whether to divide again (and therefore synthesize cyclins) is regulated by outside signals

Mitogens (Overview)

• Proteins produced by other cells that trigger the synthesis of G1/S cyclins and activation of G1/S-Cdk

• Called a “mitogen” because it activates mitosis

• A signaling cascade will trigger synthesis of G1/S-cyclins

• G1/S-Cdk is activated by binding to G1/S-cyclin

• The active G1/S-Cdk-cyclin complex stimulates the transcription of genes needed for S-phase, including S-cyclin

Mitogens (Detailed)

• Rb proteins bind to inhibit a crucial transcription factor

• When Rb is bound, this factor can’t stimulate transcription

• Once the signaling pathway (by binding of mitogens) is triggered, activated G1/S-Cdk phosphorylates Rb, inactivating Rb

• This allows the transcription factor to promote the synthesis of S-cyclin and other S-phase genes

  • First step in S-phase activation

Control of Entry into S-phase

• If mitogens are present, G1/S-Cdk is activated (b/c of G1/S-cyclin synthesis)

• This leads to the synthesis of S-cyclins (which will trigger DNA replication)

• S-Cdk-cyclin complexes will accumulate in G1

  • However, the S-Cdk-cyclin complexes are initially inactive

S-Cdk-cyclin Complexes Inactivation - CKI

• Initially, S-Cdk-cyclin complexes are inactive

• S-Cdk-cyclin complexes are bound by an inhibitor, so they can’t initially trigger S-phase

• “CKI” is a generic name for Cdk inhibitor proteins

• CKIs bind to Cdk-cyclin complexes and inhibit kinase activity

• The CKI that inhibits S-Cdk-cyclin complexes is p27

Destruction of p27 by G1/S-Cdk

• When G1/S-Cdk is activated, it phosphorylates p27

• Phosphorylated p27 can then be ubiquintinated by SCF (a “ubiquitin ligase”)

  • Note: SCF also facilitates degradation of G1/S-cyclins

• After ubiquitination, p27 is targeted for degradation by the proteasome

• Degradation of p27 activates the S-Cdk-cyclin complex

• The active S-Cdk complex can now trigger DNA replication

Initiation of DNA Replication by S-Cdk

• In G1, a “pre-replicative complex” (preRC) assembles at the origin of replication

• The PreRC contains many of the proteins needed for replication, but not all

• Once S-Cdk is activated, it recruits the rest of the replication machinery

• S-Cdk also phosphorylates some components of the preRC in order to prevent them from reloading onto origins

  • To prevent re-replication

Continuing On After S-Phase

• Once the DNA is fully replicated, S-phase is complete

• S-cyclin levels stay high for awhile because the cell wants S-Cdk to stay active to keep phosphorylating preRC components to prevent DNA re-replication

• After S-phase is complete, the cell begins synthesizing M-cyclin

Cell Cycle Transitions are Switch-like and Irreversible

• Mitogens get the process started by triggering the synthesis of G1/S and S-cyclins

• After that, a cascading series of events happens that no longer requires the mitogen

• Degradation of cyclins help make the cell cycle irreversible

Cohesin Removal Regulation

• Cohesin removal needs to be regulated such that it only happens at the metaphase-to-anaphase transition

• Cohesin removal is triggered by the same machinery that degrades M-cyclin

  • As a result, APC is required for both

• Process

  • Before anaphase, separase is held inactive by an inhibitor called securin

  • When the APC is activated, it ubiquitinates securin

  • This targets securin for degradation by the proteasome

  • Once released from inhibition, separase cleaves cohesin, triggering sister chromatid separation