#CDK

Cell Cycle Regulation

Which kinase is involved in regulating the initiation of DNA replication in S phase?

Initiation of DNA replication at licensed origins depends on two kinase families that separate licensing from firing. DDK kinase complex Dbf4-Cdc7 phosphorylates MCM2-7 helicase at N-terminal serine-threonine clusters, promoting Cdc45 recruitment. Second trigger is S-phase CDK activity supplied by cyclin E-CDK2 that peaks at G1/S border and cyclin A-CDK2 that sustains activity into S phase. Cyclin E-CDK2 phosphorylates Treslin/TICRR at threonine 969, MTBP, RecQL4, and orthologs of yeast Sld2 and Sld3, enabling their binding to BRCT repeats of TopBP1 and assembly of Cdc45-MCM-GINS active helicase known as CMG that unwinds duplex DNA and recruits DNA polymerases alpha, delta, epsilon, and Ctf4 for leading and lagging strand synthesis. CDK2 also phosphorylates licensing factors Cdc6 and Cdt1 for nuclear export and SCF-dependent proteolysis to prevent re-licensing. CDK4/6 acts indirectly in early G1 to induce cyclin E transcription via Rb phosphorylation. Inhibition of CDK2 blocks origin firing without affecting cyclin D accumulation. This circuitry is highly conserved across eukaryotes, integrating growth factor signals, DNA damage surveillance, and developmental cues, and its disruption frequently underlies oncogenesis, providing targets for checkpoint inhibitors and cancer therapeutics.

Ref: Labib, Genes & Dev 2010, Origin Activation. Limas & Cook, Genes & Dev 2019, CDK Roles S Phase.

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Cell Cycle Regulation

What is the role of Cyclins in the cell cycle?

Cyclins earned name due to fluctuating concentrations through cell cycle while CDKs remain relatively constant. Function is to activate CDKs by allosteric remodeling and to direct substrate choice through additional binding motifs. In G1, Cyclin D with CDK4/6 senses mitogens, Cyclin E with CDK2 triggers S-phase entry, Cyclin A with CDK2 drives S-phase progression and later with CDK1 primes mitosis, Cyclin B with CDK1 executes mitosis. Each cyclin contains conserved cyclin box domain and hydrophobic patch recognizing RXL motifs on substrates, plus localization signals determining nuclear or centrosomal enrichment. Binding causes CDK PSTAIRE helix rotation aligning catalytic lysine glutamate salt bridge and opening activation segment for substrate accommodation. Resulting holoenzyme phosphorylates diverse targets including Rb family proteins for transcription derepression, lamins for envelope breakdown, condensins for chromosome compaction, replication factors for origin firing control and microtubule associated proteins for spindle assembly. Ubiquitination by SCF in interphase and APC/C in mitosis ensures periodic disappearance of cyclins, creating irreversible forward direction. Thus cyclins convert basal low affinity kinase into temporal specific effector.

Ref: Murray, Cyclins: Roles in Cell Cycle and Evolution, Cell; Alberts et al., Molecular Biology of the Cell, Chapter 17, Cyclin-CDK Functions.

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Cell Cycle Regulation

Which kinase initiates DNA replication by phosphorylating helicase activators?

Authentic initiation of DNA synthesis requires coordinated activation of MCM2-7 helicase loaded at licensed origins. Two S-phase kinases perform this: Cdc7-Dbf4 complex called DDK phosphorylates N-terminal tails of MCM2, MCM4 and MCM6 promoting Cdc45 recruitment, while CDK2-Cyclin E and CDK2-Cyclin A phosphorylate Treslin, Ticrr, RecQL4 and TopBP1 creating phospho-binding sites for assembling replication machinery including Cdc45-MCM-GINS active helicase and Pol epsilon recruitment. Entry dataset incorrectly lists p53 as initiating kinase; biologically p53 acts opposite as genome guardian. Upon DNA damage, ATM/ATR stabilizes p53 by phosphorylation disrupting Mdm2 binding, p53 transactivates CDK inhibitor p21 which blocks CDK2-Cyclin E and consequently Rb phosphorylation and origin firing. Thus p53 enforces arrest rather than initiation. In unstressed cycles, rising CDK2 activity after E2F-driven Cyclin E transcription provides permissive signal for origin firing. Therefore genuine replication initiation kinases are DDK and CDK2, while p53 functions to pause cycle for repair, preventing propagation of lesions into S phase. Additional feedback loops involving polo-like kinases, phosphatases and SCF-mediated degradation reinforce irreversibility and protect against premature progression that would compromise genome integrity and viability.

Ref: Labib, Mechanism of DNA Replication Initiation by DDK and CDK, Science 2010; NCBI Bookshelf, Regulation of Origin Firing by CDK2.

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Cell Cycle Regulation

The G1/S transition is tightly regulated by:

Commitment to S phase involves transcriptional and posttranslational steps driven by Cyclin E-CDK2 activity pulse. During early G1, Cyclin D-CDK4/6 initiates partial Rb phosphorylation after mitogen induction, allowing modest E2F-dependent synthesis of Cyclin E. Rising Cyclin E binds CDK2, fully activating kinase normally restrained by inhibitors p21 and p27. Cyclin E-CDK2 hyperphosphorylates Rb at additional sites, liberating large pool of E2F1-3 which amplify expression of Cyclin A, Cdc6, MCM helicase components, dihydrofolate reductase and Pol alpha. Concurrently Cyclin E-CDK2 phosphorylates p27 at Thr187 creating phosphodegron recognized by SCF-Skp2-Cks1, leading to degradation and reinforcement of CDK2 activity. It also phosphorylates NPAT regulator of histone genes, Cdc45 recruitment factors Treslin and RecQL4, directly promoting origin firing and replication complex assembly. Cyclin E levels then fall through autophosphorylation induced recognition by SCF-Fbw7 ligase, producing transient spike. Amplification of Cyclin E in cancers causes premature S entry, centrosome overduplication, replication stress and chromosome instability. Additional feedback loops involving polo-like kinases, phosphatases and SCF-mediated degradation reinforce irreversibility and protect against premature progression that would compromise genome integrity and viability.

Ref: Hinds & Weinberg, Cell Cycle Control by Cyclin E-CDK2, Curr Opin Cell Biol; Alberts et al., Chapter 17, G1/S Transition.

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