#G1/S transition

Cell Division

Which of the following proteins is responsible for regulating the G1/S transition?

G1/S transition also known as restriction point in mammals marks commitment to DNA replication independent of extracellular mitogens thereafter. Regulation centers on retinoblastoma protein phosphorylation cascade. Mitogens induce cyclin D-CDK4/6 via Ras-MAPK pathway which initiates partial phosphorylation of Rb family proteins pRb, p107, p130 at specific serine sites, displacing histone deacetylases and allowing transcription of cyclin E gene driven by E2F1-3. Cyclin E then associates with CDK2, activated by CAK phosphorylation, forming cyclin E-CDK2 holoenzyme that hyperphosphorylates Rb at additional sites including Thr373, Ser612, causing complete dissociation and full E2F-dependent transcription of S-phase genes encoding cyclin A, MCM2-7, PCNA, DNA polymerase alpha and delta, thymidine kinase, and dihydrofolate reductase. Cyclin E-CDK2 also phosphorylates p27Kip1 Thr187 creating phosphodegron for SCF-Skp2 ubiquitination establishing positive feedback loop making transition switch-like irreversible. Loss of cyclin E arrests cells in G1 despite mitogen presence, proving essential role in governing G1/S progression. 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: Dulic et al., Cell 1992, Cyclin E-CDK2 at G1/S. Alberts 7th ed., Chapter 17, Restriction Point Control.

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Cell Cycle Regulation

In yeast, which protein is the functional equivalent of Rb in regulating the G1/S transition?

In mammalian cells, Rb represses E2F until cyclin D-CDK4/6 phosphorylates pocket domain, but budding yeast Saccharomyces cerevisiae lacks Rb ortholog and uses functionally analogous repressor Whi5 to regulate Start checkpoint analogous to Restriction Point. Whi5 contains CDK phosphorylation sites and binds heterodimeric transcription factor SBF composed of Swi4 DNA-binding protein recognizing SCB elements and Swi6 coactivator. In early G1 when CDK activity low, unphosphorylated Whi5 resides in nucleus, physically interacting with SBF at promoters of CLN1, CLN2, CLB5, and many cell wall synthesis genes, recruiting histone deacetylases Hos3 and Rpd3 to block transcription. As cell grows, Cln3 cyclin synthesized proportionally to size binds Cdc28 CDK1 and phosphorylates Whi5, triggering export via Msn5 karyopherin. SBF liberated induces Cln1 and Cln2 that further phosphorylate Whi5 in positive feedback loop analogous to cyclin E-CDK2 hyperphosphorylating Rb. Nuclear exit defines commitment to replication and budding, illustrating convergent regulatory logic. 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: Cross et al., Annu Rev Cell Dev Biol 2007, Yeast G1/S Control. NCBI Bookshelf: Whi5 Rb Analog.

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Cell Cycle Regulation

Which Cyclin-CDK complex is responsible for the G1/S transition?

Transition from growth phase G1 into DNA synthesis S phase requires activation of gene expression program and licensing factor conversion. In early G1, Cyclin D-CDK4/6 partially phosphorylates Rb after mitogen stimulation allowing limited E2F activity producing Cyclin E. Cyclin E accumulation reaches zenith in late G1 forming active kinase with CDK2. Cyclin E-CDK2 completes hyperphosphorylation of Rb at distinct sites including Ser612 Ser807, fully dissociating Rb from E2F1-E2F3 activators. Liberated E2F-DP heterodimers bind promoters with TTTCCCGC motifs driving transcription of replication genes such as Cyclin A, dihydrofolate reductase, thymidylate synthase, PCNA, Cdc6, CDT1, MCM helicase subunits and DNA polymerase alpha delta epsilon. Simultaneously Cyclin E-CDK2 phosphorylates Kip1 inhibitor p27 at Thr187 creating phosphodegron for SCF-Skp2-Cks1 ubiquitination and degradation, eliminating CDK2 inhibition. It also phosphorylates Treslin TICRR and RecQL4 facilitating Cdc45-MCM-GINS helicase activation for origin firing. Expression transient because SCF-Fbw7 targets autophosphorylated Cyclin E for destruction, restricting activity window. Amplification frequently observed in ovarian and breast cancers driving replication stress and genomic 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, Cyclin E-CDK2 Complex and G1/S Transition Control, Cell Cycle 2002; Alberts et al., Chapter 17, G1/S Regulator.

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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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