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#cyclins

2 public questions tagged with this topic.

Cell Division

What is the role of cyclins in cell cycle regulation?

Cyclins constitute regulatory subunits that impose temporal order on constitutively expressed but inactive cyclin-dependent kinases. In mammals, D-type cyclins respond to mitogens via Ras-MAPK induction, E-type governs G1/S boundary, A-type controls S and G2 progression, B-type drives mitosis. Binding of cyclin to CDK induces conformational change in T-loop exposing activation site for phosphorylation by CAK complex CDK7-cyclin H-Mat1, stabilizing fully active kinase competent for substrate phosphorylation. Substrate specificity derives from cyclin hydrophobic patch interacting with RXL motifs and subcellular localization. Cyclin D-CDK4/6 phosphorylates Rb pocket proteins, cyclin E-CDK2 phosphorylates p27 and Treslin, cyclin A-CDK2 phosphorylates ORC and Cdc6 to block re-licensing, cyclin B-CDK1 phosphorylates lamins, condensins, and nuclear pore proteins to disassemble interphase structures. Ubiquitin ligases SCF and APC/C confer switch-like inactivation by degrading cyclins at distinct cell cycle phases, producing oscillations. Without cyclin partner, CDKs exhibit negligible kinase activity, explaining functional dependency. 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: Morgan, The Cell Cycle: Principles of Control, Chapter 3: Cyclin-CDK Regulation. Alberts 7th ed., Chapter 17.

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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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