Which molecule directly targets p27 for ubiquitin-mediated degradation?
Abundance of p27 is controlled post-translationally rather than transcriptionally, illustrating CDK-regulated proteolysis that sharpens G1/S transition. Late in G1, cyclin E-CDK2 phosphorylates p27 intrinsically disordered C-terminus at threonine 187. This phosphate forms high-affinity interaction with WD40 domain of Skp2, F-box protein assembled into SCF-Skp2 E3 ligase with Skp1 adaptor, Cullin1 scaffold, Rbx1 RING finger recruiting Ubc3 E2 enzyme, plus Cks1 cofactor that bridges CDK and F-box. SCF catalyzes formation of K48-linked polyubiquitin chains on p27 lysines, marking it for rapid degradation by 26S proteasome within minutes. Transcriptional induction of Skp2 via Myc and PI3K pathways by mitogens reinforces degradation. Unlike mitotic cyclins degraded by APC/C, p27 depends distinctly on SCF family ensuring its destruction paced directly by CDK activity itself, generating positive feedback. Persistent p27 due to Skp2 loss causes G1 arrest, while Skp2 overexpression reduces p27 in many cancers. 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: Carrano et al., Nature Cell Biology 1999, SCF-Skp2 Targets p27. Alberts 7th ed., Chapter 3.