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#SCF complex

3 public questions tagged with this topic.

Cell Cycle Regulation

The SCF complex plays a crucial role in:

SCF complex, acronym for SKP1-CUL1-F-box protein, represents prototypical cullin-RING ubiquitin ligase family controlling G1/S transition through regulated proteolysis of CDK inhibitors. Core scaffold comprises elongated Cullin1 linking adaptor SKP1 and RING domain protein Rbx1 that recruits E2 ubiquitin-conjugating enzyme. Substrate specificity arises from interchangeable F-box proteins containing leucine-rich repeats or WD40 domains that bind phosphorylated degrons. In budding yeast, F-box Cdc4 recognizes Sic1 inhibitor after phosphorylation by Cln-CDK1 at multiple CDK consensus sites, while mammalian Skp2 with cofactor Cks1 recognizes p27Kip1 phosphorylated at Thr187 by cyclin E-CDK2 following mitogen signaling. SCF-catalyzed K48 polyubiquitination directs Sic1 and p27 to 26S proteasome, relieving inhibition of S-phase CDKs. This degradation derepresses cyclin E-CDK2 mediated Rb phosphorylation and origin firing, initiating DNA synthesis. Feedback loop where CDK activity promotes inhibitor destruction creates bistable switch ensuring irreversible S entry. Overexpression of Skp2 in cancers prematurely lowers p27, accelerating proliferation. 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, 2nd ed., Chapter 5: SCF Ligases and CDK Inhibitors.

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Cell Cycle Regulation

The SCF complex is primarily responsible for:

SCF complex forms prototypical cullin-RING ubiquitin ligase governing G1/S and S-phase progression, distinct from mitotic APC/C. Architecture comprises scaffold Cullin1, adaptor Skp1, RING finger Rbx1 recruiting E2 ubiquitin conjugating enzyme, and interchangeable F-box protein providing substrate specificity through WD40 or leucine-rich repeats. Activity regulated by substrate phosphorylation creating phosphodegron that interacts with F-box. During late G1, Cdk2-Cyclin E phosphorylates cyclin-dependent kinase inhibitor p27 Kip1 at Thr187, Sic1 ortholog in yeast at multiple CDK sites and p21 at Ser130, allowing recognition by F-box Skp2 with cofactor Cks1 and subsequent Lys48 polyubiquitination and proteasomal degradation. Removal lifts inhibition on CDK2-Cyclin E and CDK2-Cyclin A permitting origin firing. Similarly, Cdc6 phosphorylated for nuclear export and SCF-mediated clearance, E2F1 turned over after DNA replication. Unlike APC/C which targets D-box substrates during mitosis after activation by Cdc20/Cdh1, SCF depends on prior kinase marking, linking CDK activity and growth signals to degradation. Tumor suppressor Fbw7 component targets Cyclin E, Myc, Notch; mutations accumulate oncogenic substrates contributing to cancers. 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: Cardozo & Pagano, SCF Ubiquitin Ligases Regulate Cell Cycle, Nat Rev Mol Cell Biol 2004; Frescas & Pagano, SCF Functions in G1/S Control.

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Cell Cycle Regulation

What is the function of the SCF complex?

Ordered progression through interphase depends on timely degradation of regulatory proteins via ubiquitination. The SCF complex, meaning Skp1-Cullin-F-box, is a modular cullin-RING E3 ligase active from late G1 to early M. Core components include scaffold Cullin1, adaptor Skp1, RING finger Rbx1 and interchangeable F-box protein conferring substrate specificity. Canonical function involves CDK inhibitor p27 Kip1. When Cyclin E-CDK2 phosphorylates p27 at Thr187, phosphodegron is recognized by F-box protein Skp2 together with accessory Cks1, leading to Lys48-linked polyubiquitination and proteasomal destruction. Similar processing removes p21, p57, Sic1 in yeast and Cdc6, E2F1. Degradation eliminates CDK inhibition, allowing CDK2-Cyclin E and CDK2-Cyclin A activation for origin firing and S-phase progression. In contrast, APC/C recognizes D-box and KEN-box substrates in mitosis. SCF therefore links CDK-mediated phosphorylation to irreversible elimination, creating forward-driven cycle. Alterations in Skp2 and Fbw7 F-box proteins cause accumulation of cyclin E and Myc contributing to tumorigenesis and uncontrolled proliferation associated with poor prognosis. 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: Cardozo & Pagano, SCF Ubiquitin Ligases and Cell Cycle Control, Nat Rev Mol Cell Biol 2004; NCBI, SCF Functions.

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