Skp, Cullin, F-box containing complex (or SCF complex) is a multi-protein E3
ubiquitin ligase
A ubiquitin ligase (also called an E3 ubiquitin ligase) is a protein that recruits an E2 ubiquitin-conjugating enzyme that has been loaded with ubiquitin, recognizes a protein substrate, and assists or directly catalyzes the transfer of ubiquitin ...
complex that catalyzes the
ubiquitin
Ubiquitin is a small (8.6 kDa) regulatory protein found in most tissues of eukaryotic organisms, i.e., it is found ''ubiquitously''. It was discovered in 1975 by Gideon Goldstein and further characterized throughout the late 1970s and 1980s. Fo ...
ation of proteins destined for 26S
proteasomal degradation.
Along with the
anaphase-promoting complex
Anaphase-promoting complex (also called the cyclosome or APC/C) is an E3 ubiquitin ligase that marks target cell cycle proteins for degradation by the 26S proteasome. The APC/C is a large complex of 11–13 subunit proteins, including a cull ...
,
SCF has important roles in the
ubiquitination
Ubiquitin is a small (8.6 kDa) regulatory protein found in most tissues of eukaryotic organisms, i.e., it is found ''ubiquitously''. It was discovered in 1975 by Gideon Goldstein and further characterized throughout the late 1970s and 1980s. Fo ...
of proteins involved in the cell cycle. The SCF complex also marks various other cellular proteins for destruction.
Core components
SCF contains a variable F-box protein and three core subunits:
*
F-box protein
F-box proteins are proteins containing at least one F-box domain. The first identified F-box protein is one of three components of the SCF complex, which mediates ubiquitination of proteins targeted for degradation by the 26S proteasome.
Core c ...
(FBP) – FBP contributes to the substrate specificity of the SCF complex by first aggregating to target proteins independently of the complex. Each FBP (e.g. Skp2) may recognize several different substrates in a manner that is dependent on post-translational modifications such as phosphorylation or glycosylation. FBP then binds to Skp1 of the SCF complex using an F-box motif, bringing the target protein into proximity with the functional E2 ubiquitin-conjugating enzyme. FBP is also essential in regulating SCF activity during the course of the cell cycle. SCF levels are thought to remain constant throughout the cell-cycle. Instead, FBP affinity for protein substrates is regulated through cyclin-CDK-mediated phosphorylation of target proteins.
*
Skp1
S-phase kinase-associated protein 1 is an enzyme that in humans is encoded by the ''SKP1'' gene.
This gene encodes a protein that is a member of the SCF ubiquitin ligase protein complex. It binds to F-box proteins (proteins containing an F-box ...
– Skp1 is an adaptor protein that is essential for the recognition and binding of F-box proteins.
*
Cullin
Cullins are a family of hydrophobic scaffold proteins which provide support for ubiquitin ligases (E3). All eukaryotes appear to have cullins. They combine with RING proteins to form ''Cullin-RING ubiquitin ligases'' (CRLs) that are highly dive ...
(
CUL1) – Cullin forms the major structural scaffold of the SCF complex and links the skp1 domain to the Rbx1 domain. Different combinations of Cullin and FBPs can generate on the order of a hundred types of E3 ubiquitin ligases that target different substrates.
*
RBX1
RING-box protein 1 is a protein that in humans is encoded by the ''RBX1'' gene.
Function
This gene encodes an evolutionarily conserved protein that interacts with cullins. The protein plays a unique role in the ubiquitination reaction by heter ...
– Rbx1 contains a small, zinc-binding Really Interesting New Gene (RING) finger domain, to which the E2 ubiquitin-conjugating enzyme binds. This binding event allows the transferral of ubiquitin from E2 to a lysine residue on the target protein.
Discovery
The first hint that led to the discovery of the SCF complex came from genetic screens of ''Saccharomyces cerevisiae'', also known as budding yeast. Temperature-sensitive cell division cycle (Cdc) mutants—such as Cdc4, Cdc34, and Cdc53
—arrested in G1 with unreplicated DNA and multiple elongated buds. The phenotype was attributed to a failure to degrade Sic1, an inhibitor of S cyclin-CDK complexes.
These findings indicated that proteolysis is important in the G1/S transition.
Next, biochemical studies revealed that Cdc34 is an E2 enzyme that physically interacts with an E3 ubiquitin ligase complex containing Skp1, Cdc4, and several other proteins.
Skp1’s known binding partners—specifically Skp2, Cyclin F, and Cdc4—were found to share an approximately 40 residue motif that was coined the F-box motif. The F-box hypothesis
that followed these discoveries proposed that F-box proteins recruit substrates targeted for degradation, and that Skp1 links the F-box protein to the core ubiquitination complex.
Subsequent genetic studies in ''Caenorhabditis elegans'' later contributed to the elucidation of other SCF complex components.
Cell cycle regulation
The eukaryotic cell cycle is regulated through the synthesis, degradation, binding interactions, post-translational modifications of regulatory proteins. Of these regulatory proteins, two ubiquitin ligases are crucial for progression through cell cycle checkpoints. The anaphase-promoting complex (APC) controls the metaphase-anaphase transition, while the SCF complex controls G1/S and G2/M transitions. Specifically, SCF has been shown to regulate centriole splitting from late telophase to the G1/S transition.
SCF activity is largely regulated by post-translational modifications. For instance, ubiquitin-mediated autocatalytic degradation of FBPs is a mechanism of decreasing SCF activity.
Well-characterized cell cycle substrates of SCF complexes include:
* cyclin family proteins: Cyclin D, Cyclin E
* transcriptional regulators: Myc, E2f1, p130
* cyclin-dependent kinase inhibitors (CKIs): p27
Kip1, p21,
Wee1
Wee1 is a nuclear kinase belonging to the Ser/Thr family of protein kinases in the fission yeast ''Schizosaccharomyces pombe'' (''S. pombe'')Wee1has a molecular mass of 96 kDa and is a key regulator of cell cycle progression.
It influences ...
* centriole proteins: Cep250, Ninein
There are approximately seventy human FBPs, several of which are involved in cell cycle control as a component of SCF complexes.
Skp2 is an FBP that binds CKIs such as p27
Kip1 and p21. Skp2 binds p27
Kip1 only when two conditions are met: p27
Kip1 is phosphorylated by E/A/CKD2 and bound to Cks1. As a consequence of binding Skp2, p27
Kip1 is ubiquitinated and targeted for degradation in late G1 and early S.
SCF-Skp2 also targets p130 for degradation in a phosphorylation dependent manner.
Beta-transducin repeat-containing protein (βTRCP) is an FBP that targets emi1—an APC/C-Cdh1 inhibitor—and wee1 for degradation during early mitosis.
βTRCP recognizes these substrates after they are phosphorylated by Polo-like kinase 1 or Cyclin B-CDK1.
Fbw7, which is the human homolog of cdc4 in yeast, is an FBP that targets Cyclin E, Myc, Notch and c-Jun for degradation.
Fbw7 is stable throughout the cell cycle and is localized to the nucleus due to the presence of a nuclear localization sequence (NLS). SCF-Fbw7 targets Sic1—when at least six out of nine possible sites are phosphorylated—and Swi5 for degradation. Since Sic1 normally prevents premature entry into S-phase by inhibiting Cyclin B-CDK1, targeting Sic1 for degradation promotes S-phase entry. Fbw7 is known to be a haplo-insufficient tumor suppressor gene implicated in several sporadic carcinomas, for which one mutant allele is enough to disturb the wild type phenotype.
Fbxo4 is another tumor suppressor FBP that has been implicated in human carcinomas. SCF-fbxo4 plays a role in cell cycle control by targeting cyclin D1 for degradation.
Cyclin F is an FBP that is associated with amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD).
Mutations that prevent phosphorylation of Cyclin F alter the activity of SCF-Cyclin F, which likely affects downstream processes pertinent to neuron degeneration in ALS and FTD.
Normally, Cyclin F targets E2f1 for degradation.
Cancer
Recently, SCF complexes have become an attractive anti-cancer target because of their upregulation in some human cancers and their biochemically distinct active sites. Though many of the aforementioned FBPs have been implicated in cancer, cytotoxicity has been a limiting factor of drug development.
Skp2-targeting anti-sense oligonucleotides and siRNAs are in the drug development pipeline. Preliminary studies have shown that Skp2 downregulation can inhibit the growth of melanomas, lung cancer cells, oral cancer cells, and
glioblastoma
Glioblastoma, previously known as glioblastoma multiforme (GBM), is one of the most aggressive types of cancer that begin within the brain. Initially, signs and symptoms of glioblastoma are nonspecific. They may include headaches, personality ch ...
cells.
βTRCP-targeting siRNAs have been shown to sensitize breast cancer cells and cervical cancer cells to existing chemotherapies.
Plant hormone signaling
The plant hormone auxin binds Tir1 (Transport Inhibitor Response 1). Tir1 is an Auxin Signaling F-box Protein (AFB) that acts as an auxin receptor. Auxin-bound Tir1 stimulates binding of SCF-Tir1 to the AUX/IAA repressor. Subsequent degradation of the repressor results in activation of AUX/IAA (i.e. auxin-responsive) genes.
The plant hormone Jasmonate binds Coi1, an FBP. SCF-Coi1 then binds the JAZ transcription factor and targets it for degradation. Degradation of the JAZ transcription factor allows for the transcription of the jasmonate responsive genes.
References
Image:SCF_Morgan.jpg, Morgan, David "Protein Degradation in Cell-Cycle Control", The Cell Cycle; Principles of Control 2007
Image:APC_B-TrcP.jpg, Hartmut C. Vodermaier (2004). "APC/C and SCF: Controlling Each Other and the Cell Cycle", Current Biology, 14 (787)
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Enzymes