Skp, Cullin, F-box containing complex (or SCF complex) is a multi-protein E3 ubiquitin ligase complex that catalyzes the ubiquitination of proteins destined for 26S proteasomal degradation. [1] Along with the anaphase-promoting complex, [2] SCF has important roles in the ubiquitination of proteins involved in the cell cycle. The SCF complex also marks various other cellular proteins for destruction. [3]
SCF contains a variable F-box protein and three core subunits:
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 [6] —arrested in G1 with unreplicated DNA and multiple elongated buds. [7] The phenotype was attributed to a failure to degrade Sic1, an inhibitor of S cyclin-CDK complexes. [6] 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. [6] 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 [8] 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. [8]
The eukaryotic cell cycle [9] 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. [1]
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:
There are approximately seventy human FBPs, several of which are involved in cell cycle control as a component of SCF complexes. [10]
Skp2 is an FBP that binds CKIs such as p27Kip1 and p21. [11] Skp2 binds p27Kip1 only when two conditions are met: p27Kip1 is phosphorylated by E/A/CKD2 and bound to Cks1. As a consequence of binding Skp2, p27Kip1 is ubiquitinated and targeted for degradation in late G1 and early S. [4] 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. [2] 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. [4] Fbw7 is stable throughout the cell cycle [12] and is localized to the nucleus due to the presence of a nuclear localization sequence (NLS). [13] SCF-Fbw7 targets Sic1—when at least six out of nine possible sites are phosphorylated—and Swi5 for degradation. [14] 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. [15]
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. [4]
Cyclin F is an FBP that is associated with amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD). [16] [17] 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. [17] Normally, Cyclin F targets E2f1 for degradation.
Recently, SCF complexes have become an attractive anti-cancer target because of their upregulation in some human cancers and their biochemically distinct active sites. [18] Though many of the aforementioned FBPs have been implicated in cancer, cytotoxicity has been a limiting factor of drug development. [19]
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 cells. [19]
βTRCP-targeting siRNAs have been shown to sensitize breast cancer cells and cervical cancer cells to existing chemotherapies. [19]
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. [20]
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. [21]
Anaphase-promoting complex 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 cullin (Apc2) and RING (Apc11) subunit much like SCF. Other parts of the APC/C have unknown functions but are highly conserved.
A 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 from the E2 to the protein substrate. In simple and more general terms, the ligase enables movement of ubiquitin from a ubiquitin carrier to another thing by some mechanism. The ubiquitin, once it reaches its destination, ends up being attached by an isopeptide bond to a lysine residue, which is part of the target protein. E3 ligases interact with both the target protein and the E2 enzyme, and so impart substrate specificity to the E2. Commonly, E3s polyubiquitinate their substrate with Lys48-linked chains of ubiquitin, targeting the substrate for destruction by the proteasome. However, many other types of linkages are possible and alter a protein's activity, interactions, or localization. Ubiquitination by E3 ligases regulates diverse areas such as cell trafficking, DNA repair, and signaling and is of profound importance in cell biology. E3 ligases are also key players in cell cycle control, mediating the degradation of cyclins, as well as cyclin dependent kinase inhibitor proteins. The human genome encodes over 600 putative E3 ligases, allowing for tremendous diversity in substrates.
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.
S-phase kinase-associated protein 2 is an enzyme that in humans is encoded by the SKP2 gene.
Cullin 1, also known as CUL1, is a human protein and gene from cullin family. This protein plays an important role in protein degradation and protein ubiquitination.
RING-box protein 1 is a protein that in humans is encoded by the RBX1 gene.
F-box/WD repeat-containing protein 1A (FBXW1A) also known as βTrCP1 or Fbxw1 or hsSlimb or pIkappaBalpha-E3 receptor subunit is a protein that in humans is encoded by the BTRC gene.
F-box/WD repeat-containing protein 7 is a protein that in humans is encoded by the FBXW7 gene.
CDC34 is a gene that in humans encodes the protein Ubiquitin-conjugating enzyme E2 R1. This protein is a member of the ubiquitin-conjugating enzyme family, which catalyzes the covalent attachment of ubiquitin to other proteins.
F-box only protein 5 is a protein that in humans is encoded by the FBXO5 gene.
Cyclin-dependent kinases regulatory subunit 1 is a protein that in humans is encoded by the CKS1B gene.
Cullin 3 is a protein that in humans is encoded by the CUL3 gene.
βTrCP2 is a protein that in humans is encoded by the FBXW11 gene.
F-box/LRR-repeat protein 2 is a protein that in humans is encoded by the FBXL2 gene.
G2/mitotic-specific cyclin-F is a protein that in humans is encoded by the CCNF gene.
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 diverse and play a role in myriad cellular processes, most notably protein degradation by ubiquitination.
S-phase kinase-associated protein 1 is an enzyme that in humans is encoded by the SKP1 gene.
Cdc4 is a substrate recognition component of the SCF ubiquitin ligase complex, which acts as a mediator of ubiquitin transfer to target proteins, leading to their subsequent degradation via the ubiquitin-proteasome pathway. Cdc4 targets primarily cell cycle regulators for proteolysis. It serves the function of an adaptor that brings target molecules to the core SCF complex. Cdc4 was originally identified in the model organism Saccharomyces cerevisiae. CDC4 gene function is required at G1/S and G2/M transitions during mitosis and at various stages during meiosis.
Cdh1 is one of the substrate adaptor protein of the anaphase-promoting complex (APC) in the budding yeast Saccharomyces cerevisiae. Functioning as an activator of the APC/C, Cdh1 regulates the activity and substrate specificity of this ubiquitin E3-ligase. The human homolog is encoded by the FZR1 gene, which is not to be confused with the CDH1 gene.
Raymond Joseph Deshaies is an American biochemist and cell biologist. He is senior vice president of global research at Amgen and a visiting associate at the California Institute of Technology (Caltech). Prior to that, he was a professor of biology at Caltech and an investigator of the Howard Hughes Medical Institute. He is also the co-founder of the biotechnology companies Proteolix and Cleave Biosciences. His research focuses on mechanisms and regulation of protein homeostasis in eukaryotic cells, with a particular focus on how proteins are conjugated with ubiquitin and degraded by the proteasome.
{{cite journal}}: Cite journal requires |journal= (help)
