SCF complexes are a class of E3 ubiquitination ligase that facilitate proteasomal degradation through the catalysis of ubiquitinylation. The cell cycle relies on ubiquitin and the SCF complex plays an integral role in the regulation of this process.
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Ubiquitin ligase also regulates ubiquitinylation by stimulating the degradation of cell cycle proteins via the 26S proteasome. Dysregulation of this process can result in the progressive transformation of normal cells into cancer cells.
The Cell Cycle
Cell division (mitosis) occurs through the following phases, each of which is characterized by unique events.
- G1 phase
- S phase
- G2 phase
- M phase
In a somatic (body) cell, the cell cycle takes roughly 24 hours to completes, starting with G1. In G1, cells become enlarged, mRNA and new proteins are synthesized, and these are then used during the subsequent synthesis of DNA. DNA replication begins in S phase and requires between 10 and 12 hours to complete. DNA and protein synthesis continue throughout G2 phase. These three stages constitute interphase.
Quiescent and aging cells which are unable to replicate are said to be in resting phase or G0 where they might stay for an indefinite length of time. The process of mitosis takes place during M or mitotic phase where the chromosomes are separated into two nuclei and cytokinesis takes place.
Structure of the SCF complex
The structure of SCF ligases are variable due to the F-box protein (FBP) which determines the substrate specificity of SCF complex. FBPs mediate protein-protein interaction with roughly 50 amino acids within their structural motifs. There are approximately 70 different FBPs found in mammals.
S-Phase Kinase Associated Protein 2 (Skp2), an FBP involved in cell cycle regulation, is one of the most highly studied FBPs. Characterised by its leucine rich repeats, Skp2 belongs to the FBXL class of FBPs. There have three components: RING box protein 1 (Rbx1/Roc1, a ring-finger protein), S-Phase Kinase Associated Protein 1 (Skp1, an adaptor protein), and Cullin1 (Cul1, a scaffold protein). FBPs like Skp2 bind directly to the adaptor protein, Skp1, via an F-box motif.
Function of the SCF complex
The Cul1/Rbx1 components of the SCF complex work synergistically to catalyse the transfer of ubiquitin from E2 ubiquitin-conjugating enzymes. However, this complex requires activation by a protein called NEDD8. During interphase, a range of proteins bind to the SCF complex and begin to degrade.
At G1, retinoblastoma-like protein 2 is ubiquitinated. Other proteins, including CDC25A, p27KIP1, Cyclin E and Cdt1 are phosphorylated between the G1 and S by CDK2 which bind to the SCF complex. This prompts their subsequent degradation. As the cell cycle enters the mitotic stage, Wee1 (a G2 checkpoint kinase), is degraded alongside Emi1 (an early mitotic inhibitor) when bound to the SCF complex.