SCF complex

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Skp, Cullin, F-box containing complex (or SCF complex) is a multi-protein E3 ligase complex catalyzing the ubiquitination of proteins destined for proteasomal degradation. It has important roles in the ubiquitylation of proteins involved in the cell cycle as well as having a role in the marking various other cellular proteins for destruction. ^[1]

SCF contains three core subunits, and a number of less critical components:

F-box - (Such as cdc4) Contributes to the specificity of SCF by aggregating to target proteins independantly of the complex and and then binding to the Skp1 component, thus allowing the protein to be brought into proximity with the functional E2 protein. The F-box 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, F-box affinity for protein substrates is regulated through cdk/cyclin mediated phosphorylation of target proteins.
Skp1 - Bridging protein, forms part of the horseshoe-shaped complex, along with cullin (cul1). Skp1 is essential in the recognition and binding of the F-box.
Cullin (Cul1) forms the major structural scaffold of the SCF complex, linking the skp1 domain with the Rbx1 domain.
Rbx1 - Rbx1 contains a small zinc-binding domain called the RING Finger, to which the E2-ubiquitin conjugate binds, allowing the transferal of the ubiquitin to a cysteine residue on the target protein.