F-box proteins

The SCF ubiquitylates proteins from late Gl to early M phase. This complex consists of a core together with different so called F-box proteins, which are responsible for the substrate recognition. Typically... 

Skowyha, D., et al., F-box proteins are receptors that recruit phosphorylated substrates to the SCF ubiquitin-ligase complex. Cell, 1997, 91(2), 209-19. 

Lisztwan, j., et al., Association of human CUL-1 and ubiquitin-conjugating enzyme CDC34 with the F-box protein p45(SKP2) evidence for evolutionary conservation in the subunit composition of the CDC34-SCF pathway. Embo J, 1998, 37/2/, 368-83. 

A model of the full SCF/E2 complex [112] shows that the end of Skp2 which binds the substrate is pointed toward the Rbxl-bound E2, with a 50-A gap between the two. Models based on two other SCF structures show similar distances between the F-box protein and the E2 [109, 115]. Whether this gap can be bridged by the bound substrate is currently unclear. It has been suggested that the E2 may bind to Rbxl somewhat differently than UbcH7 is observed to bind in the c-Cbl RING/ UbcH7 complex, but it is not obvious that this can lead to a 20 A movement of the E2 toward the bound substrate as suggested [109]. 

Patton, E. E., Willems, A. R., Sa, D., Kuras, L, Thomas, D., Craig, K. L, and Tyers, M. Cdc53 is a scaffold protein for multiple Cdc34/Skpl/F-box protein complexes that regulate cell division and methionine biosynthesis in yeast. Genes Dev. 1998, 32, 692-705. 

The essential components of the SCF ubiquitin E3 ligase include Skpl, Cul-1/ Cdc53, one of many F-box proteins, and the RINC-H2-finger protein Rod (Rbxl or Hrtl) (Figure 6.2). Although initial studies did not reveal the presence of a fourth component of the SCF complex [14, 15], later work showed that a RINC-H2-finger protein, Rod, is an essential subunit of the SCF complex [3]. The SCF complex thus contains three invariable components (Rod, Cull, and Skpl) which provide a core structure to which one of the many substrate-specific subunits (F-box proteins) binds. The Rocl-Cull-Skpl core also independently interacts with the ubiquitin E2-conjugating enzyme to couple ubiquitin transfer to the substrates [3]. One of the E-box proteins binds directly to a specific substrate and such interaction facilitates the polyubiquitination of the substrate by ubiquitin... 

Skpl serves as an adaptor protein that provides a molecular link between Cull/ Rod and the F-box proteins [4, 5]. The Skpl protein contains two separate protein-interaction domains that are conserved among its family members between species [21]. The N-terminal region of Skpl (- l-70 a.a.) interacts with Cull while the C-terminal half (100-163 a.a.) binds the F-box proteins [21]. The use of Skpl as an adaptor to link the core ubiquitin E3 ligase components of Cull/Rocl with numerous and diverse substrate-targeting subunits, the F-box proteins, represents a strategy to specifically target many proteins for ubiquitination... 

The role of Skpl is to bring the substrate-targeting subunit, the F-box protein, into proximity with the Cull/Rocl/E2 complex to promote ubiquitin transfer from the E2-ubiquitin to the F-box protein-bound substrates. 

F-box proteins serve as the substrate-targeting subunit of the SCF ubiquitin E3 ligase [5]. They are structurally diverse but they all contain a relatively conserved signature motif of about 45-50 amino acids [5]. This motif, the F-box, was initially... 

The substrate recognition mechanisms discussed above suggest that phosphorylation at a particular site (or sites) is sufficient to bind the F-box proteins Skp2 or fS-... 

However, it is still possible that other mechanisms may exist to bridge the gap between substrate and E2 in the SCF-mediated ubiquitin-transfer reaction. For example, reports suggest that SCF may form higher order structures to facilitate the degradation of protein substrates. The S. pombe F-box proteins Popl and Pop2 have been shown to form heterodimers, and evidence suggests that these interactions may be important for the degradation of their in vivo substrates [62]. 

Several mechanisms have been shown to regulate the activity of the SCF complex. The expression of F-box proteins such as Skp2 is regulated by cell-cycle-dependent transcription [4]. The expression of Skp2 is high in late Gl, S, and G2/M phase but... 

Another level of control is mediated through the control of F-box protein stabilities by the SCF complex using an auto-ubiquitination mechanism. Deletion of the F-box motif of various F-box proteins such as yeast Cdc4 or j5-Trcp abolishes the interaction between Skpl/Cull and the F-box proteins [66]. Consequently the F-box proteins become more stable. This regulation may provide a means to recycle the components of SCF complexes between different F-box proteins. In addition, the levels of a particular F-box protein may be in part regulated by the balance between autoubiquitination and substrate-specific ubiquitination and thus could be sensitive to the presence of the substrates. 

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