A hitchhiker's guide to the cullin ubiquitin ligases: SCF and its kin

Abstract

The SCF (Skp1-Cullin-F-box) E3 ubiquitin ligase family was discovered through genetic requirements for cell cycle progression in budding yeast. In these multisubunit enzymes, an invariant core complex, composed of the Skp1 linker protein, the Cdc53/Cul1 scaffold protein and the Rbx1/Roc1/Hrt1 RING domain protein, engages one of a suite of substrate adaptors called F-box proteins that in turn recruit substrates for ubiquitination by an associated E2 enzyme. The cullin-RING domain-adaptor architecture has diversified through evolution, such that in total many hundreds of distinct SCF and SCF-like complexes enable degradation of myriad substrates. Substrate recognition by adaptors often depends on posttranslational modification of the substrate, which thus places substrate stability under dynamic regulation by intracellular signaling events. SCF complexes control cell proliferation through degradation of critical regulators such as cyclins, CDK inhibitors and transcription factors. A plethora of other processes in development and disease are controlled by other SCF-like complexes, including those based on Cul2-SOCS-box adaptor protein and Cul3-BTB domain adaptor protein combinations. Recent structural insights into SCF-like complexes have begun to illuminate aspects of substrate recognition and catalytic reaction mechanisms.

SCFCdc4-mediated Degradation of the Hac1p Transcription Factor Regulates the Unfolded Protein Response inSaccharomyces cerevisiae

“…cerevisiae cells contain at least 21 F-box-containing proteins (Willems et al, 2004). Of these, four (Cdc4p, Grr1p, Met30p, and Dsg1/Mdm30p) are components of SCF complexes required for degradation of various cell cycle regulatory proteins and transcription factors (for review, see Deshaies, 1999;Willems et al, 2004;Muratani et al, 2005).…”

“…It is noteworthy that the CPD motif contains the minimal consensus sequence S/T-P for phosphorylation by both cyclin-dependent kinases (CDKs) and mitogen-activated protein (MAP) kinases (Nigg, 1995), and indeed members of these kinase families are required for SCF Cdc4 -dependent degradation of several different substrates (for review, see Willems et al, 2004). Examination of the sequence of Hac1p revealed four S/T-P motifs, of which three (T 132 P, S 134 P, and S 146 P) lie within the PEST degron and are each embedded in the sequence context of a putative CPD ( Figure 3A).…”

“…Assays on deletion strain grr1⌬ and its BY4742 parent strain were performed as described in the legend to Figure 2B. Cdc53/Cul1 scaffold protein, the Skp1 adaptor protein, and an F-box protein that recruits substrates via a specific protein-protein interaction domain, such as a WD40 or leucinerich domain (for review, see Deshaies, 1999;Willems et al, 2004). We confirmed the involvement of an SCF complex in the degradation of Hac1p by measuring the stability of endogenous Hac1p in S. cerevisiae cells bearing temperaturesensitive defects in the Cdc53 or Skp1 proteins, which are core components of yeast SCF complexes (Deshaies, 1999;Xu et al, 2003;Willems et al, 2004;Vodermaier, 2004).…”

“…The data shown in Figure 6B (rows 1 and 2) demonstrate that the N-terminal 195 residues of Hac1p, like the N-terminal 47 amino acids of Cdc6p, interacted strongly with the WD40 repeat-containing fragment of Cdc4p, indicating that this region of Hac1p can act as a Cdc4-interaction domain in vivo. Deletion of the Hac1 PEST degron abrogated the interaction ( Figure 6B, row 3), supporting our earlier conclusion that the PEST sequence is necessary for Cdc4p-mediated degradation of Hac1p.All known substrates of SCF Cdc4 must be phosphorylated before they can be ubiquinated (for review, see Deshaies, 1997;Willems et al, 1999Willems et al, , 2004, and the WD40 repeat domain of Cdc4 binds with high-affinity to a consensus phosphopeptide motif I/L-I/L/P-pT-P-{K/R} 4 (where {} indicates disfavored residues) called the Cdc4 Phospho-Degron or CPD (Nash et al, 2001). It is noteworthy that the CPD motif contains the minimal consensus sequence S/T-P for phosphorylation by both cyclin-dependent kinases (CDKs) and mitogen-activated protein (MAP) kinases (Nigg, 1995), and indeed members of these kinase families are required for SCF Cdc4 -dependent degradation of several different substrates (for review, see Willems et al, 2004).…”

KEL-8 Is a Substrate Receptor for CUL3-dependent Ubiquitin Ligase That Regulates Synaptic Glutamate Receptor Turnover

“…Improper ubiquitination in the nervous system has been implicated in Parkinson's, Huntington's, and Alzheimer's diseases as well as in other neurological disorders, further supporting the importance of ubiquitination and protein turnover in proper nervous system function (Ehlers, 2004). Ubiquitination specificity is conferred by E3 ligases, including the diverse superfamily of Cullin-RING ligases (reviewed in Willems et al, 2004;Petroski and Deshaies, 2005). Cullin-RING ligases are complexes containing a RING protein, which recruits E2-conjugating enzymes, a Cullin scaffold protein, a substrate receptor, and sometimes an adaptor between the Cullin and substrate receptor.…”