SCF ubiquitin ligases in the maintenance of genome stability

Abstract

In response to genotoxic stress, eukaryotic cells activate the DNA damage response (DDR), a series of pathways that coordinate cell cycle arrest and DNA repair to prevent deleterious mutations. In addition, cells possess checkpoint mechanisms that prevent aneuploidy by regulating the number of centrosomes and spindle assembly. Among these mechanisms, ubiquitin-mediated degradation of key proteins has an important role in the regulation of the DDR, centrosome duplication and chromosome segregation. This review discusses the functions of a group of ubiquitin ligases, the SCF (SKP1-CUL1-F-box protein) family, in the maintenance of genome stability. Given that general proteasome inhibitors are currently used as anticancer agents, a better understanding of the ubiquitylation of specific targets by specific ubiquitin ligases may result in improved cancer therapeutics.

Figure 1. The SCF complex

The SCF (SKP1/CUL1/F-box protein) complexes are assembled using a CUL1 scaffold. CUL1 recruits an E2 ubiquitin conjugating enzyme through RBX1. Substrates are recruited to the complex by SKP1 and a variable F-box protein that determines substrate specificity.

Figure 2. β-TRCP and the DNA damage response

(a) β-TRCP typically recognizes substrate degrons containing DSGxxS motifs, in which certain serines or threonines are phosphorylated (bold). Most substrates (such as β-catenin, IκBα, and claspin) contain the consensus motif, whereas other substrates (such as Bora and WEE1) have variant motifs. CDC25A is recognized through an alternate phospho-degron. The three initial residues shown, SSE, represent the priming kinase site for CDC25A and are not part of the true degron that dictates binding to β-TRCP. (b) The F-box protein β-TRCP controls DNA damage-induced cell cycle arrest, cessation of DNA damage signaling, and recovery from cell cycle arrest. β-TRCP leads to the degradation of CDC25A (a CDK1 activator) in a CHK1-dependent manner. Upon removal of genotoxic stress, Claspin (a co-activator for ATR-mediated phosphorylation of CHK1) and WEE1 are degraded in a β-TRCP- and PLK1-dependent manner to stop DNA damage signaling and allow exit from cell cycle arrest. Thus, β-TRCP is involved in DNA damage-induced cell cycle arrest (through CDC25 degradation), the cessation of DNA damage signaling (through CHK1 inhibition by Claspin degradation), and the recovery from DNA damage (via degradation of WEE1) through a feedback loop involving the regulation of multiple kinases. Notably, the key kinases for β-TRCP substrate degradation, CHK1 and PLK1, are also regulated by β-TRCP through the degradation of their activators (Claspin and Bora, respectively).

Figure 3. SCF ubiquitin ligases in centriole duplication and chomosome segregation

(a) PLK4, SAS6, and CP110 are SCF substrates that are required for centrosome duplication. PLK4 is targeted by β-TRCP; SAS6 is targeted by FBXW5; and CP110 is targeted by cyclin F. Further regulation of FBXW5 is provided by APC/C-mediated degradation. PLK4 has additional substrates, including FBXW5, which may also affect centrosome duplication. (b) REST, which controls the spindle assembly checkpoint (SAC) through transcriptional regulation of MAD2, is also targeted by β-TRCP.