Control of the SCFSkp2–Cks1 ubiquitin ligase by the APC/CCdh1 ubiquitin ligase

Abstract

Skp2 and its cofactor Cks1 are the substrate-targeting subunits of the SCF(Skp2-Cks1) (Skp1/Cul1/F-box protein) ubiquitin ligase complex that regulates entry into S phase by inducing the degradation of the cyclin-dependent kinase inhibitors p21 and p27 (ref. 1). Skp2 is an oncoprotein that often shows increased expression in human cancers; however, the mechanism that regulates its cellular abundance is not well understood. Here we show that both Skp2 and Cks1 proteins are unstable in G1 and that their degradation is mediated by the ubiquitin ligase APC/C(Cdh1) (anaphase-promoting complex/cyclosome and its activator Cdh1). Silencing of Cdh1 by RNA interference in G1 cells stabilizes Skp2 and Cks1, with a consequent increase in p21 and p27 proteolysis. Depletion of Cdh1 also increases the percentage of cells in S phase, whereas concomitant downregulation of Skp2 reverses this effect, showing that Skp2 is an essential target of APC/C(Cdh1). Expression of a stable Skp2 mutant that cannot bind APC/C(Cdh1) induces premature entry into S phase. Thus, the induction of Skp2 and Cks1 degradation in G1 represents a principal mechanism by which APC/C(Cdh1) prevents the unscheduled degradation of SCF(Skp2-Cks1) substrates and maintains the G1 state.

Mutant B-RAF signaling and cyclin D1 regulate Cks1/S-phase kinase-associated protein 2-mediated degradation of p27Kip1 in human melanoma cells

“…The mechanism of B-RAF-cyclin D1 effects on p27 Kip1 appears distinct from the Rb-mediated competition for Skp2 binding that also enhances p27 Kip1 levels (Ji et al, 2004) and a non-catalytic role for Cdk2 in the regulation of Skp2-p27 Kip1 complexes ( Zhu et al, 2004). As expression of Skp2 is regulated in a cell cycle-dependent manner (Bashir et al, 2004;Wei et al, 2004), knockdown of B-RAF or cyclin D1 likely prevents the expression of Skp2 in late G1/S. This notion is consistent with studies by showing that overexpression of p107 downregulates Skp2 levels in fibroblast cells (Rodier et al, 2005).…”

“…Thus, our data indicate that mutant B-RAF and cyclin D1 control expression of Cks1, which in turn stabilizes Skp2 levels. Regulation of Skp2 through this pathway likely occurs through inhibiting Skp2 ubiquitylation, and may involve the anaphasepromoting complex/cyclosome-Cdh1 complex that is known to degrade Skp2 (Bashir et al, 2004;Wang et al, 2004;Wei et al, 2004).…”

BubR1 is involved in regulation of DNA damage responses

“…As p21 is also degraded by the proteasome-mediated pathway (Bloom et al, 2003), it is interesting to know whether elevated APC/C Cdc20 activity would modulate its poly-ubiquitylation in BubR1-deficient cells. A recent study shows that APC/C Cdh1 participates in the regulation of SCF(Skp2-Cks1) activity, which in turn is responsible for the degradation of p21 and p27 (Bashir et al, 2004). As Skp2 is a target of APC/C Cdh1 , it would be interesting to test whether there is a cross-talk between APC/C Cdc20 and SCF.…”

Skp2 regulates the antiproliferative function of the tumor suppressor RASSF1A via ubiquitin-mediated degradation at the G1–S transition

“…The F-box protein Skp2 has been implicated in the ubiquitin-dependent degradation of p27, E2F-1, free cyclin E and FOXO1 (Marti et al, 1999;Nakayama et al, 2000;Huang et al, 2005). The APC-Cdh1 complex degrades Skp2 at the onset of G 1 phase, after which Skp2 begins to accumulate late in G 1 and its abundance peaks during S and G 2 phases (Bashir et al, 2004;Wei et al, 2004).…”