The cyclosome, a large complex containing cyclin-selective ubiquitin ligase activity, targets cyclins for destruction at the end of mitosis.

Abstract

The ubiquitin-mediated degradation of mitotic cyclins is required for cells to exit from mitosis. Previous work with cell-free systems has revealed four components required for cyclin-ubiquitin ligation and proteolysis: a nonspecific ubiquitin-activating enzyme E1, a soluble fraction containing a ubiquitin carrier protein activity called E2-C, a crude particulate fraction containing a ubiquitin ligase (E3) activity that is activated during M-phase, and a constitutively active 26S proteasome that degrades ubiquitinated proteins. Here, we identify a novel approximately 1500-kDa complex, termed the cyclosome, which contains a cyclin-selective ubiquitin ligase activity, E3-C. E3-C is present but inactive during interphase; it can be activated in vitro by the addition of cdc2, enabling the transfer of ubiquitin from E2-C to cyclin. The kinetics of E3-C activation suggest the existence of one or more intermediates between cdc2 and E3-C. Cyclosome-associated E3-C acts on both cyclin A and B, and requires the presence of wild-type N-terminal destruction box motifs in each cyclin. Ubiquitinated cyclins are then rapidly recognized and degraded by the proteasome. These results identify the cyclosome-associated E3-C as the component of the cyclin destruction machinery whose activity is ultimately regulated by cdc2 and, as such, the element directly responsible for setting mitotic cyclin levels during early embryonic cell cycles.

An ATM- and ATR-dependent checkpoint inactivates spindle assembly by targeting CEP63

“…Plk1 is also thought to play a role in priming sister chromatids for separase cleavage by phosphorylating the SCC1 cohensin subunit after spindle assembly checkpoint inactivation (van Gugt and Medema, 2005). During metaphase APC/Cdc20 also ubiquitylates Cyclin B, the following destruction of Cyclin B inactivates Cdk1 (Glotzer et al, 1991;Sudakin et al, 1995;Peters, 2006).…”

Two F‐box/WD‐repeat proteins Pop1 and Pop2 form hetero‐ and homo‐complexes together with cullin‐1 in the fission yeast SCF (Skp1‐Cullin‐1‐F‐box) ubiquitin ligase

“…In addition to a classical E3 such as Ubr1, and the HECT domaincontaining proteins (Huibregtse et al 1995;Varshavsky 1996), novel E3s have been identified as crucial regulatory factors for cell cycle progression. These include the APC/cyclosome (the Anaphase Promoting Complex, Hershko et al 1994;Irniger et al 1995;King et al 1995;Sudakin et al 1995) and the SCF complex (Skp1-Cullin-1/Cdc53-F-box, Feldman et al 1997;Skowyra et al 1997).…”

Prostate carcinoma cell death resulting from inhibition of proteasome activity is independent of functional Bcl-2 and p53

“…A condensed list of these moderators includes various cyclins (Sudakin et al, 1995), the tumor suppressor protein p53 , cdc25 (Nefski and Beach, 1996), c-mos (Ishida et al, 1993) and the cyclin-dependent kinase inhibitor p27 Kip1 (Pagano et al, 1995). The proteasome also regulates the timed degradation of important transcription factors including c-jun (Trier et al, 1994), c-myc (Ciechanover et al, 1991) and STAT1 (Kim and Maniatis, 1996).…”