1993). cyclin B amounts decrease on exit from M phase but Cdc2 levels remain constant. egg extracts The cyclin BCCdc2 kinase is usually a universal regulator of M phase (for review, observe Nurse 1990; Nigg 1995). Its activation induces access into M phase and its inactivation is necessary for exit from M phase. The activity of cyclin BCCdc2 kinase is usually regulated primarily by the formation of a complex between the catalytic Cdc2 subunit and the cyclin B regulatory subunit. This complex is usually stabilized by phosphorylation of Cdc2 on Thr 161, and is kept inactive by inhibitory phosphorylation of Cdc2 on Thr 14 and Tyr 15 (for evaluate, observe Nigg 1995). The amount of Cdc2 remains relatively constant throughout the cell cycle, whereas cyclin B accumulates during interphase, reaching a peak at metaphase, and is suddenly destroyed at the exit from M phase (for review, observe King et al. 1996; Townsley and Ruderman 1998). The differences in the stability of Cdc2 and cyclin Cdh15 B at the end of M phase is seen even though the two proteins are tightly associated in a complex prior to cyclin B degradation. Although CXCR2-IN-1 it has not been previously exhibited, it is probable that this cyclin BCCdc2 complex must dissociate prior to the degradation of cyclin B subunit so that Cdc2 might escape degradation. However, when the dissociation of cyclin B and Cdc2 takes place and the actual steps CXCR2-IN-1 involved in the regulation of this dissociation are not known. The degradation of the mitotic B-type cyclins is performed by ubiquitinCproteasome-mediated proteolysis. The N-terminal region of cyclin B contains a conserved motif called the destruction box, which serves as a signal for ubiquitination and is necessary for cell cycle-regulated proteolysis (Glotzer et al. 1991). The formation of ubiquitin conjugates requires the concerted activity of a series of enzymes that first activate ubiquitin (E1) and then identify and transfer ubiquitin (E2 and E3) to proteins destined for turnover (for evaluate, observe Hershko and Ciechanover 1998). Cyclin B is usually polyubiquitinated by a specific E3, the multicomponent 20S complex known as the APC/C (anaphase-promoting complex/cyclosome; for review, observe Townsley and Ruderman 1998). Neither the activities of E1 nor a specific E2 (E2-C or UBC10) for cyclin B ubiquitination switch during the cell cycle (Hershko et al. 1994; CXCR2-IN-1 King et al. 1995; Sudakin et al. CXCR2-IN-1 1995), but the E3-like activity of the APC/C is the target of cell cycle-dependent regulation depending on association with Cdc20 (for review, observe Townsley and Ruderman 1998; Morgan 1999). Even though APC/C-dependent polyubiquitination of cyclin B alone may occur in an in vitro reconstitution system (King CXCR2-IN-1 et al. 1995), it normally occurs in vivo in the cyclin BCCdc2 complex. However, it has not been determined whether the polyubiquitination by the APC/C is usually involved in dissociation of the cyclin BCCdc2 complex or only in targeting cyclin B for degradation. Polyubiquitinated proteins that are destined for turnover are acknowledged and degraded by the 26S proteasome (for review, observe Baumeister et al. 1998; Rechsteiner 1998). The 26S proteasome can be divided into three subcomplexes. A core subcomplex, the 20S proteasome, is usually a cylindrical stack consisted of four rings and exhibits proteolytic activity. Each end of the 20S cylinder is usually capped.