89in a stable state of intermediate activity, but only in one of the two extreme steady
states—‘on’ or ‘off’ (Trunnell et al. 2011 ). Studies in frog egg extracts suggest that
Cdk1 stimulates the activity of Xenopus Polo-like kinase 1 (Plx1) that in turn ampli-
fies the autoamplification loop by acting in a stimulatory and inhibitory manner on
Cdc25 and Myt1, respectively (Abrieu et al. 1998 ). Cdk1/cyclin-B activation begins
in the cytoplasm, most prominently at centrosomes. Yet, to induce nuclear envelope
breakdown (NEBD), Cdk1/cyclin-B must accumulate in the nucleus, and this
depends on the phosphorylation of cyclin-B at a nuclear export signal sequence
resulting in decreased export rates (Yang et al. 1998 ). Notably, like its activation,
nuclear translocation of Cdk1/cyclin-B at late prophase occurs in an abrupt manner.
Recent studies identified a spatial positive feedback where nuclear Cdk1/cyclin-B
promotes the translocation of Cdk1/cyclin-B from the cytoplasm into the nucleus
(Santos et al. 2012 ). Thus, the irreversible and switch-like onset of M-phase seems
to be mediated by positive feedback loops acting on both the activity and localisa-
tion of Cdk1/cyclin-B.
3.3 Control of the Cell Cycle Through Ubiquitin-Mediated
Proteolysis
Exit from mitosis depends on the inactivation of Cdk1/cyclin-B, which is primarily
mediated by the destruction of cyclin-B via the ubiquitin-proteasome pathway.
Selective ubiquitylation of cyclin-B in mitosis requires the ubiquitin E3 ligase
anaphase- promoting complex/cyclosome (APC/C) (Irniger et al. 1995 ; King et al.
1995 ; Sudakin et al. 1995 ). The APC/C is a multisubunit complex made up of 14
known proteins, including the really interesting new gene (RING) finger protein
Apc11 and the Cullin-like subunit Apc2. Unlike homologous to E6AP C-terminus
(HECT) ligases, the APC/C does not form a thioester bond with ubiquitin but rather
promotes the direct transfer of ubiquitin from the E2 to substrate proteins. Poly-
ubiquitylation of APC/C substrates occurs in a sequential manner using two differ-
ent E2 enzymes: UbcH10 (Xenopus: UbcX) binds to the RING domain of Apc11
and initiates the ubiquitylation of substrate proteins by conjugating ubiquitin to the
ε-amino group of a lysine residue in the substrate protein (Brown et al. 2014 ),
whereas Ube2S catalyses chain elongation by assembling Lys11-linked ubiquitin
chains (Jin et al. 2008 ; Kelly et al. 2014 ). The mitotic APC/C is activated through
association with either of the two known activator subunits—cell division cycle 20
(Cdc20 also known as Fizzy (FZY)) and Cdc20 homologue 1 (Cdh1 also known as
fizzy related (FZR)) (Visintin et al. 1997 ). Both activators bind substrates through
their WD-40 propeller repeats, which recognise three motifs—the destruction box
(D-box) (consensus: RXXLXXXXN) and/or the KEN box (consensus:
KENXXXN) and/or the ABBA motif (Di Fiore et al. 2015 ; Pfleger and Kirschner
2000 ; Glotzer et al. 1991 ). The core APC/C subunit, Apc10, together with Cdc20 or
Cdh1, forms the bipartite substrate receptor.
3 Regulation of Cell Division