phase and G 2 /M phase transition events, and cyclin B is implicated at the G 2 /M transition [13,50–52]. Cy-
clin F, which is most closely related to A and B, also fluctuates with the cell cycle peaking at G 2 but it
lacks the destruction box motif of mitotic cyclins and contains PEST sequences as do G 1 cyclins [48]. Cy-
clins C, D, and E were identified in humans based on their ability to complement a yeast mutant lacking
G 1 -type (CLN) cyclins [52–54]. D cyclins do not fluctuate with the cell cycle but are responsive to growth
factors and nutrient supply, and cyclins C and E accumulate periodically, peaking at different times in G 1.
Cyclin C levels rise about twofold early in G 1 and decrease slowly through S, G 2 , and M phases [54].
Studies indicate that cyclin C may be associated with the transcription apparatus and may be involved in
relaying growth-regulatory signals [23]. Cyclin E peaks in late G 1 and is involved at the G 1 /S transition
[53]. Cyclin G has neither a destruction box nor a PEST sequence and does not fluctuate with the cell cy-
cle. Rather, it responds to growth stimuli and is a transcriptional target of the p53 tumor suppressor pro-
tein [55,56]. Cyclin H is most closely homologous to cyclin C and is implicated in the control of multi-
ple cell cycle transitions [32]. Cyclin I was isolated from human brain but also expressed in skeletal and
heart muscle [57]. Its expression is not cell cycle dependent. Cyclin K was isolated in a yeast two-hybrid
screen with Cdk9 [58]. Table 2 lists the cyclins in vertebrates, their known Cdk partners, and possible
function(s) of the cyclin-cdk complex.
III. ONSET OF M PHASE
Research in the past several years with yeast and mammalian systems using various approaches indicates
that the onset of M phase is regulated by a mechanism that is common to all eukaryotic cells [14,59]. The
regulation of the G 2 /M transition was the first to be elucidated and is the best understood in yeast and ver-
tebrates. Regulation is coupled to mechanisms monitoring time, cell mass, growth rate, and the comple-
tion of chromosome replication [19]. Entry into mitosis is characterized by a structural reorganization of
the cell including chromosome condensation, disassembly of the nuclear lamina and other intermediate
filament systems, arrest of membrane traffic and nuclear envelope breakdown, reorganization of micro-
tubules to form a mitotic spindle apparatus, and rearrangements of the actomyosin cytoskeleton for cell
rounding and cytokineses [19].
A. Key Proteins Involved in G 2 /M Phase Transition
- p34 Protein Kinase
p34 protein kinase was identified genetically in the fission yeast (Saccharomyces pombe) as the product
of a cell division cycle gene (cdc2) that encodes a 34-kDa protein [60–62]. Homologues of this gene have
been found in budding yeast (p34CDC28) [63–65], several vertebrates [15,22], invertebrates [4], and plants
[66,67] and are shown to be highly conserved both structurally and functionally among all eukaryotes.
p34 protein kinase genes from evolutionarily distant multicellular organisms including vertebrates and
plants have been shown to complement yeast mutants in this gene [22,66]. Hence, it is considered to be a
universal regulator of mitosis in eukaryotic cells [13,14,59]. In vertebrates, Cdk1/mitotic-cyclin complex
(MPF) is the center of regulation but other proteins are also involved (Figure 1). The molecular mecha-
nism of M-phase induction involves activation of Cdk1. When activated, kinase activity is directed
against serine and threonine residues in substrates. The consensus phosphorylation target is S/T-P-X-Z
(Xpolar amino acid, Z basic amino acid) [68]. Table 3 lists some substrates of Cdk/cyclin com-
plexes. Several of these are M-phase substrates, some of which have been shown to be in vivo as well as
in vitro substrates. Lamins, histone H1, nucleolin, caldesmon, and the regulatory light chain of myosin II
are examples of in vivo substrates involved in the M-phase transition [21,68].
- Cyclins and Regulation of p34 Protein Kinase Activity
Figure 1 is a model of MPF activation during the G 2 /M-phase transition. The level of p34 protein kinase
is fairly constant during the cell cycle of dividing cells in yeast [69] and vertebrates [38]. However, the
activity of this kinase increases significantly prior to the onset of M phase [13,14]. In animals, cyclin B
accumulates during G 2 , associates with Cdk1, and is abruptly destroyed at mitosis [70,71]. Cyclin B ac-
cumulation and association with Cdk1 have been shown to be required for MPF activation but are not suf-
ficient to activate MPF [70]. The activity of MPF is also regulated by phosphorylation and dephosphory-
232 REDDY AND DAY