The way components of SPF are regulated is being elucidated. An Rb-like protein has been tied to
control of cyclin A expression [201]. Protein kinase C has been implicated in the regulation of CAK ac-
tivity toward cyclin/Cdk2 complexes [202]. In human fibroblasts, the activation of protein kinase C in
late G 1 causes cell cycle arrest at least in part through down-regulation of CAK-mediated Cdk2 phos-
phorylation. The suppression of CAK activity was accompanied by a decrease in the message levels of
both cyclin H and Cdk7 (the components of CAK).
Cyclin C is a third cyclin type isolated by rescue of G 1 cyclin minus yeast [54]. Its activity had been
assigned to G 1. An eighth Cdk (Cdk8) was isolated in a search for human protein kinases with a possible
role in cell cycle control [23]. This kinase was shown to associate with cyclin C in vitro and probably in
vivo. The cyclin C/Cdk8 pair is structurally related to SRB10-SRB11, a Cdk/cyclin pair shown to be a
part of the RNA polymerase II holoenzyme of S. cerevisiae[139]. It is proposed that cyclin C/Cdk8 might
be functionally associated with the mammalian transcription apparatus and perhaps be involved in relay-
ing growth-regulatory signals [23].
- Inhibitors of Cyclin/Cdk Complexes
As discussed before, the sequential formation, activation, and subsequent inactivation of a series of cy-
clin/Cdk complexes (Figure 2) govern progression of cells through the cell cycle. Studies have identified
regulatory proteins that bind to cyclin/Cdk complexes and inhibit their activity. These proteins are termed
Cdk inhibitory proteins (CKIs) [203]. The first CKIs were identified in yeast [204–206]. CKIs with dif-
ferent roles have also been identified in vertebrates. One of these CKIs, p21, was identified in three sep-
arate studies. It was isolated as a protein that interacted with cyclin/Cdk2 complexes and inhibited their
activity [207], a protein whose expression was increased in senescent cells [208], and a protein that could
be induced by the tumor repressor p53 [209]. p21 inhibits cyclin A/Cdk2, cyclin A/Cdk1, cyclin E/Cdk2,
cyclin D1/Cdk4, and cyclin D2/Cdk4 complexes [85,210]. The tumor suppressor p53 is involved in reg-
ulation of p21. The p21 promoter has a p53 binding site and p21 transcription is activated by wild-type
p53, which appears to be essential to the p53-mediated arrest of the cell cycle in G 1 in response to DNA
damage [200]. The tumor suppressor pRb might also be transcriptionally activated by p53 and there may
be a direct protein-protein interaction between p53 and pRB [85]. In light of the phosphorylation of pRb
by cyclin/Cdk complexes causing the release of transcription factor E2F at the G 1 /S transition, p53 may
be involved in arrest by increasing the p21 inhibitor of cyclin/Cdk complexes, increasing the amount of
pRb and directly interacting with it and in some way limiting its ability to be phosphorylated. Various vi-
ral oncogene products can promote cell growth by abrogating activity of either p53 or pRb. p21 is also
regulated in other ways. Three studies in muscle indicate that p21 is up-regulated in a non–p53-depen-
dent manner during skeletal muscle differentiation [211–213] or by signal transducers and activator of
transcription (STAT) proteins.
A second CKI, p27, also interacts with multiple Cdks including cyclin E/Cdk2, cyclin D/Cdk4, and
cyclinA/Cdk2 complexes [22,214]. The antimitogenic factor transforming growth factor (TGF) in-
duces reversible arrest of target cells in late G 1. This arrest has been correlated with inhibition of the cy-
clin E/Cdk2 complex [215], and in cells arrested by TGF, cyclin E/Cdk2 is associated with p27 [216].
Cyclin D/Cdk4 complexes also interact with p27 and may sequester it [217]. As reviewed by Sherr [191],
p27 is titrated by cyclin D/Cdk complexes, inhibiting them until a threshold is exceeded. TGFinhibits
Cdk4 synthesis, which would raise the effective level of “free” p27 allowing inhibition of cyclin E/Cdk2
activity.
Both p21 and p27 preferentially associate with the cyclin/Cdk complex rather than with the individ-
ual kinases. Two closely related proteins, p16 and p15, that are structurally and functionally distinct from
p21 and p27 target Cdk4 and Cdk6 subunits and prevent their binding to cyclins [22]. p16 and p15 are
representatives of a family of 15 to 20-kDa proteins with loosely conserved ankyrin motifs, some mem-
bers of which have been isolated and are differentially expressed in response to a number of antiprolifer-
ative signals [185].
B. G 1 /S Phase Plant Proteins
Studies indicate that plants have several distinct p34-like protein kinases [218,219]. Two cdc2 homo-
logues from alfalfa (CDC2A and CDC2B) appear to regulate different phases of the cell cycle. CDC2A
could complement only G 2 /M transition, whereas CDC2B complemented G 1 /S function [219]. A study
CELL CYCLE REGULATION IN PLANTS 243