negatively regulating their activities. Phosphorylation of pRb by D cyclins reverses the growth-suppres-
sive effect of pRb by releasing E2F. Active E2F can then trigger transcription of E2F-driven genes, which
probably include cyclin E and cyclin A [193]. D-type cyclins contain a motif LXCXE (X any amino
acid) through which they bind pRb [155]. This domain is also present in viral proteins that bind pRb. D-
type cyclins have also been shown to associate with proliferating cell nuclear antigen (PCNA), an acidic,
nonhistone nuclear protein and an auxiliary protein of DNA polymerase-shown to be present only in
proliferating mammalian cells and not in nondividing cells [194].
Cdk2 is considered to be the Cdk most directly involved in DNA replication [195]. Cyclins E and A
sequentially activate Cdk2. The Cdk activity required for DNA replication has been defined as S
phase–promoting factor (SPF) [196]. In the early Xenopusembryo, cyclin E/Cdk2 is sufficient to support
entry into S phase, but later in development, cyclin A/Cdk2 provides a significant additional quantity of
SPF [196]. Activity of the cyclin E/Cdk2 complex is dependent on phosphorylation of Thr 160 (in human
cyclin E) by cyclin H/Cdk7. Mammalian cells that fail to proliferate because of loss of anchorage show a
decrease in phosphorylation of this threonine [197]. Overexpression of cyclin E causes premature entry
into S phase [198], and microinjection of antibodies against cyclin E prevents S phase initiation [199].
Cdk2 complexes are negatively regulated by T14/Y15 phosphorylation [72], and a mammalian CDC25
homologue (CDC25A) regulates G 1 progression [200].
Cyclin E is degraded once cells enter S phase and Cdk2 forms complexes with cyclin A (Figure 2)
[199]. Cyclin A activates Cdk2 shortly after cyclin E and is essential for progression through S phase [22].
Both cyclin E and A complexes associate with the retinoblastoma (Rb)-related protein p107 and the tran-
scription factor E2F [49]. Cyclin E/Cdk2 interaction with E2F activate transcription, whereas
cyclinA/Cdk2 phosphorylation results in the loss of E2F binding activity [193].
242 REDDY AND DAYFigure 2 Cyclin and cyclin-dependent kinase complexes in the vertebrate cell cycle. Rb (retinoblastoma pro-
tein) is known to be phosphorylated by a cyclin D/Cdk4. p107 is a homologue of Rb, and E2F is a transcription
elongation factor. Cyclin H/Cdk7 complex is the Cdk activating kinase (Cak). WEE1/MYT1 are kinases and
CDC25 is a phosphatase involved in inactivation/activation of cyclinB/Cdk1.