Functional studies have been done on a few of the cloned cyclins. The established functional test for
cyclins is injection of mRNA or protein into Xenopusoocytes [21,39]. The G 2 -arrested oocytes undergo
germinal vesicle breakdown in a dose-dependent manner when injected with cyclin mRNA. This test was
used for Glyma;CycB1;1, Arath;CycB1;1, and the maize cyclins [124,146,161]. Using a relatively new
test for function, the Arabidopsiscyclin Arath;CycB1;2; the tobacco cyclins Nicta;CycA1;1, Nicta;
CycA2;1, and Nicta;CycB1;1; the maize cyclin Zeama;CycA1;2; the alfalfa cyclin Medsa;CycA2;1; and
the rice cyclins Orysa;CycB2;1, Orysa;CycB2;2, and Orysa;CycA1;1 were able to complement a yeast
CLN (G 1 ) cyclin-minus mutant [149,153,162,164,169]. Although the cyclins complement G 1 cyclins, mi-
totic cyclins in animals can also replace the function of the G 1 cyclins in yeast [54]. A study using a
p34cdc2/cyclinB-like kinase from Chlamydomonasinjected into Tradescantia virginianastamen hairs
demonstrated the effects of this complex on plant cell division [180]. Microinjection caused rapid disas-
sembly of the preprophase band of microtubules and chromatin condensation and nuclear envelope break-
down were accelerated, similar to the initiation of nuclear division by the maturation- or mitosis-promot-
ing factor of animal cells.
- Other Proteins Involved in the G 2 /M Transition
A plant homologue of a SUC1/CKS1-type protein (CKS1At) was isolated in ArabidopsisusingCDC2aAt
(a Cdk kinase) as a bait in a yeast two-hybrid screen [181]. Studies of human SUC1/CKS1gene suggest
that the SUC1/CKS1 protein may function as a docking factor for both positive and negative regulators
of Cdk complexes [182]. CKS1At could bind both CDC2aAt and CDC2bAt and could rescue yeast mu-
tant in the cks1 gene. Mutants of CDC 2aAt and CDC2bAt in the region involved in the interaction of hu-
man Cdk2 with the CKS1Hs protein [182] abolished the binding of CKS1At with CDC2aAt [144].
Plant homologues of the kinetochore protein SKP1, which is required for cell cycle progression in
mammals, have been identified in Arabidopsis[183]. Human SKP1 associates with the cyclin A/Cdk2
complex, and in yeast it is shown to be an essential part of the ubiquitin complex that marks proteins for
destruction. An SKP1 homologue was identified in orchids and Arabidopsisand its expression was highly
correlated with meristem activity [183].
A WEE1 homologue (ZmWee1) was isolated from maize [184]. WEE1 is a kinase whose phospho-
rylation provides negative regulation of Cdks (see earlier). Overexpression of ZmWEE1 in yeast inhib-
ited cell division, and recombinant ZmWEE1 could inhibit activity of a cyclin-dependent kinase from
maize [184].
IV. PROGRESSION THROUGH G 1 AND S PHASES
In yeast, the same protein kinase (p34cdc2/CDC28) is responsible for regulating START, a point in G 1
where a cell commits itself to DNA synthesis and the G 2 /M transition. G 1 cyclins (CLN genes) interact
with p34cdc2/CDC28 to drive the cell through this G 1 restriction point (START) to enter into S phase
[16]. However, in multicellular organisms progression through G 1 and S phase seems to be much more
complex and appears to be controlled by a family of Cdks that are structurally related to p34 kinase
[24,27,115,185–187]. Each of the Cdks seems to associate with a specific type of cyclin(s) to be activated
and appears to be involved in a specific phase of the cell cycle (Figure 2 and Table 2).
A. Proteins Involved in Animal G 1 and S Phases
- Cyclin / Cdk Interactions
Two major players in G 1 are the D cyclins and cyclin E. The D-type cyclins (D1, 2, 3) interact with Cdk4
or Cdk6 and act in middle to late G 1 [88,188,189]. The D-type cyclins are good candidates for activating
the Cdk required to pass through the R point (Figure 2). In mammalian cells, the D-type cyclins are in-
duced in a cell lineage–specific manner and are synthesized as long as growth factor stimulation persists
and are degraded rapidly when mitogens are withdrawn [190]. Cyclin D-bound Cdk4 undergoes activat-
ing phosphorylation by CAK (cyclin H/Cdk7) on its conserved threonine (Table 1) [191].
The D-type cyclins can bind directly to retinoblastoma protein (Rb) in complexes with either Cdk4
or Cdk6 [191]. The expression of D1 is also dependent on the presence of pRb, suggesting the existence
of a regulatory loop between pRb and cyclin D1 [192]. pRb binds to transcription factors such as E2F,
CELL CYCLE REGULATION IN PLANTS 241