Cell Cycle Control 
suggesting that oncogenes act by perturbing nor- 
mal cellular pathways. In general, very little is 
known about how proto-oncogenes work, but the 
specific involvement of mos in cell cycle control 
is the clearest example of a specific function for 
any proto-oncogene in a defined cellular process. 
The mos gene encodes a serine/threonine pro- 
tein kinase, indicating the existence of a substrate 
for phosphorylation by mos that can lead to acti- 
vation of MPF as well as stabilization of cyclin in 
the metaphase arrest of meiosis II. The transition 
between meiosis I and II is also perhaps the only 
well-documented case in which cdc2 kinase ac- 
tivity declines without accompanying cyclin B 
degradation. 
This year Linda Roy observed that introduction 
of protein synthesis inhibitors into the system at 
the transition between meiosis I and II leads to 
the immediate destruction of cyclin B, which 
would otherwise be stable, thereby establishing 
that the stability of cyclins between meiosis I and 
II requires continuous synthesis of protein. Ini- 
tially we expected that the protein synthesis re- 
quired for cyclin stability would be exemplified 
by the synthesis of the mos proto-oncogene ki- 
nase itself, since that kinase is known to comprise 
one of the major newly synthesized proteins dur- 
ing maturation. However, Dr. Roy found that, in 
fact, the mos kinase remains fully active even 
when B-type cyclins are degraded. This implies 
that there is another protein whose synthesis is 
required for cyclin stability besides the mos 
proto-oncogene . 
Dr. Roy also did the converse experiment by 
microinjecting antisense oligodeoxynucleotides 
against the mos kinase mRNA into cells just prior 
to entry into meiosis I. By this procedure she was 
able to ablate greatly the level of mos proto- 
oncogene expression. However, under these con- 
ditions, cyclin B does not undergo degradation 
between meiosis I and II, thereby establishing 
that mos is not necessary for the stability of cyclin 
B between meiosis I and II. These results suggest 
that other components besides the mos kinase are 
required for the unusual cyclin stability between 
meiosis I and II, and clearly the identification of 
these other components merits attention. 
Gj -*■ S Regulation 
The cell cycle restriction point in Gj governing 
the Gi ^ S transition that involves cdc2-like ki- 
nases has been termed START in yeast and the R 
point in mammalian cells. In both budding yeast 
and fission yeast, it is quite clear that the genuine 
cdc2 gene product mediates both the Gj and G2 
control points. Recently it has become evident 
that the regulation of the Gj ^ S transition in 
vertebrate cells is considerably more compli- 
cated than in yeast. One aspect of this complexity 
involves the presence of another cdc2-like gene 
in the Gj phase that may mediate events at the 
Gj -* S transition. This gene was originally discov- 
ered in Xenopus and given the name Egl but has 
since been renamed cyclin-dependent kinase 2, 
or cdk2. The idea that cdk2 might be a form of 
cdc2 specialized for Gj control has come from 
the finding by others that cdk2 itself will not 
complement mutations in the Saccharomyces cere- 
visiae cdc2 cognate gene CDC28 that affect the 
G2 ^ M restriction point, but will substitute at least 
partially for the Gj function of when coex- 
pressed with a Gj cyclin from human cells. 
This year we have made a major effort to study 
the biochemistry and regulation of cdk2 in the 
Xenopus embryonic cell system. Toward this end 
we have developed an antibody against the pro- 
tein and used it to show that the protein kinase 
activity of cdk2 oscillates in the cell cycle with a 
periodicity similar to cdc2. In the Xenopus em- 
bryonic cell cycle, cdk2 is not associated with 
either cyclin A or cyclin B, but instead with two 
proteins of 36 and 48 kDa. This suggested the 
possibility that cdk2 would be regulated by phos- 
phorylation, perhaps in ways similar to the 
known regulation by phosphorylation of cdc2 ki- 
nase itself. 
Recently we showed that in fact cdk2 is a phos- 
phoprotein, that it is phosphorylated on tyrosine 
and serine residues, and that phosphorylation of 
these residues changes during the cell cycle. In 
particular, the presence of phosphotyrosine cor- 
relates with less-active forms of the protein, but 
the major site of phosphorylation was determined 
by tryptic phosphopeptide mapping to be in a 
phosphopeptide distinct from the one containing 
tyrosine 15, so characteristic of cdc2 kinase. De- 
spite this diff^erence in the sites of phosphoryla- 
tion, however, it would appear that the same en- 
zyme responsible for the dephosphorylation of 
cdc2 is also active on cdk2. 
By continuing to study both cdc2 kinase in the 
G2 ^ M transition and cdk2 kinase in the Gj ^ S 
transition, it should be possible to achieve a com- 
prehensive understanding of cell cycle control at 
the two main restriction points present in eukar- 
yotic cells. 
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