Cellular Interactions
471
deiminase-like protein is associated with
the cytoskeletal sheets and it becomes ac-
tive shortly after egg activation where it
may serve to aid in the reorganization of
the cytoskeletal Flaments in the sheets.
3
Formation of the Fertilization-competent
Egg
3.1
CaM KII
Several
reports
have
determined
that
CaM KII is present in the fertilization-
competent mouse egg and it also is
enriched on the meiotic spindle. A co-
factor required for activation of CaM KII,
calmodulin, is also present in the egg, but
in contrast, very little of it is associated
with CaM KII or the meiotic spindle in the
metaphase II egg. Calmodulin, in associ-
ation with CaM KII activity, which results
from the basal level of [Ca
2
+
]
i
in the egg,
has been demonstrated in fertilization-
competent eggs by biochemical assays. As
will be discussed later, the level of CaM KII
activity greatly increases after fertilization.
3.2
PKC in the Mammalian Egg
PKC is another kinase that functions to
convert the mammalian oocyte into the
fertilization-competent egg. Activation of
PKC has been reported to inhibit the disas-
sembly of the germinal vesicle. Thus, one
might expect that the activity of PKC would
either remain constant, or be reduced, at
the time of germinal vesicle disassem-
bly. Several different PKC isotypes exist
in fertilization-competent eggs of mice, al-
though there are differences between those
represented at the protein level compared
to those represented at the RNA level.
PKC
λ
and PKC
µ
are detected at the RNA
level, while at the protein level PKC
α
,
δ
,
λ
,
µ
,and
ξ
are detected in the metaphase
II egg. The protein level may have more
isotypes because the proteins may be of
maternal origin. Gangeswaran and Jones
also examined mouse PKC isotypes both
during the resumption of meiosis to pro-
duce the fertilization-competent egg and
in the egg itself. However, they did not
detect PKC isotypes
α
,
β
,
γ
,
ε
,
η
,
θ
,and
ζ
but detected PKC isotypes
δ
and
λ
in
germinal vesicle stage oocytes at both the
p
r
o
t
e
inan
dmRNAl
e
v
e
linm
e
t
a
ph
a
s
e
II eggs. Raz et al. detected protein and
mRNA for PKC
α
,
β
,
γ
,
ε
,
µ
,
λ
,and
ζ
in
oocytes and metaphase II eggs from rat.
Only conventional isotypes of PKC (i.e.
PKC
α
,
β
,and
γ
)wou
ldbecand
ida
tesfor
signaling agents that acted proximate to
the fertilization-induced [Ca
2
+
]
i
since only
the conventional isotypes of PKC require
calcium as a cofactor for activation. Other
PKC isotypes may be active at the time
of fertilization, but the function of these
isotypes may not be dependent on the
generation of a calcium signal.
3.3
MAPK and MPF
Two important kinases that function to
convert the oocyte into the fertilization-
competent egg are MP± and MAP kinase.
At about the time that the oocyte nu-
cleus (i.e. the germinal vesicle) begins to
disassemble, the activity of both kinases
appears. There is evidence to suggest that
in mammalian development, MAP kinase
activity appears Frst, followed by MP± ac-
tivity, or there is simultaneous activation
of MAP kinase and MP±. Data from other
investigators have indicated a reverse se-
quence. As Cyclin B1 is synthesized and
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