Cellular Interactions
473
Inoue et al. accelerated germinal vesicle
disassembly by microinjection of active
MAP kinase into the germinal vesicle,
which suggests that MAP kinase acts
directly on components in the nucleus to
mediate nuclear disassembly.
T
h
ea
c
t
i
v
ef
o
rmo
fM
A
Pk
i
n
a
s
ei
s
enriched on the entire meiotic spindle
in the fertilization-competent egg arrested
at meiotic metaphase II. To demonstrate
the presence of the active form of the
kinase, an immunocytochemical assay was
performed using antibodies that bind sites
on MAP kinase that are phosphorylated
only when the kinase is active. By using
an antibody that recognized both forms of
the kinase (i.e. total MAP kinase and active
MAP kinase), the distribution of both the
active and total forms of the kinase was
compared. Other investigators have also
reported association of total MAP kinase
with the mammalian meiotic spindle;
however, only the poles of the meiotic
spindle were reported by the investigators
to bind MAP kinase. This may have
been observed because the spindles were
physically isolated from the metaphase II
egg before CaM KII was measured by
immunolocalization, and this may have
resulted in restriction of MAP kinase to
the poles.
4
Activation of the Egg
4.1
Remodeling through Changes in Activity
of Calcium/Calmodulin-dependent Protein
Kinase II
As previously mentioned, CaM KII’s activ-
ity increases after fertilization in mouse
eggs; however, until recently little was
known concerning the function of this
activity in mammalian eggs. Johnson et al.
demonstrated that CaM KII is enriched
throughout the meiotic spindle in the
fertilization-competent mouse egg. When
either the egg or activated egg is detergent
extracted to remove soluble and freely dif-
fusing components from the cell, CaM KII
remains behind and appears to be tightly
associated with the meiotic spindle mi-
crotubules. In contrast, a concentration of
calmodulin cannot be detected in associa-
tion with the spindle in the metaphase II
egg after detergent extraction, except for
a window of time that is approximately
5 min after egg activation. After activa-
tion of an egg with calcium ionophore,
calmodulin transiently associates with the
detergent-resistant spindle 5 min after the
ionophore treatment, but is absent by
15 min posttreatment. Calmodulin would
bind to the increase in [Ca
2
+
]
i
in the egg
caused by a calcium ionophore treatment.
Subsequently, one would anticipate that
this calcium/calmodulin complex would
activate CaM KII by associating with the
kinase. This reaction could be representa-
tive of the 5 min after ionophore treatment
when the transient calmodulin is bind-
ing to CaM KII, which is attached to the
detergent-resistant spindle. This proposal
is supported by Johnson et al., who demon-
strated that the level of CaM KII activity
almost doubles within 5 min after egg ac-
tivation, the same window of time that
calmodulin is tightly associated with the
meiotic spindle. In support of this Fnding,
it was demonstrated by a recent investiga-
tion that CaM KII was active during this
time interval and that a large portion of the
CaM KII that is activated is enriched on
the meiotic spindle.
The results described above suggest that
activation of CaM KII is speciFcally in-
volved with some function of the meiotic
spindle. The transition of chromosomes
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