Cellular Interactions
475
the chromosomes do not transit normally
from metaphase into anaphase. Midzone
microtubules will not form since the chro-
mosomes do not transit properly into
anaphase, which could lead to an incorrect
positioning of CaM KII. CaM KII would
not be activated even if the kinase did
translocate correctly to the contractile ring
of the forming second polar body because
the [Ca
2
+
]
i
levels would not have been
elevated. Contractile ring closure may be
inhibited in the absence of CaM KII activa-
tion. Without a complete separation from
the egg, the second polar body may in-
stead become absorbed into the activated
egg after a period of time.
As noted in a previous section, CaM KII
is tightly associated with the meiotic spin-
dle. However, other cytoplasmic signaling
agents including cyclin, c-mos, and MAP
kinase have been reported to be associated
with at least parts of the spindle in a vari-
ety of different systems. The presence of,
or localization to, a particular site within
a cell does not indicate that a kinase is
in its active form. In fact, location of the
greatest concentration of a kinase may dif-
fer from the site where the active kinase is
positioned, as was shown in the study by
Hatch and Capco that examined the dis-
tribution of CaM KII and MAP kinase in
mouse eggs. This study makes use of anti-
bodies that only recognize the active form
of the kinase and other antibodies that rec-
ognize both the active and inactive forms
of the kinase (i.e. the total kinase). The
spatial distribution of total CaM KII and
total MAP kinase was compared and indi-
cated that the kinases colocalized on the
meiotic spindle in the metaphase II egg
and on the midzone microtubules after
egg activation. However, very little active
MAP kinase was present on the midzone
microtubules and was only localized on
the spindle for 25 min after egg activation.
At later postactivation stages, total MAP
kinase was still enriched at the midzone
microtubules, but at that same location
active MAP kinase was not present. In
comparison, CaM KII was constantly asso-
ciated with the spindle microtubules and
later the midzone microtubules, whereas
active CaM KII was enriched on the spindle
only at 5 and 25 min after egg activation,
and then enriched on the contractile ring
of the second polar body 55 min after egg
activation.
Colocalization of CaM KII and MAP
kinase on the meiotic spindle provides
the opportunity for interaction between
the two kinases. This may be particularly
important for CaM KII since it becomes
active on the spindle 5 min after egg
activation at the same time that MAP
kinase also is active and associated with
the
meiotic
spindle.
An
investigation
demonstrated that inhibition of CaM KII
activity caused a decrease in both the
amount of total MAP kinase and active
MAP kinase on the spindle. This study
suggested that activation of CaM KII
(within 5 min of egg activation) serves
to potentiate MAP kinase in the active
state. There are two forms of MAP kinase
in mouse eggs ERK1 and ERK2. Both
forms contain consensus phosphorylation
sites for CaM KII. These are not the
same sites that activate MAP kinase, but
phosphorylation by CaM KII on these sites
may prevent degradation of MAP kinases.
As will be discussed in a later section,
it has been proposed that the activity
of MAP kinase inhibits pronuclei from
forming precociously in the zygote, and
after MAP kinase activity drops, pronuclei
immediately form. Thus, as the means
of regulating the timing of the formation
of pronuclei, egg activation induces the
activation of CaM KII. This CaM KII
activity serves to boost MAP kinase activity
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