472
Cellular Interactions
associated with p34
cdc2
to make active
MPF, oocytes arrested in prophase I of
meiosis are recruited to resume meiosis.
Cyclin degradation is initiated as the oocyte
transits between meiosis I and meiosis II.
New synthesis of cyclin replenishes the
diminished concentration of cyclin and
maintains the activity of MPF. This bal-
ance of continued degradation and new
synthesis of cyclin accompanies the for-
mation of the fertilization-competent egg
and explains why, when protein synthe-
sis inhibitors are applied to eggs, the
eggs subsequently activate. This is be-
cause cyclin continues to be degraded as
normal but the inhibitor blocks any new
cyclin synthesis. Subsequently, the arrest
at meiotic metaphase II cannot be main-
ta
inedbecauseo
ftheleve
lo
fMPF
.The
integrity of a molecular scaffold of the
cell, the meiotic spindle, plays an im-
portant role in the degradation of MPF
within the fertilization-competent egg at
meiotic metaphase II. Cyclin degradation
is inhibited and the egg remains arrested
in meiotic metaphase II if the meiotic
spindle is disrupted with a microtubule-
disassembling agent.
In
contrast,
the
degradation of MPF is much slower be-
tween meiosis I and meiosis II and it does
not appear to be linked to the integrity
of the spindle microtubules. This suggests
that in a metaphase II egg, the presence of
a molecular scaffold (i.e. the meiotic spin-
dle) is associated with components that
provide a mechanism to increase the ef±-
ciency of a variety of reactions including
cyclin degradation. The colocalization of
components on molecular scaffolds in the
cell will be discussed below.
Several different MAP kinase pathways
have been identi±ed in various cell types
functioning through different MAP ki-
nases, for example, ERK 1/2, JNK/SAPK,
p38, and ERK 5. The ERK 1/2 pathway has
been identi±ed in oocytes and eggs. In this
pathway, the upstream activator of MAP ki-
nase is MEK 1/2 (i.e. MAP kinase kinase).
Further upstream in this kinase cascade in
eggs is c-mos (MAP kinase kinase kinase),
whereas in other cell types it is typically
one of the Raf isotypes. When oocytes are
triggered to resume meiosis from the ar-
rest at prophase I, MAP kinase is activated.
MAP kinase is dependent on protein syn-
thesis to be initially activated because if
puromycin, an inhibitor of protein synthe-
sis, is applied MAP kinase does not become
active in mouse or rat oocytes. However,
once MAP kinase is activated, protein syn-
thesis is no longer necessary to maintain
MAP kinase activation. C-mos is present
in mammalian eggs and is considered a
part of cytostatic factor (a factor in the cyto-
plasm that maintains the arrest at meiotic
metaphase II). It is important to note that c-
mos is thought to be an upstream activator
of MAP kinase and in the absence of c-
mos activity, MAP kinase does not become
active. Evidence of this activation pathway
also comes from studies where the c-mos
gene has been disrupted. In such mice,
MAP kinase does not become active. Raf,
which is a known upstream activator of
MAP kinase, is also present in these eggs,
but Raf activity was not detected when
MAP kinase became active. The activation
of MAP kinase requires phosphorylation
on both tyrosine and threonine residues
to become an active serine/threonine ki-
nase and the application of phosphatase
inhibitors speeds MAP kinase activation.
The major changes in organizational
state of the microtubules (i.e. the switch
from
the
interphase
to
the
M-phase
con±guration of microtubules) as well as
the disassembly of the germinal vesicle
that is required for the formation of
the metaphase II egg appear to be due
to MAP kinase activation in the oocyte.
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