Calcium Biochemistry
151
binding domain from their interaction
with the regulatory segment comprising
the core of the CaM binding domain. Such
an arrangement could be of general rel-
evance for the activation mechanism of
CaM-dependent enzymes. The structure
further suggests that binding of CaM and
subsequent reorientation of the ATP bind-
ing domain would open up the substrate
binding domain that would permit Thr177
to be accessible for the activating CaMKK.
Regulation of Activity
The regulation of
the activity of CaMKI and CaMKIV is a
rather complex process and involves three
major steps:
1. Binding of Ca
2
+
/CaM to the enzyme
2. Phosphorylation
of
either
Thr177
(CaMKI) or Thr196 (CaMKIV) located
in the activation loop of the kinases by
CaMKK
3. Autophosphorylation of serine resi-
due(s) at the N-terminus of the enzyme
CaMKII on one hand and CaMKI and
CaMKIV on the other are activated by
different mechanisms. As discussed be-
fore, CaMKII is a multisubunit, homo- or
hetero-oligomer enzyme complex that is
activated by autophosphorylation between
two neighbouring subunits. On the other
hand, CaMKI and CaMKIV do not seem
to multimerize, and both enzymes are
activated by CaMKK; whether both are
activated by the same kinase kinase or
by different enzymes is not entirely clear.
In CaMKIV, the critical residue phospho-
rylated by CaMKK is Thr196, which is
located in the activation loop of the kinase
and is homologous to Thr177 in CaMKI.
Accessibility of the two threonine residues
is brought about by the binding of calmod-
ulin, that is, binding of calmodulin not only
releases autoinhibition of I or IV but also
enables the enzymes to be phosphorylated
at this critical residue. This phosphoryla-
tion results in a 10- to 20-fold activation in
the case of CaMKIV. Furthermore, it was
reported for CaMKIV that Ser11 and Ser12
could be autophosphorylated in a slow pro-
cess as indicated before, but CaMKK could
considerably enhance this process by phos-
phorylating Thr196.
CaMKIV and Ca
2
+
-dependent Gene
Expression
One of the consequences of
elevated calcium in the cell, especially
in the nucleus, is the induction of gene
expression.
Since
transcription
factors
such as CREB, CREM
τ
,A
T
F
-
1
,S
R
F
,
and ETS-1 are among the best substrates
for CaMKIV, and CaMKIV has been
localized to the nucleus, the enzyme could
have direct access to transcription factors
to regulate their function in a Ca
2
+
-
dependent manner. Thus, it has been
shown in different cell lines that CaMKIV
is involved in the regulation of expression
of immediate early genes (IEGs) either
through CREB or through SRF.
Originally, CREB was identi±ed as a
transcription factor induced in a cAMP-
dependent
process,
but
later
studies
demonstrated that several kinases activat-
ing CREB phosphorylated the transcrip-
t
i
onf
a
c
t
o
ra
tth
es
am
es
e
r
in
er
e
s
i
du
e
,
Ser133. In contrast to CaMKIV, CaMKII
phosphorylates next to Ser133 also Ser142,
which has an inhibitory effect on gene
expression.
CaM-dependent
Protein
Kinase
Kinase
(CaMKK)
As indicated before, CaMKI
and
IV
are
activated
by
calmodulin-
dependent protein kinase(s), the CaMKK.
Two isoforms of CaMKK are known to
date,
α
and
β
which are encoded by dif-
ferent genes. Both isoforms are organized
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