Neuronal plasticity is governed in part by well-orchestrated
intracellular signaling pathways. The orchestration involves
the where and when individual molecules become activated.
Ca2+/calmodulin-dependent protein
kinase II (CaM-kinase II) is the most abundant protein
kinase in nerve cells and its role in regulating neuronal
plasticity is well documented.
Inhibiting the kinase through either genetic knockout
strategies or pharmacological treatments negatively impacts
one of the widest- studied models of neuronal plasticity,
long-term potentiation. A unique feature of CaM-kinase
II is that the molecule catalyzes the covalent modification
of itself through an autophosphorylation event and this
autophosphorylation impacts the functionality of the enzyme.
Each CaM-kinase II molecule can therefore exist in different
stable states adding significant, but fascinating, complexity
to the enzyme population within neurons.
CaM-kinase II is a complex of multiple subunits that
forms an oligomeric structure and it is the interactions
between subunits of a holoenzyme that is proposed to underlie
the autophosphorylation mechanism. In addition, the multimeric
structure of the kinase provides numerous opportunities
for protein-protein interactions either between CaM-kinase
II and other proteins, such as the NMDA receptor, or possibly
between CaM-kinase II holoenzymes. The hypothesis is put
forth that the combined molecular features of CaM- kinase
II subserve a Ca2+/calmodulin regulated
scaffolding function leading to the additional idea that
CaM-kinase II is a structural protein.
The talk addressed several of these issues. First, work
describing the solution of the three-dimensional structure
of the kinase at a resolution of approximately 27 angstroms
was presented. CaM-kinase II is a dodecameric (12-subunit)
molecule that has dimensions of ~20 nm x 20 nm. The 12
subunits are arranged in a tail-to-tail fashion producing
a molecule with a dense central core and catalytic domains
extending away from the core on short stalks. A comparison
of the three-dimensional structure of each CaM-kinase
II gene family member (a, b, g and d) reveals significant
homology in overall architecture. Thus the differences
in the biology ascribed to these different isoforms cannot
be due to differences in their overall architecture. The
distances between and rigidity of the catalytic domains
suggested by these three-dimensional reconstructions leaves
open the question, from a structural viewpoint, how two
subunits within one holoenzyme can be brought into contact
with each other to autophosohorylate.
With the three-dimensional structure in hand it is now
possible to paint CaM-kinase II into electron micrographs
of synaptic contacts as a means of providing a visual
framework to constrain thinking about the molecules associated
with the synaptic architecture. The second half of the
talk addressed the mechanism of CaM-kinase II self-ssociation
in living cells. The multimeric nature of the holoenzymes
provides ample opportunities for interactions that lead
to macromolecular assemblies of CaM-kinase II molecules.
Live cell imaging indicates that enzyme clusters assemble
and disassemble on the minute time scale in a stimulus-
dependent manner. Binding of the activator Ca2+/CaM
is essential for the expression of this property as is
the holoenzyme structure. One possibility is that self-assembly
of CaM-kinase II holoenzymes may provide synaptic contacts
with a previously unidentified Ca2+/CaM-regulated
structural element to help stabilize synaptic contacts.
Another consequence of self- association would be to limit
the diffusion of CaM-kinase II away from its point of
activation, tagging that spot (i.e., a particular synapse)
with activated CaM-kinase II molecules.
Overall, the hope of the presentation is to provide new
data from CaM- kinase II structure-function studies that
will stimulate novel hypotheses about the role of this
enzyme in the regulation of neuronal function. Incorporating
temporal and spatial changes in the kinase's localization
and providing a spatial framework for possibly relating
the enzyme's function at synaptic contacts is a further
goal.
