Biomedical Engineering Reference
In-Depth Information
fractions. The expression of recombinant CLIC5B in cultured cells led to an
increased channel activity in whole cell membrane preparations, and the modifica-
tion of the protein altered the activity [
107
,
108
].
However, there are several other members in the CLIC family of proteins, which
have been termed as CLIC1, 2, 3, 4, 5, and 6 [
109
,
110
]. There exists a high degree
of similarity in their C-termini, but their N-termini are divergent in both length and
sequence. In addition to these six CLICs, which are mostly found in vertebrates
[
109
], there are a few more which are found in invertebrates, for example,
Dm
CLIC
(in
Drosophila melanogaster
), EXC4 and EXL1 (in
Caenorhabditis elegans
) and
At
DGAR1-4 (in
Arabidopsis thaliana
)[
11
].
2.5.1 Structure of CLICs
All CLICs share structural homology with members of omega glutathione S
transferase (GST) superfamily [
110
,
111
]. Unlike other ion channels, they all
exist in dimorphic form: either as a soluble globular protein, or as an integral
membrane protein suggested to form functional ion channels. The functional ion
channels are formed by undergoing a structural transition from the water-soluble
form. This transition is, however, a complex process, which is due to the many
degrees of freedom and the balance between enthalpic and entropic contribution
to the free energy from the polypeptide chain and solvent molecules. To form
a functional channel in the membrane by any CLIC protein, it is predicted that there
should be at least four molecules, since CLIC proteins have only one transmem-
brane domain and thus a single molecule cannot form a functional ion channel.
Positively charged residues form two selectivity rings in the pore region of the
channel indicating their importance in maintaining selectivity.
2.5.2 Biophysical Properties of CLICs
Because of the unavailability of method to directly access intracellular organelles
and due to limited electrophysiological techniques for examining intracellular ion
channels, no detailed study on the biophysical properties of CLICs so far could be
made. So far all electrophysiological studies carried out in planar bilayers have
indicated that CLIC proteins predominantly form non-selective ion channels
[
112
-
115
] with different conductance levels. However, all CLICs form functional
ion channels that vary in their conductance in different systems and within the
same system [
115
]. The single putative transmembrane domain present in them
lines the pore region of the channels, targets the protein to the membrane, and plays
an active role in ion transport. However, it is suggested that CLICs may be
functional only in specific cholesterol-rich microdomains in membranes. These
