Biomedical Engineering Reference
In-Depth Information
having distinct properties, such as the presence of a single transmembrane domain
and the dimorphic existence: either as a soluble globular protein or as an integral
membrane protein. They are relatively new class of putative ion channel proteins
that differ from other classes of channels in their primary structure, and in the
transmembrane region of their tertiary structure. We now thus discuss here all
important classes of Cl
channels as mentioned.
2.1 The CLC Chloride Channels
CLC Cl
channels form a large gene family that is found in bacteria, archae, and
eukaryotes. They constitute the only class of Cl
channels that are conserved from
bacteria to man. Mammals have nine different CLC genes, which, based on
homology, are grouped into three branches. The first branch contains ClC-1,
ClC-2, ClC-Ka, and ClC-Kb that exert their function in the plasma membrane.
The second branch is composed of ClC-3, ClC-4, and ClC-5, and the third branch
has ClC-6 and ClC-7. These two branches function primarily in intracellular
membranes.
All CLC Cl
channels have a dimeric structure formed from two homologous
pore forming proteins having molecular weights between ~60 and 110 kDa. This
dimeric structure of CLC channels was deduced from electrophysiological analysis
of wild-type (WT) and mutant channels [
12
-
16
], using sedimentation analysis and
chemical cross-linking [
14
,
17
], which has now been confirmed by crystal
structures [
18
,
19
]. However, this dimeric structure of Cl
channels was first
indicated by the biophysical analysis of a channel reconstituted from
Torpedo
electric organ [
12
]. The compelling evidence for a dimeric structure of CLC
channels was provided by single-channel analysis of mutant/WT ClC-0 heteromers
[
15
,
16
] as well as of ClC-0/ClC-1 and ClC-0/ClC-2 concatemers [
20
]. Even
a bacterial CLC protein obtained from
E. coli
was shown to be a dimmer [
21
].
Some CLC-proteins (e.g., ClC-1 and ClC-2) can form heterodimers [
20
,
22
]
in vitro, but it is not clear whether this also happens in vivo.
By a detailed biophysical study, Miller [
23
] proposed a “double-barrel” model
for ClC-0 channels, which stated that this channel has two identical pores, which
gate independently, but which can also be closed together by a slower, common
gate. Also, there is a common “gate” that closes both the pores at the same time.
ClC-1 and ClC-2 were also reported to behave as “double-barreled” channels
[
22
,
24
]. A brief review of CLC channel gating and the “double barrel” model
has been presented by Est´vez and Jentsch [
25
] and by Jentsch et al. [
9
].
2.1.1 Topological Features of CLC Channels
A definitive picture of topological features of CLC channels has been identified by
their crystal structure [
18
]. Some previous biochemical studies on CLC topology
