Chemistry Reference
In-Depth Information
4
Ion Channels
Toru Ide, Minako Hirano, and Yuko Takeuchi
4.1
Introduction
Single-molecule imaging techniques are very powerful tools for investigating
biomolecules. These techniques have beenapplied to study various types ofmolecules
and have revealed many novel properties. Funatsu et al. observed, for the
first time,
interactionbetween a singlemyosinmolecule and singleATPmolecules using a TIRF
(total internal re
ection
uorescence)microscope [1]. They combinedsinglemolecule
imaging techniques with singlemoleculemanipulation techniques using a scanning
probe or optical tweezers observing important properties ofmotor proteins not shown
bymulti-molecular experiments [2]. These techniques, whichwere initially developed
to study motility of the acto-myosin system, have spread rapidly throughmany
fields.
Nowthemotionof other typesofmotorproteins, suchaskinesinandF
0
,F
1
-ATPase, are
routinely measured using these techniques.
In contrast to water-soluble proteins, application of these techniques to
ion-channel proteins is not as advanced because it is much more dif
cult to image
single ion-channels while recording their functions simultaneously. This is due to the
fact that, unlikemostwater-solubleproteins, it isdif
cult tomaintainchannel function
with isolated ion-channel proteins. Lipidmolecules are indispensable formaintaining
the channel activity. Thismeans that recording channel function requires the addition
of a large number of lipid molecules that may produce optical noise. On the other
hand, experimental techniques to investigate single ion-channel function, namely the
single channel recording techniques, are well established compared to the technolo-
gies applied to measure functions of other types of proteins. Measurement of motor
protein motility, for example, requires highly specialized equipment, such as an
apparatus to measure small displacements of beads or a special kind of atomic force
microscope. These problems are minimal when investigating single ion-channel
function, because of the availability of commercial equipment.
Our present purpose is to develop an experimental apparatus for simultaneous
optical and electrical observation of single ligand bindings to a single channel protein,
