Molecular Mechanisms of Ion Channel Function 
Richard W. Aldrich, Ph.D. — Associate Investigator 
Dr. Aldrich is also Associate Professor of Molecular and Cellular Physiology at Stanford University. He re- 
ceived his B.S. degree in biology from the University of Arizona and his Ph.D. degree in neuroscience from 
Stanford. He carried out postdoctoral research at Yale University with Knox Chandler and Charles Stevens. 
Before returning to Stanford, he was Assistant Professor of Molecular Neurobiology at Yale. Among his 
awards are a Searle Scholars Award and the Young Investigator Award of the Society for Neuroscience. 
ION channels are the molecular units of electri- 
cal signaling in cells. They are proteins that 
regulate the movement of ions, such as sodium, 
calcium, and potassium, into and out of cells. 
They are responsible for the conversion of exter- 
nal sensory signals to the electrical language of 
the nervous system and the integration of these 
signals to generate appropriate behavior. In addi- 
tion, ion channels are important for the genera- 
tion and regulation of the heart beat, for contrac- 
tion of muscles, and for the release of hormones 
in the bloodstream. A large variety of ion channel 
types are found in the body. They are specialized 
to select for certain species of ions and to open 
and close in response to a number of different 
stimuli, such as the binding of a neurotransmitter 
molecule or a change in the voltage that exists 
across a cell's membrane. Our laboratory is inter- 
ested in the molecular mechanisms of ion chan- 
nel function. One of our major goals is to under- 
stand the conformational changes that occur as 
the channels respond to appropriate stimuli. 
Voltage-gated ion channels are an important 
functional class. As their name implies, they can 
open in response to changes in the electrical po- 
tential across the cell membrane, a property cru- 
cial for the generation of electrical signals and 
their transmission throughout the body. These 
molecules have a way to measure the electrical 
potential and open accordingly. In addition, 
some of them inactivate, or become unavailable 
for opening after use. In recent years we have 
studied the molecular mechanisms of inactiva- 
tion of a class of potassium channels that were 
cloned in Drosophila. 
These channels are products of the Shaker 
gene. They exhibit the fastest inactivation of any 
potassium channels yet cloned. William Zagotta 
and I began by using single-channel recording 
methods to study the gating properties of wild- 
type Shaker channels in their native tissue. Such 
methods allow us to record the behavior of a sin- 
gle-channel molecule as it opens and closes on a 
millisecond time scale. 
We determined that the conformational 
changes associated with opening the channel de- 
pended strongly on the membrane voltage and 
therefore involved a substantial rearrangement of 
an electrically charged part of the channel in the 
membrane. On the other hand, the inactivation 
process did not involve significant charge rear- 
rangement. This result, combined with our abil- 
ity to alter inactivation by internal enzymes, led 
us to the hypothesis that inactivation involved a 
conformational change on the inside of the mem- 
brane that blocked the flow of potassium ions 
through the channel. 
Dr. Zagotta, Toshinori Hoshi, and I further stud- 
ied this hypothesis by making altered channels 
with recombinate DNA methods and expressing 
the normal and altered channels in frog oocytes. 
Our results demonstrated that the first 10 or so 
hydrophobic amino acids, and the positively 
charged amino acids in positions 16 through 19, 
are important in the inactivation mechanism. By 
altering these amino acids, we can change the 
inactivation rate in a graded manner up to about 
20 times slower than normal. 
Deletion and insertion mutations in an adjacent 
region alter the inactivation rate as a function of 
the length of the remaining amino acid chain, 
suggesting that this region acts as a spacer. Our 
results are strikingly consistent with the "ball and 
chain" model of inactivation originally proposed 
for the voltage-dependent sodium channel by 
Armstrong and Bezanilla in 1977. 
The following model of the molecular mecha- 
nism of inactivation emerges from our results. 
The amino terminus of the Shaker channel acts as 
an agonist for inactivation. The inactivation re- 
ceptor is located elsewhere on the cytoplasmic 
side of the molecule. When the inactivation ago- 
nist is bound to the receptor, the channel closes. 
The first 1 0 or so hydrophobic amino acids of the 
Shaker protein either form the core of the inacti- 
vation particle or provide the hydrophobic inter- 
actions necessary for the particle to bind to the 
inactivation receptor. The positively charged resi- 
dues electrostatically interact with the receptor, 
which is likely to be negatively charged. Muta- 
tions in this region presumably disrupted the in- 
activation particle and precluded inactivation. 
Amino acid residues 21 and beyond form the 
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