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 
received 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, the Young Investigator Award of the Society 
for Neuroscience, and the Young Investigator Award of the Biophysical Society. 
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 for the integration of 
these signals to generate appropriate behavior. In 
addition, ion channels are important for the gen- 
eration and regulation of the heartbeat, for con- 
traction of muscles, and for the release of hor- 
mones in the bloodstream. A very 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 neuro- 
transmitter molecule or a change in the voltage 
that exists across a cell's membrane. Our labora- 
tory is interested in the molecular mechanisms of 
ion channel function. One of our major goals is to 
understand the conformational changes that oc- 
cur as the channels open and close in response 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 for 
the transmission of information throughout the 
body. These molecules have a way to measure the 
electrical potential and open accordingly. In ad- 
dition, a number of these channels inactivate or 
become unavailable for opening after use. In re- 
cent years we have studied the molecular mecha- 
nisms of inactivation of a class of potassium chan- 
nels 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 al- 
low us to record the behavior of a single-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 recombinant DNA methods and expressing 
the normal and altered channels in frog oocytes. 
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 mechanism of 
inactivation emerges from our results. The amino 
terminus of the Shaker channel acts as an inacti- 
vation particle or internal plug for the channel. 
When the inactivation particle is bound to the 
receptor, the channel closes. We tested this 
model further by applying a solution containing 
free synthetic inactivation particle to the inside 
face of mutant channels that did not inactivate. In 
the presence of the synthetic inactivation parti- 
cle, the mutant channels regained inactivation, 
consistent with the ball and chain mechanism. 
Ruth Murrell-Lagnado and I have used syn- 
thetic peptide inactivation particles with altered 
amino acid composition to determine the impor- 
tant features that influence an ability to bind to 
the internal mouth of the channel. The naturally 
occurring amino acid sequence can be divided 
into an uncharged region and a highly charged 
region. The charged amino acids interact with 
negative charges at or near the mouth of the chan- 
nel to influence the rate of binding. The more 
positive charges in the charged region, the faster 
the binding rate. Negative charges in the charged 
region inhibit binding. A surprising result is that 
the net charge in this region seems to be the im- 
portant factor for the binding rate, regardless of 
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