Molecular Studies of Voltage-Sensitive 
Potassium Channels 
Lily Y.Jan, Ph.D. — Investigator 
Dr. Jan is also Professor of Physiology and Biochemistry at the University of California, San Francisco. 
During her graduate study at the California Institute of Technology with Jean Paul Revel and Max 
Delbriick, Dr. Jan localized the visual pigment rhodopsin at the ultrastructural level. Because she studied 
high-energy theoretical physics before becoming a biology student, her Ph.D. degree was in physics and 
biophysics. She stayed at CalTech to do postdoctoral research with Seymour Benzer and began to 
collaborate with her husband, Yuh Nungjan. Their first collaboration resulted in the identification of the 
Shaker locus as a potential structural gene for a potassium channel. Before accepting faculty appointments 
at UCSF, the Jans worked in Stephen Kuffler's laboratory at Harvard Medical School. 
VOLTAGE-sensitive potassium channels proba- 
bly constitute the most diverse and wide- 
spread class of ion channels. More than 30 differ- 
ent types of potassium channels have been 
characterized. They dilFer in their voltage sensi- 
tivity, their kinetic properties, and their sensitiv- 
ity to second messengers within the cell. Potas- 
sium channels have been found in almost every 
eukaryotic cell type examined, in both the ani- 
mal and the plant kingdoms. They are important 
for a wide range of physiological functions, in- 
cluding insulin release due to raised glucose lev- 
els, proliferation of lymphocytes induced by mi- 
togens, and the movements of leaflets in plants or 
the opening and closing of leaf stomatal pores. In 
the mammalian nervous system, potassium chan- 
nels control excitability and the strength of sig- 
naling between nerve cells. Indeed, some of the 
potassium channels have been implicated as 
playing a role in learning and memory. 
To study how the diversity of potassium chan- 
nels arises and how they serve the wide variety of 
cellular functions, one needs to study these chan- 
nels biochemically as well as biophysically. How- 
ever, they are difficult to purify because they are 
rather heterogeneous and inaccessible. For this 
reason we have taken advantage of the well- 
developed genetic technologies applicable to the 
fruit fly Drosophila melanogaster. In this organ- 
ism, if a gene (say, coding for a potassium chan- 
nel) can be identified by the abnormalities 
caused by its mutations, one can clone it for mo- 
lecular studies of the gene product. 
More than a decade ago, Yuh Nungjan (HHMI, 
University of California, San Francisco), Mike 
Dennis, and I found that mutations at the Shaker 
locus cause prolonged transmitter release from 
the motor nerve terminal, probably because of a 
defect in potassium channel function. The locus 
was subsequently cloned by Diane Papazian, Tom 
Schwarz, and Bruce Tempel in our laboratory. It 
codes for proteins that contain multiple stretches 
of hydrophobic amino acids that can potentially 
span the cell membrane; several proteins that 
diff^er in sequence flanking these stretches of hy- 
drophobic amino acids are generated because of 
alternative splicing of the primary transcript. 
Leslie Timpe demonstrated that RNA encoding 
for these Shaker proteins of known deduced se- 
quence, when injected into frog oocytes, causes 
the functional expression of potassium channels 
of different kinetic properties. Since the different 
Shaker proteins are present in different regions 
of the fly brain, they are likely to give rise to dif- 
ferent subtypes of A channels. 
Starting with the Shaker gene in the fruit fly, 
Bruce Tempel isolated a gene in the mouse that 
codes for a potassium channel in the mouse 
brain. This protein is 65 percent identical in its 
sequence to the Drosophila potassium channel 
protein. In frog oocytes, the gene produces potas- 
sium channels that do not inactivate rapidly. Anal- 
ysis of the distribution of amino acid residues that 
appear to be essential and have been totally con- 
served over 600 million years has provided some 
clues to the channel's structure. Now over 1 5 dif- 
ferent mammalian potassium channel genes have 
been characterized by a number of laboratories. 
One of these (the rat Shall gene), cloned and 
characterized by Tim Baldwin, Meei-Ling Tsaur, 
and George Lopez in our laboratory, produces a 
rapidly inactivating potassium channel in frog 
oocytes and is expressed in the heart as well as 
the brain. The high degree of conservation among 
these mammalian channels and the fruit fly potas- 
sium channels reiterates the point that any experi- 
mental organism, as long as it is amenable to the 
specific type of experimentation, will reveal in- 
formation of medical interest. 
Having cloned some of the potassium channel 
genes, we can now ask how this channel works. 
How does it detect a voltage change across the 
cell membrane and, responding, open? How does 
it "inactivate" after it opens? How does it discrim- 
inate between sodium and potassium ions and 
show exquisite selectivity? To probe these ques- 
tions, we have altered specific residues of the po- 
tassium channel to see how the various functions 
are affected. Studies in our laboratory (Ehud 
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