Fundamental Mechanisms of Ion Channel Proteins 
from a kidney cDNA library and that shows a re- 
markable molecular property: a very small polypep- 
tide of only 130 amino acids — some 10-fold 
smaller, for instance, than the voltage-dependent 
Na^ channel. We have constructed and expressed a 
fully synthetic gene for this "mink" channel, using 
the degeneracy of the genetic code to build a large 
number of unique restriction sites throughout the 
coding sequence. Thus positioned to perform rou- 
tine cassette mutagenesis, we are initiating a search 
for functional domains of the channel and develop- 
ing direct tests to settle whether this gene does in 
fact code for an ion channel at all, a basic question 
that has not yet been answered rigorously. On the 
basis of point mutants that lead to changes in ion 
selectivity, we consider that mink is a structural 
gene for an ion channel. 
High-Level Expression of Ion 
Channel Proteins 
Over the past few years, ion channel genes 
have been cloned at a rapidly accelerating pace, 
and they can be expressed in systems that allow 
high-sensitivity electrophysiological assays. 
But channels are proteins, and it would be de- 
sirable to have a high-level expression system 
available for producing milligram-scale protein 
for cloned channel genes. We have found that 
the baculovirus-Sf9 transient expression system 
works well with the Shaker K"^ channel. By in- 
fecting a cell line derived from the armyworm 
caterpillar with a Shaker-recombinant baculo- 
virus that normally attacks the alfalfa-looper, 
we have shown that electrophysiologically nor- 
mal currents can be observed and that the 
Shaker protein is the major membrane protein 
produced. We are presently testing the wider 
generality of this system and using this insect- 
based "channel factory" as a source for purifica- 
tion and functional reconstitution of the Shaker 
channel protein. 
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