Molecular Mechanisms of Ion Channel Function 
which of the amino acids are charged. Con- 
versely, the uncharged region is important for de- 
termining the unbinding rate. This work was sup- 
ported in part by the National Institutes of Health 
and the National Institute of Mental Health. 
Shaker potassium channels also exhibit a 
slower inactivation process, which can be seen 
both in wild-type channels and after the faster 
inactivation process has been removed by muta- 
genesis. The slow inactivation does not require 
intact fast inactivation. Although it also does not 
involve rearrangement of charge in the mem- 
brane, this slower inactivation seems to involve a 
mechanism different from the fast process. In col- 
laboration with Kathleen Choi and Gary Yellen, 
we have found that the slow inactivation is af- 
fected by external agents, suggesting that the 
conformational changes for this process involve 
external structures. Dr. Hoshi, Jose Lopez Barneo, 
and I have found that the rate of this type of inac- 
tivation is profoundly affected by external ions. 
Normally sodium is the predominant positively 
charged ion outside the cell. When a small 
amount of potassium is added to the external so- 
lution, as could occur during epileptic activity in 
the brain or other pathological states, the inacti- 
vation is significantly slowed. We have studied 
the dependence of the inactivation rate on the 
type of external ion present and have determined 
that the ability of an ion species to slow inactiva- 
tion correlates with its permeability in the chan- 
nel. Ions that get into the channel better from the 
outside inhibit inactivation more effectively. 
These results are consistent with a hypothesis 
whereby a channel that is occupied by an ion can- 
not undergo slow inactivation. 
The slow inactivation process occurs by greatly 
different rates in variants of the Shaker channel 
with differences in structure at the carboxyl end 
of the protein. We have made mutations in both 
of these variants and have localized the differ- 
ence responsible for the differences in slow inac- 
tivation to a single hydrophobic amino acid in a 
membrane-spanning region of the channel mole- 
cule. Other amino acid substitutions at this posi- 
tion have dramatic effects on gating, with larger 
hydrophobic amino acids leading to slower 
inactivation. 
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