Inactivation of the potassium ion channel. The amino-terminal region of the K⁺ chain is critical for inactivation. (A) The wild-type Shaker K⁺ channel displays rapid inactivation after opening. (B) A mutant channel lacking residues 6 through 46 does not inactivate. (C) Inactivation can be restored by adding a peptide consisting of residues 1 through 20 at a concentration of 100 μM. [Data from W. N. Zagotta, T. Hoshi, and R. W. Aldrich, Science 250:568–571, 1990.]