Voltage-activated K+ channels are a family of closely related membrane proteins that differ in their gating behavior, conductance, and pharmacology. A prominent and physiologically important difference among K+ channels is their rate of inactivation. Inactivation rates range from milliseconds to seconds, and K+ channels with different inactivation properties have very different effects on signal integration and repetitive firing properties of neurons. The cloned Shaker B (H4) potassium channel is an example of a K+ channel that inactivates in a few milliseconds. Recent experiments have shown that removal of an N-terminal region of the Shaker protein by site-directed deletion practically abolishes this fast inactivation, but the modified channel does still inactivate during a prolonged depolarization lasting many seconds. Here we report that this remnant inactivation must occur by a distinct mechanism from the rapid inactivation ofthe wild-type Shaker channel. Like the inactivation of another K+ channel [Grissmer, S. & Calahan, M. (1989) Biophys. J. 55, 203-206], this slow inactivation is retarded by the application ofa channel blocker, tetraethylammonium, to the extracellular side of the channel. By contrast, the fast inactivation of the wild-type Shaker channel is sensitive only to intracellular application of tetraethylammonium. Intracellular tetraethylammonium slows down the fast inactivation process, as though it competes with the binding of the inactivation particle.Recent experiments on the rapidly inactivated Shaker K+ channel suggest that the N-terminal part of this protein may act as a blocking particle that blocks current through the channel and thus produces fast inactivation (1, 2). The model of a blocking particle tethered to the rest of the channel (picturesquely termed the "ball-and-chain" model) was proposed many years ago to explain the properties of inactivation of voltage-activated Na' channels (3, 4). Inactivation of both Na' and Shaker K+ channels can be removed by mild protease treatment of the intracellular side of the membrane (1, 5, 6). Inactivation of both types of channels can also be mimicked by the application of intracellular compounds that block current through the channel after the activation gates have opened (7)(8)(9)(10)(11)(12)(13). One such compound is a soluble 20-amino acid peptide with the sequence ofthe N terminus ofthe Shaker protein, which mimics the fast inactivation of this channel when applied to the intracellular side of a Shaker channel from which a region near the N terminus has been genetically removed (14). These results suggest that the N terminus of the Shaker protein is a strong candidate for the tethered "ball" and that the inactivation process is localized to the intracellular portion of the channel.These results on the rapid inactivation of Shaker stand in contrast to studies of the much slower inactivation process seen in a delayed-rectifier K+ channel in lymphocytes. This inactivation process, which occurs over a period of hundreds of millisecond...

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