K-TERMINAL AND C-TERMINAL DOMAINS( CHANNEL INACTIVATION MEDIATED BY THE CONCERTED ACTION OF THE CYTOPLASMIC N)

We have examined the molecular mechanism of rapid inactivation gating in a mouse Shal K+ channel (mKv4.1). The results showed that inactivat ion of these channels follows a complex time course that is well appro ximated by the sum of three exponential terms. Truncation of an amphip athic region at the N-terminus (residues 2-71) abolished the rapid pha se of inactivation (tau = 16 ms) and altered voltage-dependent gating. Surprisingly, these effects could be mimicked by deletions affecting the hydrophilic C-terminus. The sum of two exponential terms was suffi cient to describe the inactivation of deletion mutants. In fact, the t ime constants corresponded closely to those of the intermediate and sl ow phases of inactivation observed with wild-type channels. Further an alysis revealed that several basic amino acids at the N-terminus do no t influence inactivation, but a positively charged domain at the C-ter minus (amino acids 420-550) is necessary to support rapid inactivation . Thus, the amphipathic N-terminus and the hydrophilic C-terminus of m Kv4.1 are essential determinants of inactivation gating and may intera ct with each other to maintain the N-terminal inactivation gate near t he inner mouth of the channel. Furthermore, this inactivation gate may not behave like a simple open-channel blocker because channel blockad e by internal tetraethylammonium was not associated with slower curren t decay and an elevated external K+ concentration retarded recovery fr om inactivation.