EFFECTS OF EXTERNAL CATIONS AND MUTATIONS IN THE PORE REGION ON C-TYPE INACTIVATION OF SHAKER POTASSIUM CHANNELS

After removal of N-type inactivation in Shaker K channels another inac tivation process remains (C-type inactivation). The C-type inactivatio n time course is reversibly slowed when external [K+] increases. The e ffect of K+ is mimicked by Rb+ and, with less potency, by the less per meant ions Na+, Cs+, and NH4+. These results, which can be explained b y the foot-in-the-door model of gating, could reflect the variable int eraction of cations with amino acids in the ion-conducting pore. Mutat ions at position 449 (near the outer mouth of the pore) produce drasti c changes in C-type inactivation kinetics and in its interaction with monovalent cations. Replacement of threonine in the wild-type by gluta mic acid or lysine leads to a hundred-fold acceleration of inactivatio n (time constant approximately 25 ms). In contrast, placing valine at this position results in channels that do not inactivate in 45 s. More over, high K+, besides slowing down the inactivation kinetics, produce s an increase in current amplitude despite a concomitant decrease in K + driving force. This second effect, which is larger in mutants with f aster inactivation kinetics, is caused by an increase in the number of channels that open on depolarization. Thus, C-type inactivation is a process influenced by the ionic composition of the external milieu whi ch strongly depends on the amino acid at position 449 in the pore regi on. These findings may help to explain the variability in inactivation kinetics observed in the various types of K channels.