Two mechanisms of K+-dependent potentiation in Kv2.1 potassium channels

Elevation of external [K+] potentiates outward K+ current through several v oltage-gated K+ channels. This increase in current magnitude is paradoxical in that it occurs despite a significant decrease in driving force. We have investigated the mechanisms involved in K+-dependent current potentiation in the Kv2.1 K+ channel. With holding potentials of -120 to -150 mV, which completely removed channels from the voltage-sensitive inactivated state, e levation of external [K+] up to 10 mM produced a concentration-dependent in crease in outward current magnitude. In the absence of inactivation, curren ts were maximally potentiated by 38%. At more positive holding potentials, which produced steady-state inactivation, K+-dependent potentiation was enh anced. The additional K+-dependent potentiation (above 38%) at more positiv e holding potentials was precisely equal to a K+-dependent reduction in ste ady-state inactivation. Mutation of two lysine residues in the outer vestib ule of Kv2.1 (K356 and K382), to smaller, uncharged residues (glycine and v aline, respectively), completely abolished K+-dependent potentiation that w as not associated with inactivation. These mutations did not influence stea dy-state inactivation or the K+-dependent potentiation due to reduction in steady-state inactivation. These results demonstrate that K+-dependent pote ntiation can be completely accounted for by two independent mechanisms: one that involved the outer vestibule lysines and one that involved K+-depende nt removal of channels from the inactivated state. Previous studies demonst rated that the outer vestibule of Kv2.1 can be in at least two conformation s, depending on the occupancy of the selectivity filter by KC (Immke, D., M . Wood, L. Kiss, and S. J. Kern. 1999. J. Gen. Physiol. 113:819-836; Immke, D., and S. J. Kern. 2000. J. Gen. Physiol. 115:509-518). This change in co nformation was functionally defined by a change in TEA sensitivity. Similar to the K+-dependent change in TEA sensitivity, the lysine-dependent potent iation depended primarily (>90%) on Lys-356 and was enhanced by lowering in itial K+ occupancy of the pore. Furthermore, the K+-dependent changes in cu rrent magnitude and TEA sensitivity were highly correlated. These results s uggest that the previously described K+-dependent change in outer vestibule conformation underlies the lysine-sensitive, K+-dependent potentiation mec hanism.