Allosteric effects of external K+ ions mediated by the aspartate of the GYGD signature sequence in the Kv2.1 K+ channel

K+ channels achieve exquisite ion selectivity without jeopardizing efficient permeation by employing multiple, interacting K+-binding sites. Introduction of a cadmium ( Cd2+)-binding site in the external vestibule of Kv2.1 ( drk1), allowed us to functionally characterize a binding site for external monovalent cations. Permeant ions displayed higher affinity for this site than non-permeant monovalent cations, although the selectivity pro. le was different from that of the channel. Point mutations identified the highly conserved aspartate residue immediately following the selectivity filter as a critical determinant of the antagonism between external K+ and Cd2+ ions. A conservative mutation at this position ( D378E) significantly affected the open-state stability. Moreover, the mean open time was found to be modulated by external K+ concentration, suggesting a coupling between channel closing and the permeation process. Reducing the Rb+ conductance by mutating the selectivity filter to the sequence found in Kv4.1, also significantly reduced the effectiveness of Rb+ ions to antagonize Cd2+ inhibition, thereby implicating the selectivity filter as the site at which K+ ions exert their antagonistic effect on Cd2+ block. The equivalent of D378 in KcsA, D80, takes part in an inter-subunit hydrogen-bond network that allows D80 to functionally interact with the selectivity filter. The results suggest that external K+ ions antagonize Cd2+ inhibition ( in I379C) and modulate the mean open time ( in the wild-type Kv2.1) by altering the occupancy pro. le of the K+-binding sites in the selectivity filter.