ATP inhibition of K-ATP channels: control of nucleotide sensitivity by theN-terminal domain of the Kir6.2 subunit

1. To gain insight into the role of the cytoplasmic regions of the Kir6.2 s ubunit in regulating channel activity, we have expressed the sulphonylurea receptor SUR1 with Kir6.2 subunits containing systematic truncations of the N- and C-termini. Up to 30 amino acids could be truncated from the N-termi nus, and up to 36 amino acids from the C-terminus without loss of functiona l channels in co-expression with SUR1. Furthermore, Kir6.2 Delta C25 and Ki r6.2 Delta C36 subunits expressed functional channels in the absence of SUR 1. 2. In co-expression with SUR1, N-terminal truncations increased K-i.ATP ([A TP] causing half maximal inhibition of channel activity) by as much as 10-f old, accompanied by an increase in the ATP-insensitive open probability whe reas the C-terminal truncations did not affect the ATP sensitivity of co-ex -pressed channels. 3. A mutation in the near C-terminal region, K185Q, reduced ATP sensitivity of co-expressed channels by approximately 30-fold, and on the Kir6.2 Delta N2-30 background, this mutation decreased ATP sensitivity of co-expressed channels by approximately 400-fold. 4. Each of these mutations also reduced the sensitivity to inhibition by AD P, AMP and adenosine tetraphosphate. 5. The results can be quantitatively explained bq assuming that the N-termi nal deletions stabilize the ATP-independent open state, whereas the Kir6.2K 185Q mutation may alter the stability of ATP binding. These two effects are energetically additive, causing the large reduction of ATP sensitivity in the double mutant channels.