Involvement of the N-terminus of Kir6.2 in the inhibition of the K-ATP channel by ATP

1. ATP-sensitive potassium (K-ATP) channels are composed of pore-forming Ki r6.2 and regulatory SUR subunits. A truncated isoform of Kir6.2, Kir6.2 Del ta C26, expresses ATP-sensitive channels in the absence of SUR1, suggesting the ATP-inhibitory site lies on the Kir6.2 subunit. 2. We examined the effect on the channel ATP sensitivity of mutating the ar ginine residue at position 50 (R50) in the N-terminus of Kir6.2, by recordi ng macroscopic currents in membrane patches excised from Xenopus oocytes ex pressing wild-type or mutant Kir6.2 Delta C26. 3. Substitution of R50 by serine, alanine or glycine reduced the K-i for AT P inhibition from 117 mu M to 800 mu M, 1.1 mM and 3.8 mM, respectively. Th e single-channel conductance and kinetics were unaffected by any of these m utations. Mutation to glutamate, lysine, asparagine, glutamine or leucine h ad a smaller effect (K-i, similar to 300-400 mu M). The results indicate th at the side chain of the arginine residue at position 50 is unlikely to con tribute directly to the binding site for ATP, and suggest it may affect ATP inhibition by allosteric interactions. 4. Mutation of the isoleucine residue at position 49 to glycine (I49G) redu ced the channel ATP sensitivity, while the mutation of the glutamate residu e at position 51 to glycine (E51G) did not. 5. When a mutation in the N-terminus of Kir6.2 Delta C26 that alters ATP se nsitivity (R50S; K-i, 800 mu M) was combined with one in the C-terminus (E1 79Q; K-i, 300 mu M), the K-i for the apparent ATP sensitivity was increased to 2.8 mM. The Hill coefficient was also increased. This suggests that the N- and C-termini of Kir6.2 may co-operate to influence channel closure by ATP.