Altered functional properties of K-ATP channel conferred by a novel splicevariant of SUR1

1. ATP-sensitive potassium (K-ATP) channels are composed of pore-forming (K ir6.x) and regulatory sulphonylurea receptor (SURx) subunits. We have isola ted a novel SUR variant (SUR1b Delta 33) from a hypothalamic cDNA library. This variant lacked exon 33 and introduced a frameshift that produced a tru ncated protein lacking the second nucleotide binding domain (NBD2). It was expressed at low levels in hypothalamus, midbrain, heart and the insulin-se creting beta-cell line MIN6. 2. We examined the properties of K-ATP channels composed of Kir6.2 and SUR1 b Delta 33 by recording macroscopic currents in membrane patches excised fr om Xenopus oocytes expressing these subunits. We also investigated the effe ct of truncating SUR1 at either the start (SUR1bT1) or end (SUR1bT2) of exo n 33 on K-ATP channel properties. 3. Kir6.2/SUR1b Delta 33 showed an enhanced open probability (P-o = 0.6 at -60 mV) and a reduced ATP sensitivity (K-i, 88 mu M), when compared with wi ld-type channels (P-o = 0.3; K-i, 22 mu M). However, Kir6.2/SUR1bT1 and Kir 6.2/SUR1bT2 resembled the wild-type channel in their P-o and ATP sensitivit y. 4. Neither MgADP, nor the K+ channel opener diazoxide, enhanced Kir6.2/SUR1 b Delta 33, Kir6.2/SUR1bT1. or Kir6.2/SUR1bT2 currents, consistent with the idea that these agents require an intact NBD2 for their action. Sulphonylu reas blocked K-ATP channels containing any of the three SUR variants, but i n excised patches the extent of block was less than that. for the wild-type channel. In intact cells, the extent of sulphonylurea block of Kir6.2/SUR1 b Delta 33 was greater than that in excised patches and was comparable to t hat found for wild-type channels. 5. Our results demonstrate that NBD2 is not essential for functional expres sion or sulphonylurea block, but is required for K-ATP channel activation b y K+ channel openers and nucleotides. Some of the unusual properties of Kir 6.2/SUR1b Delta 33 resemble those reported for the K-ATP channel of ventrom edial hypothalamic (VMH) neurones, but the fact that this mRNA is expressed at low levels in many other tissues makes it less likely that SUR1b Delta 33 serves as the SUR subunit for the VMH K-ATP channel.