Dominant mutations in ABCC8 accounted for 12 percent of cases of neonatal diabetes in the study group. Diabetes results from a newly discovered mechanism whereby the basal magnesium-nucleotide-dependent stimulatory action of SUR1 on the Kir pore is elevated and blockade by sulfonylureas is preserved.

ATP-sensitive potassium channels, termed KATP channels, link the electrical activity of cell membranes to cellular metabolism. These channels are heteromultimers of sulfonylurea receptor (SUR) and KIR6.X subunits associated with a 1:1 stoichiometry as a tetramer (SUR/KIR6.X forms the pores, whereas SUR regulates their activity. Changes in [ATP]i and [ADP]i gate the channel. The diversity of KATP channels results from the assembly of SUR and KIR6.X subtypes KIR6.1-based channels differ from KIR6.2 channels mainly by their smaller unitary conductance. SUR1- and SUR2-based channels are distinguished by their differential sensitivity to sulfonylureas, whereas SUR2A-based channels are distinguished from SUR2B channels by their differential sensitivity to diazoxide. Mutations that result in the loss of KATP channels in pancreatic beta-cells have been identified in SUR1 and KIR6.2. These mutations lead to familial hyperinsulinism. Understanding the mutations in SUR and KIR6.X is allowing insight into how these channels respond to nucleotides, sulfonylureas, and potassium channel openers, KCOs.

Adenosine 5'-triphosphate-sensitive potassium (KATP) channels couple metabolic events to membrane electrical activity in a variety of cell types. The cloning and reconstitution of the subunits of these channels demonstrate they are heteromultimers of inwardly rectifying potassium channel subunits (KIR6.x) and sulfonylurea receptors (SUR), members of the ATP-binding cassette (ABC) superfamily. Recent studies indicate that SUR and KIR6.x associate with 1:1 stoichiometry to assemble a large tetrameric channel, (SUR/KIR6.x)4. The KIR6.x subunits form the channel pore, whereas SUR is required for activation and regulation. Two KIR6.x genes and two SUR genes have been identified, and combinations of subunits give rise to KATP channel subtypes found in pancreatic beta-cells, neurons, and cardiac, skeletal, and smooth muscle. Mutations in both the SUR1 and KIR6.2 genes have been shown to cause familial hyperinsulinism, indicating the importance of the pancreatic beta-cell channel in the regulation of insulin secretion. The availability of cloned KATP channel genes opens the way for characterization of this family of ion channels and identification of additional genetic defects.

Structure-function analyses of K؉ channels identify a common pore architecture whose gating depends on diverse signal sensing elements. The "gatekeepers" of the long, ATP-inhibited K IR 6.0 pores of K ATP channels are ABC proteins, SURs, receptors for channel opening and closing drugs. Several competing models for SUR/K IR coupling exist. We show that SUR TMD0, the N-terminal bundle of five transmembrane helices, specifically associates with K IR 6.2, forcing nearly silent pores to burst like native K ATP channels and enhancing surface expression. Inclusion of adjacent submembrane residues of L0, the linker between TMD0 and the stimulatory nucleotide-and drug-binding ABC core, generates constitutively active channels, whereas additional cytoplasmic residues counterbalance this activation establishing a relationship between the mean open and burst times of intact pores. SUR fragments, lacking TMD0, fail to modulate K IR . TMD0 is thus the domain that anchors SUR to the K IR pore. Consistent with data on chimeric ABCC/K IR s and a modeled channel structure, we propose that interactions of TMD0-L0 with the outer helix and N terminus of K IR bidirectionally modulate gating. The results explain and predict pathologies associated with alteration of the 5 ends of clustered ABCC8 (9)/ KCNJ11 (8) genes.ADP/ATP-dependent (SUR/K IR 6.0) 4 , K ATP channels encoded by the ABCC8(SUR1)/KCNJ11(K IR 6.2) and ABCC9(SUR2)/ KCNJ8(K IR 6.1) gene clusters (1-3) present the puzzle of how disparate subunits, from two large superfamilies of membrane transport proteins, are coupled. K IR 6.2 pores, normally retained in the ER 1 (4), display a low maximal open probability in ligand-free solutions (P O(max) ), when forced to the cell surface by deleting the C-terminal RKR retention signal (5). Assembly with SURs increases surface expression, the P O(max) , and sensitivity to inhibitory ATP, and induces responsiveness to stimulatory MgADP, K ATP openers, inhibitory sulfonylureas, and other insulin secretagogues (6). ATP acts through the K IR cytoplasmic domains to close the pore, while MgADP acts through the nucleotide-binding domains (NBDs) in the ABC core to stimulate the channel (5, 7). In vivo, SUR/K IR 6.0 coupling machinery can overcome the power of saturating concentrations of inhibitory ATP (8), indicating a novel "gatekeeper" (9) that exerts activating forces at multiple points on the long (10, 11) K ATP pore. The recent structure of a related ABC transporter shows the SUR core (gray in Fig. 1A) consists of two bundles of six transmembrane helices (TMDs) fused to NBDs (12). SURs, and certain ABCC proteins (13), have an additional TMD0-L0 module, for which there is no three-dimensional template. Several competing models for SUR/K IR coupling have been suggested (14 -17), but attempts, using chimeric subunits, to discover which domains of SUR interact with K IR have been unsuccessful. Here we used SUR fragments to generate "mini-K ATP " that validate our original model (17) and reveal the mechanism of SUR/K IR coupling. EXPERIMENTAL...

In beta cells from the pancreas, ATP-sensitive potassium channels, or K ATP channels, are composed of two subunits, SUR1 and K IR 6.2, assembled in a (SUR1/ K IR 6.2) 4 stoichiometry. The correct stoichiometry of channels at the cell surface is tightly regulated by the presence of novel endoplasmic reticulum (ER) retention signals in SUR1 and K IR 6.2; incompletely assembled K ATP channels fail to exit the ER/cis-Golgi compartments. In addition to these retrograde signals, we show that the C terminus of SUR1 has an anterograde signal, composed in part of a dileucine motif and downstream phenylalanine, which is required for K ATP channels to exit the ER/cis-Golgi compartments and transit to the cell surface. Deletion of as few as seven amino acids, including the phenylalanine, from SUR1 markedly reduces surface expression of K ATP channels. Mutations leading to truncation of the C terminus of SUR1 are one cause of a severe, recessive form of persistent hyperinsulinemic hypoglycemia of infancy. We propose that the complete loss of beta cell K ATP channel activity seen in this form of hyperinsulinism is a failure of K ATP channels to traffic to the plasma membrane.