Kazumitsu Ueda

MgADP Antagonism to Mg2+-independent ATP Binding of the Sulfonylurea Receptor SUR1

Pancreatic β-cell ATP-sensitive potassium (KATP) channels play an important role in the regulation of glucose-induced insulin secretion. The β-cell KATP channel comprises two subunits, the sulfonylurea receptor SUR1, a member of the ATP-binding cassette (ABC) superfamily, and Kir6.2, a member of the inward rectifier K+ channel family. The activity of the KATP channel is under complex regulation by the intracellular ATP and ADP. To understand the roles of the two nucleotide-binding folds (NBFs) of SUR1 in the regulation of KATP channel activity, we introduced point mutations into the core consensus sequence of the Walker A or B motif of each NBF of SUR1 and characterized ATP binding and ADP or MgADP antagonism to it. SUR1 was efficiently photolabeled with 8-azido-[α-32P]ATP and 8-azido-[γ-32P]ATP in the presence or absence of Mg2+ or vanadate. NBF1 mutations impaired ATP binding, but NBF2 mutations did not. MgADP strongly antagonized ATP binding, and the NBF2 mutation reduced MgADP antagonism. These results show that SUR1, unlike other ABC proteins, strongly binds ATP at NBF1 even in the absence of Mg2+ and that MgADP, through binding at NBF2, antagonizes the Mg2+-independent high affinity ATP binding at NBF1.

CHAPTER 27 – THE SULFONYLUREA RECEPTOR: AN ABCC TRANSPORTER THAT ACTS AS AN ION CHANNEL REGULATOR

The sulfonylurea receptor (SUR) serves as the regulatory subunit of the ATP-sensitive potassium (K ATP ) channel, endowing it with the ability to respond to changes in cell metabolism. The K ATP channel is a hetero-octameric complex of four Kir6.x and four SUR subunits. Kir6.x belongs to the family of inwardly rectifying K channels and assembles into tetramers to form the channel. Binding of ATP to the intracellular domains of Kir6.x produces channel inhibition. Associated with each Kir6.x subunit is a regulatory subunit, the sulfonylurea receptor (SUR). SUR has two large intracellular domains, containing consensus sequences for nucleotide binding and hydrolysis, which are known as the nucleotide binding domains (NBDs). Interaction of Mg-nucleotides with these NBDs mediates activation of the K ATP channel. The SUR subunit also binds therapeutic drugs, such as the sulfonyl ureas, which inhibit K ATP channel activity, and the K ATP channel openers that stimulate channel activity. Kir6.2 serves as the pore-forming subunit, but it associates with different SUR subunits: for example, SUR1 in pancreas and brain, SUR2A in heart, and skeletal muscle and SUR2B in a variety of tissues including brain and smooth muscle. In some smooth muscles, the K ATP channel comprises Kir6.1 in association with SUR2B. This chapter focuses on the physiological role of the K ATP channel and how this is impaired in disease. It then discusses the molecular composition of the K ATP channel, and details its regulation by nucleotides, metabolism and other signaling molecules. Finally, what is known of the pharmacological properties of the channel is summarized.