MgADP antagonism to Mg2+-independent ATP binding of the sulfonylurea receptor SUR1

Pancreatic beta-cell ATP-sensitive potassium (K-ATP) channels play an important role in the regulation of glucose-induced insulin secretion. The beta-cell K-ATP 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 K-ATP, 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 K-ATP 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-[alpha-P-32]ATP and 8-azido-[gamma-P-32]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.