The essential role of the Walker A motifs of SUR1 in K-ATP channel activation by Mg-ADP and diazoxide

The ATP-sensitive K-channel (K-ATP channel) plays a key role in insulin secretion from pancreatic beta-cells. It is closed by glucose metabolism, which stimulates insulin secretion, and opened by the drug diazoxide, which inhibits insulin release, Metabolic regulation is mediated by changes in ATP and Mg-ADP, which inhibit and potentiate channel activity, respectively, The beta-cell K-ATP channel consists of a pore-forming subunit, Kir6.2, and a regulatory subunit, SUR1. We have mutated (independently or together) two lysine residues in the Walker A (W-A) motifs of the first (K719A) and second (K1384M) nucleotide-binding domains (NBDs) of SUR1, These mutations are expected to inhibit nucleotide hydrolysis, Our results indicate that the W-A lysine of NBD1 (but not NBD2) is essential for activation of K-ATP currents by diazoxide, The potentiatory effects of Mg-ADP required the presence of the W-A lysines in both NBDs, Mutant currents were slightly more sensitive to ATP than wild-type currents, Metabolic inhibition led to activation of wildtype and K1384M currents, but not K719A or K719A/K1384M currents, suggesting that there may be a factor in addition to ATP and ADP which regulates K-ATP channel activity.