Fig. 7. NBD1 of SURx binds ATP or ADP, and has little or no ATPase activity (Bienengraeber et al., 2000; Matsuo et al., 2000). NBD2 of SURx binds MgATP or MgADP depending on their ratio and concentration. Bound MgATP is hydrolysed to MgADP. MgADP activates the KATP channel via NBD2. The S2R mutation of SUR1 and SUR2B abolishes channel activation by Mg-nucleotides completely. In contrast, the S1R mutation in SURx reduces channel activation by MgADP or MgATP, possibly by influencing transduction at NBD2. The S2R mutation of SUR2A does not alter KATP channel activation by MgADP, suggesting that the SUR2A tail prevents (or compensates for) the effect of the S2R mutation on signal transduction.