Differential nucleotide regulation of K_ATP channels by SUR1 and SUR2A

ATP-sensitive potassium (K_ATP) channels are heterooctamers of an inward rectifier potassium channel (Kir6) and a sulfonylurea receptor (SUR, a member of the ATP-binding cassette (ABC) transporter family). In the pancreatic β-cell K_ATP channels are dynamically active, and transgenic expression of overactive Kir6.2 mutants leads to severe neonatal diabetes and death, while in the ventricular cardiomyocyte they are closed except under conditions of severe metabolic inhibition, and similarly overactive transgenes are without gross phenotypic consequence. This discrepancy may arise in part from differences at the molecular level between the two SUR isotypes that constitute the regulatory subunit of the K_ATP channel in those tissues: SUR1 in the pancreas, SUR2A in the heart. K_ATP channels generated from coexpression of Kir6.2 with SUR1 exhibit greater MgADP stimulation than channels generated from coexpression of Kir6.2 with SUR2A. This difference persists when the open state stability of the channel is enhanced by application of PIP₂, consistent with each isotype transducing an intrinsically different energetic contribution to the channel pore. When expressed as isolated, affinity-purified protein constructs, NBF2 of SUR1 exhibits increased in vitro ATP hydrolysis compared to NBF2 of SUR2A. This biochemical difference may underlie the increased MgADP stimulation exhibited by SUR1-containing channels vs. SUR2A-containing channels, which may in turn contribute to physiological differences, observed at the tissue level, between pancreatic and cardiac K_ATP channels.