Interaction of the cytosolic domains of the Kir6.2 subunit of the KATP channel is modulated by sulfonylureas

Abstract

The NH2- and COOH-termini of the ATP-sensitive potassium (KATP) channel pore-forming subunit, Kir6.2, both lie intracellularly and interact with one another. To study this interaction, cyan fluorescent protein (CFP) and yellow fluorescent protein (YFP) were fused to the NH2- and COOH-termini of Kir6.2, respectively (CFP-Kir6.2-YFP). These fluorescent proteins have sufficient spectral overlap to allow distance-dependent fluorescence resonance energy transfer (FRET). When CFP- Kir6.2-YFP was expressed in human embryonic kidney cells and illuminated at 440 nm to excite CFP, significant fluorescence was recorded at 535 nm, the peak of the YFP emission spectrum. This indicated that FRET was occurring and thus that the NH2- and COOH-termini of Kir6.2 lie in close proximity to one another. The emission ratio, F535/F480, was increased by co-expression of SUR2A, but not SUR1, suggesting that SUR2A but not SUR1 influences the Kir6.2 NH2- and COOH-terminal interaction. This interaction was reduced by the sulfonylureas tolbutamide and gliclazide, but not by the pore blocker barium. The properties of the tolbutamide response indicate that the drug disrupts the interaction between the NH2- and COOH-termini of Kir6.2 by binding to a low-affinity site on Kir6.2.