Résumé
The ATP-sensitive potassium channel (KATP) of pancreatic β-cells is composed of the sulphonylurea-binding protein, SUR1, and the inwardly rectifying K+ channel subunit, Kir6.2. We have characterized two novel isoforms of rat SUR1 in the RINm5F insulin-secreting cell line.
SUR1A2 is an allelic variant with a single amino acid change in the first nucleotide-binding domain. Coinjection of SUR1A2 plus Kir6.2 into Xenopus oocytes or expression of a SUR1A2–Kir6.2 tandem in HEK-293 cells yielded large currents with characteristics similar to the wild-type KATP channel.
SUR1BΔ31, detected in several human tissues, is a splice variant of the rat SUR1 gene that lacks exon 31 of the corresponding human SUR1 gene. SUR1BΔ31 lacks the TM16–TM17 transmembrane-spanning helices leading to a protein with a different transmembrane topology. Coinjection of SUR1BΔ31 plus Kir6.2 into Xenopus oocytes or expression of the Kir6.2/SUR1BΔ31 tandem construct in HEK-293 cells did not result in any current, and a surface expression assay indicated that this channel does not reach the plasma membrane.
SUR1A2 and SUR1A1 proteins expressed in HEK-293 cells display similar binding affinities for [3H]-glibenclamide, while SUR1BΔ31 shows a 500-fold lower affinity.
These findings confirm that TM16–TM17 of SUR1 are important for high-affinity glibenclamide binding and that their deletion impairs trafficking of the KATP channel to the surface membrane.