SUBUNIT STOICHIOMETRY OF THE PANCREATIC BETA-CELL ATP-SENSITIVE K+ CHANNEL

We have investigated the subunit stoichiometry of the pancreatic beta- cell ATP-sensitive K+ (K-ATP) channel (SUR1/Kir6.2 channel) by constru cting cDNA encoding a single polypeptide (beta alpha polypeptide) cons isting of a SUR1 (beta) subunit and a Kir6.2 (alpha) subunit. Rb-86(+) efflux and single-channel properties of COS1 cells expressing beta al pha polypeptides were similar to those of COS1 cells coexpressing or m onomers and beta monomers. Coexpression of beta alpha polypeptides wit h alpha monomers inhibited the K+ currents, while coexpression with be ta monomers did not. We then constructed another single polypeptide (b eta alpha(2)) consisting of a beta submit and a dimeric repeat of the alpha subunit. Rb-86(+) efflux from COS1 cells expressing beta alpha(2 ) polypeptides was small, but was restored by supplementation with bet a monomers. These results indicate that the activity of K-ATP channels is optimized when the alpha and beta subunits are coexpressed with a molar ratio of 1:1. Since inward rectifier K+ channels are thought to function as home- or hetero-tetramers, this suggests that the K-ATP ch annel functions as a multimeric protein, most likely a hetero-octamer composed of a tetramer of the Kir6.2 subunit and a tetramer of the SUR 1 subunit. (C) 1997 Federation of European Biochemical Societies.