Assembly, maturation, and turnover of KATP channel subunits

ATP-sensitive K+, or K-ATP, channels are comprised of K(IR)6.x and sulfonylurea receptor (SUR) subunits that assemble as octamers, (K-IR/SUR)(4). The assembly pathway is unknown. Pulse-labeling studies show that when K(IR)6.2 is expressed individually, its turnover is biphasic; similar to60% is lost with t(1/2) similar to 36 min. The remainder converts to a long-lived species (t(1/2) similar to 26 h) with an estimated half-time of 1.2 h. Expressed alone, SUR1 has a long half-life, similar to25.5 h. When K(IR)6.2 and SUR1 are co-expressed, they associate rapidly and the fast degradation of KIR6.2 is eliminated. Based on changes in the glycosylation state of SUR1, the half-time for the maturation of K-ATP channels, including completion of assembly, transit to the Golgi, and glycosylation, is similar to 2.2 h. Estimation of the turnover rates of mature, fully glycosylated SUR1 associated with K(IR)6.2 and of K(IR)6.2 associated with Myc-tagged SUR1 gave similar values for the half-life of K-ATP channels, a mean value of similar to 7.3 h. K-ATP channel subunits in INS-1 beta-cells displayed qualitatively similar kinetics. The results imply the octameric channels are stable. Two mutations, K(IR)6.2 W91R and SUR1 DeltaF1388, identified in patients with the severe form of familial hyperinsulinism, profoundly alter the rate of KIR6.2 and SUR1 turnover, respectively. Both mutant subunits associate with their respective partners but dissociate freely and degrade rapidly. The data support models of channel formation in which K(IR)6.2-SUR1 heteromers assemble functional channels and are inconsistent with models where SUR1 can only assemble with K(IR)6.2 tetramers.