Assembly, Maturation, and Turnover of KATP Channel Subunits

Abstract

ATP-sensitive K+, or KATP, channels are comprised of KIR6.x and sulfonylurea receptor (SUR) subunits that assemble as octamers, (KIR/SUR)4. The assembly pathway is unknown. Pulse-labeling studies show that when KIR6.2 is expressed individually, its turnover is biphasic; ∼60% is lost with t1/2 ∼36 min. The remainder converts to a long-lived species (t1/2, ∼26 h) with an estimated half-time of 1.2 h. Expressed alone, SUR1 has a long half-life, ∼25.5 h. When KIR6.2 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 KATP channels, including completion of assembly, transit to the Golgi, and glycosylation, is ∼2.2 h. Estimation of the turnover rates of mature, fully glycosylated SUR1 associated with K IR6.2 and of KIR6.2 associated with Myc-tagged SUR1 gave similar values for the half-life of KATP channels, a mean value of ∼7.3 h. KATP channel subunits in INS-1 β-cells displayed qualitatively similar kinetics. The results imply the octameric channels are stable. Two mutations, KIR6.2 W91R and SUR1 AF1388, 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 IR6.2-SUR1 heteromers assemble functional channels and are inconsistent with models where SUR1 can only assemble with KIR6.2 tetramers.