Modulation of hERG 1a Trafficking by hERG 1b Subunits in Heart

Abstract

In human heart the rapid component of the delayed rectifier potassium current (IKr), is an important contributor to repolarization. IKr exists as heterotetrameric channel complexes composed of hERG 1a/1b subunits. hERG 1b is defined by a shorter and unique N-terminus that lacks the Per-Arnt-Sim or ether-a-go-go, domain with implications for its role in the faster deactivation of hERG 1a/1b currents compared to homomeric hERG 1a currents. Decreases in IKr, due to either congenital mutations in hERG 1a/1b subunits or acquired in diabetes, delay repolarization, leading to long QT syndrome (LQTS), a condition that increases the likelihood of fatal arrhythmias. The mechanism by which hERG 1a/1b subunits traffic in heart is unknown. Here, we tested the hypothesis that hERG 1b subunits modulate the surface expression of hERG 1a in heart. hERG 1a/1b subunits were tagged intra-cellularly with YFP/CFP and extra-cellularly with a 13-residue high affinity α-bungarotoxin binding site (BBS). BBS-tagged hERG 1a-YFP expressed alone or with 1b-CFP generated functional currents. Cell surface BBS-tagged channel subunits were selectively detected with quantum dot. Tagged hERG 1a/1b subunits expressed in rat ventricular myocytes using adenovirus were co-localized and targeted to the surface sarcolemma, t-tubules and intercalated disks, similar to native channels. Optical pulse chase assays revealed for the first time that co-expression of hERG 1b increases the surface expression of hERG 1a in heart. Our data emphasize that hERG 1b is an important modulator of IKr. Understanding this process is likely to provide crucial insights for the development of targeted therapies for LQTS.