The N-linker region of hERG1a upregulates hERG1b potassium channels

Abstract

A major physiological role of hERG1 (human Ether-a-go-go-Related Gene) potassium channels is to repolarize cardiac action potentials. Two isoforms, hERG1a and hERG1b, associate to form the native cardiac IKr current in vivo. Inherited mutations in hERG1a or hERG1b cause prolonged cardiac repolarization, Long QT Syndrome and sudden death arrhythmia. hERG1a subunits assemble with and enhance the number of hERG1b subunits at the plasma membrane, but the mechanism for the increase in hERG1b by hERG1a is not well understood. Here, we report that the hERG1a N-terminal PAS (Per-Arnt-Sim) domain-N-linker region expressed in trans with hERG1b markedly increased hERG1b currents and increased biotin-labelled hERG1b protein at the membrane surface. hERG1b channels with a deletion of the 1b domain did not have a measurable increase in current or biotinylated protein when co-expressed with hERG1a PAS domain-N-linker regions indicating that the 1b domain was required for the increase in hERG1b. Using a biochemical pull-down interaction assay and a FRET hybridization experiment, we detected a direct interaction between the hERG1a PAS domain-N-linker region and the hERG1b N-terminal 1b domain. Using engineered deletions and alanine mutagenesis, we identified a short span of amino acids at positions 216-220 within the hERG1a N-linker region that were necessary for the upregulation of hERG1b. Taken together, we propose that direct structural interactions between the hERG1a N-linker region and the hERG1b N-terminal 1b domain increase hERG1b at the plasma membrane. Mechanisms that enhance hERG1b current would be anticipated to shorten action potentials, which could be anti-arrhythmic, and may point toward hERG1b or the hERG1a N-linker as molecular targets for therapy for Long QT syndrome. Significance StatementPotassium (K) ion channels help to control the membrane voltage in cardiac cells and are responsible for maintaining the normal rhythm of the heart. A decrease in the number of K channels causes cardiac arrhythmia and sudden death. One K channel (KCNH2b, hERG1b) weakly forms channels at the cell membrane. Here we describe a new mechanism for upregulation of hERG1b. An N-terminal domain of a related K channel (KCNH2a, hERG1a), expressed in trans as a separate piece, interacts with hERG1b and increases the number of hERG1b channels at the membrane. A loss of hERG channels is pro-arrhythmic and an increase in hERG channels is considered anti-arrhythmic, thus the hERG1a N-terminal domain is a potential therapeutic for cardiac arrhythmias.