Defective Human Ether-à-go-go-related Gene Trafficking Linked to an Endoplasmic Reticulum Retention Signal in the C Terminus

Abstract

Mutations in the humanEther-à-go-go-Related gene (HERG), encoding the protein underlying the cardiac K⁺ current, I_Kr, cause chromosome 7-linked long QT syndrome (LQT2). In this study, we show that deletion of the C-terminal 147 amino acids (HERG_Δ147) abolished I_Kr, whereas a larger, 159-amino acid deletion (HERG_Δ159) identified in an LQT2 kindred did generate I_Kr, albeit with reduced amplitude compared with the wild type. The 12 amino acids present in HERG_Δ147 and absent in HERG_Δ159 include a potential endoplasmic reticulum (ER) retention signal, RGR, which when mutated to LGL (HERG_Δ147-LGL) restored I_Kr. Streptavidin selection of biotin-labeled surface proteins showed good expression of wild-type and HERG_Δ159 at the cell surface and low expression of HERG_Δ147-LGL and HERG_Δ147. Additionally, a 100-amino acid peptide spanning the RGR triplet can rescue the defect in HERG_Δ147 when co-expressed as an ER-targeted minigene. Failure of HERG trafficking is known to cause LQT2, and this identified a molecular mechanism underlying this defect. Further, our data indicate that a key function of the C-terminal 104 amino acids is to mask the RGR ER retention signal, which becomes exposed when mutations truncate the HERG C terminus.