encodes a nuclear-targeted polypeptide that mediates hERG1 channel gating and expression.

encodes hERG1, the voltage-gated potassium channel that conducts the rapid delayed rectifier potassium current (I) in human cardiac tissue. hERG1 is one of the first channels expressed during early cardiac development, and its dysfunction is associated with intrauterine fetal death, sudden infant death syndrome, cardiac arrhythmia, and sudden cardiac death. Here, we identified a hERG1 polypeptide (hERG1) that is targeted to the nuclei of immature cardiac cells, including human stem cell-derived cardiomyocytes (hiPSC-CMs) and neonatal rat cardiomyocytes. The nuclear hERG1 immunofluorescent signal is diminished in matured hiPSC-CMs and absent from adult rat cardiomyocytes. Antibodies targeting distinct hERG1 channel epitopes demonstrated that the hERG1 signal maps to the hERG1 distal C-terminal domain. deletion using CRISPR simultaneously abolished I and the hERG1 signal in hiPSC-CMs. We then identified a putative nuclear localization sequence (NLS) within the distal hERG1 C-terminus, 883-RQRKRKLSFR-892. Interestingly, the distal C-terminal domain was targeted almost exclusively to the nuclei when overexpressed HEK293 cells. Conversely, deleting the NLS from the distal peptide abolished nuclear targeting. Similarly, blocking α or β1 karyopherin activity diminished nuclear targeting. Finally, overexpressing the putative hERG1 peptide in the nuclei of HEK cells significantly reduced hERG1a current density, compared to cells expressing the NLS-deficient hERG1 or GFP. These data identify a developmentally regulated polypeptide encoded by , hERG1, whose presence in the nucleus indirectly modulates hERG1 current magnitude and kinetics.