KCNE3 Truncation Mutants Reveal a Bipartite Modulation of KCNQ1 K⁺ Channels

The five KCNE genes encode a family of type I transmembrane peptides that assemble with KCNQ1 and other voltage-gated K⁺ channels, resulting in potassium conducting complexes with varied channel-gating properties. It has been recently proposed that a triplet of amino acids within the transmembrane domain of KCNE1 and KCNE3 confers modulation specificity to the peptide, since swapping of these three residues essentially converts the recipient KCNE into the donor (Melman, Y.F., A. Domenech, S. de la Luna, and T.V. McDonald. 2001. J. Biol. Chem. 276:6439–6444). However, these results are in stark contrast with earlier KCNE1 deletion studies, which demonstrated that a COOH-terminal region, highly conserved between KCNE1 and KCNE3, was responsible for KCNE1 modulation of KCNQ1 (Tapper, A.R., and A.L. George. 2000 J. Gen. Physiol. 116:379–389.). To ascertain whether KCNE3 peptides behave similarly to KCNE1, we examined a panel of NH₂- and COOH-terminal KCNE3 truncation mutants to directly determine the regions required for assembly with and modulation of KCNQ1 channels. Truncations lacking the majority of their NH₂ terminus, COOH terminus, or mutants harboring both truncations gave rise to KCNQ1 channel complexes with basal activation, a hallmark of KCNE3 modulation. These results demonstrate that the KCNE3 transmembrane domain is sufficient for assembly with and modulation of KCNQ1 channels and suggests a bipartite model for KCNQ1 modulation by KCNE1 and KCNE3 subunits. In this model, the KCNE3 transmembrane domain is active in modulation and overrides the COOH terminus' contribution, whereas the KCNE1 transmembrane domain is passive and reveals COOH-terminal modulation of KCNQ1 channels. We furthermore test the validity of this model by using the active KCNE3 transmembrane domain to functionally rescue a nonconducting, yet assembly and trafficking competent, long QT mutation located in the conserved COOH-terminal region of KCNE1.