Secondary Structure of the MiRP1 (KCNE2) Potassium Channel Ancillary Subunit

Abstract

MiRP1 (encoded by the KCNE2 gene) is one of a family of five single transmembrane domain voltage-gated potassium (Kv) channel ancillary subunits currently under intense scrutiny to establish their position in channel complexes and elucidate alpha subunit contact points, but its structure is unknown. MiRP1 mutations are associated with inherited and acquired cardiac arrhythmia. Here, synthetic peptides corresponding to human MiRP1 (full-length and separate domains) were structurally analyzed using FTIR and CD spectroscopy. The N-terminal (extracellular) domain was soluble and predominantly non-ordered in aqueous media, but predominantly alpha-helical in L-alpha-lysophosphatidylcholine (LPC) micelles. The MiRP1 transmembrane domain was predominantly a mixture of alpha-helix and non-ordered structure in LPC micelles, with a minor contribution from non-aggregated beta-strand. The intracellular C-terminal domain was insoluble in aqueous solution; reconstitution into non-aqueous environments resulted in solubility and adoption of increasing amounts of alpha-helix, with the solvent order sodium dodecyl sulphate < dimyristoyl L-alpha-phosphatidylcholine (DMPC) < LPC < trifluoroethanol. Correlation of secondary structure changes with lipid transition temperature during heating suggested that the MiRP1 C-terminus incorporates into DMPC bilayers. Full-length MiRP1 was soluble in SDS micelles and calculated to contain 34% alpha-helix, 23% beta-strand and 43% non-ordered structure in this environment, as determined by CD spectroscopy. Thus, MiRP1 is highly dependent upon hydrophobic interaction via lipid and/or protein contacts for adoption of ordered structure without nonspecific aggregation, consistent with a role as a membrane-spanning subunit within Kv channel complexes. These data will provide a structural framework for ongoing mutagenesis-based in situ structure-function studies of MiRP1 and its relatives.