A shared mechanism for lipid- and beta-subunit-coordinated stabilization of the activated K+ channel voltage sensor

The low-dielectric plasma membrane provides an energy barrier hindering transmembrane movement of charged particles. The positively charged, voltage-sensing fourth transmembrane domain (S4) of voltage-gated ion channels must surmount this energy barrier to initiate channel activation, typically necessitating both membrane depolarization and interaction with membrane lipid phospho-head groups (MLPHGs). In contrast, and despite containing S4, the KCNQ1 K(+) channel alpha subunit exhibits predominantly constitutive activation when in complexes with transmembrane beta subunits, MinK-related peptide (MiRP) 1 (KCNE2) or MiRP2 (KCNE3). Here, using a 2-electrode voltage clamp and scanning mutagenesis of channels heterologously expressed in Xenopus laevis oocytes, we discovered that 2 of the 8 MiRP2 extracellular domain acidic residues (D54 and D55) are important for KCNQ1-MiRP2 constitutive activation. Double-mutant thermodynamic cycle analysis revealed energetic coupling of D54 and D55 to R237 in KCNQ1 S4 but not to 10 other native or introduced polar residues in KCNQ1 S4 and surrounding linkers. MiRP2-D54 and KCNQ1-R237 also similarly dictated susceptibility to the inhibitory effects of MLPHG hydrolysis, whereas other closely situated polar residues did not. Thus, by providing negative charge near the plasma membrane extracellular face, MiRP2 uses a lipomimetic mechanism to constitutively stabilize the activated KCNQ1 voltage sensor.