Recruitment of Neuronal KCNQ2/3 Channels to Membrane Microdomains Depends on Palmitoylation of Alzheimer's Disease-Related Protein BACE1

Abstract

{beta}-secretase 1 ({beta}-site amyloid precursor protein (APP)-cleaving enzyme 1, BACE1) plays a crucial role in the amyloidogenesis of Alzheimers Disease (AD). BACE1 was also discovered to act like an auxiliary subunit to modulate neuronal KCNQ2/3 channels independent of its proteolytic function. BACE1 is palmitoylated at its carboxyl-terminal region, which brings BACE1 to ordered, cholesterol-rich membrane microdomains (lipid rafts). However, the physiological consequences of this specific localization of BACE1 remain elusive. Using spectral Forster Resonance Energy Transfer (FRET), BACE1 and KCNQ2/3 channels were confirmed to form a signaling complex, a phenomenon that was relatively independent of the palmitoylation of BACE1. Nevertheless, palmitoylation of BACE1 was required for recruitment of KCNQ2/3 channels to lipid-raft domains. Two fluorescent probes designated L10 and S15, were used to label lipid-raft and non-raft domains of the plasma membrane, respectively. Coexpressing BACE1 substantially elevated the FRET between L10 and KCNQ2/3 whereas the BACE1-4C/A quadruple mutation failed to produce this effect. In contrast, BACE1 had no significant effect on the FRET between S15 probes and KCNQ2/3 channels. A reduction of BACE1-dependent FRET between raft-targeting L10 probes and KCNQ2/3 channels by applying cholesterol-extracting reagent methyl-{beta}-cyclodextrin (M{beta}CD), raft-disrupting general anesthetics, or pharmacological inhibitors of palmitoylation all supported our hypothesis of the palmitoylation-dependent and raft-specific localization of KCNQ2/3 channels. Furthermore, mutating the four carboxyl-terminal cysteines (4C/A) of BACE1 abolished the BACE1-dependent increase of FRET between KCNQ2/3 and a lipid raft-specific protein caveolin 1. Collectively, we propose how the AD-related protein BACE1 underlies the localization of a neuronal potassium channel.