Heterodimerization within the TREK Channel Subfamily

Abstract

TREK1, TREK2, and TRAAK form the TREK subfamily of two-pore-domain K+ (K2P) channels. Despite sharing up to 78% sequence identity and showing overlapping expression profiles in the nervous system, these channels show major differences in their regulation by physiological stimuli, such as phosopholipids or external pH. For instance, TREK1 is inhibited by external acidification while TREK2 is activated. Here, we investigated the ability of the members of the TREK subfamily to assemble to form functional heteromeric channels with novel properties. Using single molecule pull-down and subunit counting in the plasma membrane of living cells, we show that TREK1, TREK2 and TRAAK readily co-assemble to form heterodimers. TREK1 and TREK2 can each heterodimerize with TRAAK, but do so less efficiently than with each other. We functionally characterized the different heterodimers and found that all combinations form outwardly-rectifying potassium-selective channels but with variable voltage-sensitivity and regulation. TREK1/TREK2 heterodimers show low levels of activity at physiological pH but, unlike their corresponding homodimers, are activated by both acidic and alkaline conditions. Modelling based on recent crystal structures, along with mutational analysis, suggests that each subunit within a TREK1-TREK2 channel can be regulated independently of each other via titratable histidines and their interaction with nearby charged residues. TREK1/TRAAK heterodimers is also functional and unlike the TRAAK homodimer is able to be regulated by PLD2 which binds the TREK1 moiety. Together, this study greatly expands the number of channels within the TREK subfamily and defines how heterodimerization can alter the physiological properties of K2P channels.