Unsynchronised subunit motion in single trimeric sodium-coupled aspartate transporters

Excitatory amino acid transporters (EAATs) are secondary transport proteins that mediate the uptake of glutamate and other amino acids(1). EAATs fulfil an important role in neuronal signal transmission by clearing the excitatory neurotransmitters from the synaptic cleft after depolarization of the postsynaptic neuron. An intensively studied model system for understanding the transport mechanism of EAATs is the archaeal aspartate transporter Glt(Ph)(2-6). Each subunit in the homotrimeric Glt(Ph) supports the coupled translocation of one aspartate molecule and three Na+ ions(2) as well as an uncoupled flux of Cl- ions(7). Recent crystal structures of Glt(Ph)(3,5,6,8) revealed three possible conformations for the subunits, but it is unclear whether the motions of individual subunits are coordinated to support transport. Here, we report the direct observation of conformational dynamics in individual Glt(Ph) trimers embedded in the membrane by applying single-molecule fluorescence resonance energy transfer (FRET). By analysing the transporters in a lipid bilayer instead of commonly used detergent micelles, we achieve conditions that approximate the physiologically relevant ones. From the kinetics of FRET level transitions we conclude that the three Glt(Ph) subunits undergo conformational changes stochastically and independently of each other.