Water and urea permeation pathways of the human excitatory amino acid transporter EAAT1

Glutamate transport is coupled to the co-transport of 3 Na⁺ and 1 H⁺ followed by the counter-transport of 1 K⁺. In addition, glutamate and Na⁺ binding to glutamate transporters generates an uncoupled anion conductance. The human glial glutamate transporter EAAT1 (excitatory amino acid transporter 1) also allows significant passive and active water transport, which suggests that water permeation through glutamate transporters may play an important role in glial cell homoeostasis. Urea also permeates EAAT1 and has been used to characterize the permeation properties of the transporter. We have previously identified a series of mutations that differentially affect either the glutamate transport process or the substrate-activated channel function of EAAT1. The water and urea permeation properties of wild-type EAAT1 and two mutant transporters were measured to identify which permeation pathway facilitates the movement of these molecules. We demonstrate that there is a significant rate of L-glutamate-stimulated passive and active water transport. Both the passive and active L-glutamate-stimulated water transport is most closely associated with the glutamate transport process. In contrast, L-glutamate-stimulated [¹⁴C]urea permeation is associated with the anion channel of the transporter. However, there is also likely to be a transporter-specific, but glutamate independent, flux of water via the anion channel.