The discovery of slowness: low-capacity transport and slow anion channel gating by the glutamate transporter EAAT5

Biophys J. 2011 Jun 8;100(11):2623-32. doi: 10.1016/j.bpj.2011.04.034.


Excitatory amino acid transporters (EAATs) control the glutamate concentration in the synaptic cleft by glial and neuronal glutamate uptake. Uphill glutamate transport is achieved by the co-/countertransport of Na(+) and other ions down their concentration gradients. Glutamate transporters also display an anion conductance that is activated by the binding of Na(+) and glutamate but is not thermodynamically coupled to the transport process. Of the five known glutamate transporter subtypes, the retina-specific subtype EAAT5 has the largest conductance relative to glutamate uptake activity. Our results suggest that EAAT5 behaves as a slow-gated anion channel with little glutamate transport activity. At steady state, EAAT5 was activated by glutamate, with a K(m)= 61 ± 11 μM. Binding of Na(+) to the empty transporter is associated with a K(m) = 229 ± 37 mM, and binding to the glutamate-bound form is associated with a K(m) = 76 ± 40 mM. Using laser-pulse photolysis of caged glutamate, we determined the pre-steady-state kinetics of the glutamate-induced anion current of EAAT5. This was characterized by two exponential components with time constants of 30 ± 1 ms and 200 ± 15 ms, which is an order of magnitude slower than those observed in other glutamate transporters. A voltage-jump analysis of the anion currents indicates that the slow activation behavior is caused by two slow, rate-limiting steps in the transport cycle, Na(+) binding to the empty transporter, and translocation of the fully loaded transporter. We propose a kinetic transport scheme that includes these two slow steps and can account for the experimentally observed data. Overall, our results suggest that EAAT5 may not act as a classical high-capacity glutamate transporter in the retina; rather, it may function as a slow-gated glutamate receptor and/or glutamate buffering system.

Publication types

  • Research Support, N.I.H., Extramural
  • Research Support, Non-U.S. Gov't

MeSH terms

  • Biological Transport
  • Electric Conductivity
  • Excitatory Amino Acid Transporter 5 / metabolism*
  • Glutamates / chemistry
  • Glutamates / metabolism
  • HEK293 Cells
  • Humans
  • Indoles / chemistry
  • Indoles / metabolism
  • Ion Channel Gating*
  • Kinetics
  • Lasers
  • Photolysis
  • Sodium / metabolism


  • 4-methoxy-7-nitroindolinyl-glutamate
  • Excitatory Amino Acid Transporter 5
  • Glutamates
  • Indoles
  • Sodium