We have investigated the functional impact of a naturally occurring mutation of 
the human glutamate transporter GLT1 (EAAT2), which had been detected in a 
patient with sporadic amyotrophic lateral sclerosis. The mutation involves a 
substitution of the putative N-linked glycosylation site asparagine 206 by a 
serine residue (N206S) and results in reduced glycosylation of the transporter 
and decreased uptake activity. Electrophysiological analysis of N206S revealed a 
pronounced reduction in transport rate compared with wild-type, but there was no 
alteration in the apparent affinities for glutamate and sodium. In addition, no 
change in the sensitivity for the specific transport inhibitor dihydrokainate 
was observed. However, the decreased rate of transport was associated with a 
reduction of the N206S transporter in the plasma membrane. Under ionic 
conditions, which favor the reverse operation mode of the transporter, N206S 
exhibited an increased reverse transport capacity. Furthermore, if coexpressed 
in the same cell, N206S manifested a dominant negative effect on the wild-type 
GLT1 activity, whereas it did not affect wild-type EAAC1. These findings provide 
evidence for a role of the N-linked glycosylation in both cellular trafficking 
and transport function. The resulting alteration in glutamate clearance capacity 
likely contributes to excitotoxicity that participates in motor neuron 
degeneration in amyotrophic lateral sclerosis.