Glutamate Transporter Protein Structure And Function

The excitatory amino acid transporter EAAC1 is an electrogenic Na+-and K+-gradient driven transporter. In addition, the transporter mediatesin the presence of Na+ and glutamate an anion conductance uncoupled fromthe transport of the glutamate. The first two N-terminal domains,important for forming the conductance mode, are extracellularly borderedby positively charged arginine residues, R39 and R61, being completelyconserved throughout the transporter family. Also the conservedtryrosine residue Y98 could be important for the Cl- conductance. We haveinvestigated by measurements of glutamate uptake and glutamate-inducedcurrents the effects of mutation of the arginines and the tyrosine toalanine. The mutation R39A hardly affects transport and channel mode.The mutation R61A, on the other hand, reduces the activity of transportbut stimulates the channel conductance. In addition, the apparent Kmvalues for glutamate uptake and for the glutamate-activated current arereduced. Glutamate stimulation of current seems to be associated witha voltage-dependent step, and the apparent valence of charge moved duringbinding is reduced in the R61A mutant. The mutation Y98A leads to reducedfunction with reduced apparent Km value for glutamate, and with strongreduction of the selectivity ration between NO3 and Cl- of the conductancemode.