C-terminal interactions modulate the affinity of GLAST glutamate transporters in salamander retinal glial cells

1. Proteins that interact with the intracellular carboxy termini of neurotr ansmitter- and voltage-gated ion channels are known to control the subcellu lar localization of the channels, localize other proteins near those channe ls, and modulate channel activity. By contrast, little is known about the c ontrol of neurotransmitter transporter function by interacting proteins. 2. To competitively disrupt interactions of the C- and N-termini of the GLA ST glutamate transporter with other proteins, we dialysed whole-cell patch- clamped retinal glia with peptides identical to the eight amino acids at th e C- or N-termini of the transporter, and compared the effect on transporte r-mediated currents with dialysis of scrambled versions of the same peptide s. 3. Dialysis with the N-terminus peptide had no effect on the maximum glutam ate-evoked current nor on the glutamate affinity of the transporter. Dialys is with the C-terminus peptide had no effect on the maximum current, but in creased the affinity of the transporter for glutamate (compared with scramb led C-terminus peptide, and with N- and scrambled N-terminus peptides: K-m decreased from 16 to 11 mu m)). 4.These data suggest that disruption of an interaction between an intracell ular protein and the last eight amino acids of the GLAST C-terminus, which have some similarity to the PDZ binding domain of ion channel C-termini, in creases the glutamate affinity of GLAST. Thus, the interacting protein decr eases the affinity of GLAST transporters. 5. Removing the GLAST C-terminus interaction increases the transporter curr ent by 40 % at low glutamate concentrations. Thus, this interaction may sig nificantly slow the removal of low concentrations of glutamate from the ext racellular space, and affect the kinetics of retinal cell light responses.