Two Discontinuous Segments in the Carboxyl Terminus Are Required for Membrane Targeting of the Rat -Aminobutyric Acid Transporter-1 (GAT1)*

Like all members of the Na /Cl -dependent neurotransmitter transporter family, the rat -aminobutyric acid transporter-1 (GAT1) is sorted and targeted to specialized domains of the cell surface. Here we identify two discontinuous signals in the carboxyl terminus of GAT1 that cooperate to drive surface expression. This conclusion is based on the following observations. Upon deletion of the last 37 amino acids, the resulting GAT137 remained trapped in the endoplasmic reticulum. The presence of 10 additional residues (GAT127) sufficed to support the interaction with the coat protein complex II component Sec24D; surface expression of GAT127 reached 50% of the wild type level. Additional extensions up to the position 3 (GAT13) did not further enhance surface expression. Thus the last three amino acids (AYI) comprise a second distal signal. The sequence AYI is reminiscent of a type II PDZ-binding motif; accordingly substituting Glu for Ile abrogated the effect of this motif. Neither the AYI motif nor the last 10 residues rescued the protein from intracellular retention when grafted onto GAT137 and GAT132; the AYI motif was dispensable for targeting of GAT1 to the growth cone of differentiating PC12 cells. We therefore conclude that the two segments act in a hierarchical manner such that the proximal motif (VMI) supports endoplasmic reticulum export of the protein and the distal AYI motif places GAT1 under the control of the exocyst.