Excitatory amino-acid carrier 1 (EAAC1) is a high-affinity Na+-dependent L-glutamate/D, L-aspartate cell-membrane transport protein 1. It is expressed in brain as well as several non-nervous tissues. In brain, EAAC1 is the primary neuronal glutamate transporter(2,3). It has a polarized distribution in cells and mainly functions perisynaptically to transport glutamate from the extracellular environment(2-4). In the kidney it is involved in renal acidic amino-acid re-absorption and amino-acid metabolism(5-7). Here we describe the identification and characterization of an EAAC1-associated protein, GTRAP3-18. Like EAAC1, GTRAP3-18 is expressed in numerous tissues(8,9). It localizes to the cell membrane and cytoplasm, and specifically interacts with carboxy-terminal intracellular domain of EAAC1. Increasing the expression of GTRAP3-18 in cells reduces EAAC1-mediated glutamate transport by lowering substrate affinity. The expression of GTRAP3-18 can be upregulated by retinoic acid, which results in a specific reduction of EAAC1-mediated glutamate transport. These studies show that glutamate transport proteins can be regulated potently and that GTRAP can modulate the transport functions ascribed to EAAC1. GTRAP3-18 may be important in regulating the metabolic function of EAAC1.