Sodium-independent, proton-coupled amino acid uptake at the apical membrane of the small intestine model cell line Caco-2 was originally demonstrated by Thwaites et al. (1993), and it is now evident that members of the SLC36 (PAT) family mediate the proton-coupled uptake of glycine, alanine and proline (reviewed by Boll et al. 2004). The CG1139 gene from Drosophila melanogaster encodes a protein with a predicted open reading frame of 451 amino acids with 33% sequence identity to both mouse PAT1 and PAT2. Here we provide evidence that CG1139 encodes a proton-coupled amino acid transport system analogous to the mammalian PAT transporters. The CG1139 clone was obtained from the Drosophila Genomics Resource Centre and expressed in Xenopus laevis oocytes. Membrane potential and the uptake of 50μM [³H]-Ala were measured using a similar approach to that previously described (Meredith 2004). The membrane potential of oocytes injected with CG1139 cRNA was initially shown to depolarize in response to a mixture of 20 common L-amino acids. Further experiments revealed that the amino acids responsible for this response were alanine, glycine and proline, and that there was a larger depolarization at pH_out 5.5 than 7.4 (Fig. 1A). No depolarization of membrane potential was seen in control (non-injected) oocytes. Further characterization of transport by CG1139 using 50μM [³H]-alanine: uptake was stimulated approximately 2.5-fold by extracellular acidification (78.8 ± 2.33 versus 31.5 ± 2.41 pmoles/oocyte/hr, pH_out 5.5 versus 7.4 respectively, n=5 oocytes, p<0.001 StudentÆs t test), with an apparent Km for alanine of 1.2 ± 0.2 mM at pH_out 5.5 (Figure 1B). Thus the affinity of CG1139 for alanine falls between that of the lower affinity mouse PAT1 and the higher affinity PAT2 isoforms (Km of 7.5 and 0.26 mM respectively when expressed in Xenopus oocytes, Boll et al. 2002). We conclude that CG1139 encodes for a proton-coupled amino acid transporter with a substrate specificity for alanine, glycine and proline, similar to that of the mammalian PAT family.