We have cloned and characterized the opt1 gene of Drosophila melanogaster. This gene encodes a protein with significant similarity to the PTR family of oligopeptide transporters. The OPT1 protein is localized to the apical epithelial membrane domains of the midgut, rectum, and female reproductive tract. The opt1 message is maternally loaded into developing oocytes, and OPT1 is found in the alpha-yolk spheres of the developing embryo. It is also found throughout the neuropil of the central nervous system, with elevated expression within the alpha- and beta-lobes of the mushroom bodies. Transport activity was examined in HeLa cells transiently expressing OPT1. This protein is a high-affinity transporter of alanylalanine; the approximate Km constant is 48.8 microM for this substrate. OPT1 dipeptide transport activity is proton dependent. The ability of selected beta-lactams to inhibit alanylalanine transport suggests that OPT1 has a broad specificity in amino acid side chains and has a substrate requirement for an alpha-amino group. Together these data suggest an important role for OPT1 in regulating amino acid availability.