A Novel H+-coupled Oligopeptide Transporter (OPT3) from Caenorhabditis elegans with a Predominant Function as a H+ Channel and an Exclusive Expression in Neurons

We have cloned and functionally characterized a novel, neuron-specific, H+-coupled oligopeptide transporter (OPT3) from Caenorhabditis elegans that functions predominantly as a H+ channel. Theopt3 gene is ∼4.4 kilobases long and consists of 13 exons. The cDNA codes for a protein of 701 amino acids with 11 putative transmembrane domains. When expressed in mammalian cells and in Xenopus laevis oocytes, OPT3 cDNA induces H+-coupled transport of the dipeptide glycylsarcosine. Electrophysiological studies of the transport function of OPT3 inXenopus oocytes show that this transporter, although capable of mediating H+-coupled peptide transport, functions predominantly as a H+ channel. The H+channel activity of OPT3 is ∼3–4-fold greater than the H+/peptide cotransport activity as determined by measurements of H+ gradient-induced inward currents in the absence and presence of the dipeptide using the two-microelectrode voltage clamp technique. A downhill influx of H+ was accompanied by a large intracellular acidification as evidenced from the changes in intracellular pH using an ion-selective microelectrode. The H+ channel activity exhibits aK0.5H of 1.0 μm at a membrane potential of -50 mV. At the level of primary structure, OPT3 has moderate homology with OPT1 and OPT2, two other H+-coupled oligopeptide transporters previously cloned from C. elegans. Expression studies using theopt3::gfp fusion constructs in transgenicC. elegans demonstrate that opt3 gene is exclusively expressed in neurons. OPT3 may play an important physiological role as a pH balancer in the maintenance of H+ homeostasis in C. elegans.