Located between the inner and outer membranes of Gram-negative bacteria, periplasmic binding proteins (PBPs) scavenge or sense diverse nutrients in the environment by coupling to transporters or chemotaxis receptors in the inner membrane. Their three-dimensional structures have been deduced in atomic detail with the use of X-ray crystallography, both in the free and liganded state. PBPs consist of two large lobes that close around the bound ligand, resembling a Venus flytrap. This architecture is reiterated in transcriptional regulators, such as the lac repressors. In the process of evolution, genes encoding the PBPs have fused with genes for integral membrane proteins. Thus, diverse mammalian receptors contain extracellular ligand binding domains that are homologous to the PBPs; these include glutamate/glycine-gated ion channels such as the NMDA receptor, G protein-coupled receptors, including metabotropic glutamate, GABA-B, calcium sensing, and pheromone receptors, and atrial natriuretic peptide-guanylate cyclase receptors. Many of these receptors are promising drug targets. On the basis of homology to PBPs and a recently resolved crystal structure of the extracellular binding domain of a glutamate receptor ion channel, it is possible to construct three-dimensional models of their ligand binding domains. Together with the extensive information available on the mechanism of ligand binding to PBPs, such models can serve as a guide in drug discovery.

Purpose:The proton-dependent oligopeptide transporters (POT) gene family currently consists of ~70 cloned cDNAs derived from diverse organisms. In mammals, two genes encoding peptide transporters, PepT1 and PepT2 have been cloned in several species including humans, in addition to a rat histidine/peptide transporter (rPHT1). Because the Candida elegans genome contains five putative POT genes, we searched the available protein and nucleic acid databases for additional mammalian/human POT genes, using iterative BLAST runs and the human expressed sequence tags (EST) database. The apparent human orthologue of rPHT1 (expression largely confined to rat brain and retina) was represented by numerous ESTs originating from many tissues. Assembly of these ESTs resulted in a contiguous sequence covering ~95% of the suspected coding region. The contig sequences and analyses revealed the presence of several possible splice variants of hPHT1. A second closely related human EST-contig displayed high identity to a recently cloned mouse cDNA encoding cyclic adenosine monophosphate (cAMP)-inducible 1 protein (gi:4580995). This contig served to identify a PAC clone containing deduced exons and introns of the likely human orthologue (termed hPHT2). Northern analyses with EST clones indicated that hPHT1 is primarily expressed in skeletal muscle and spleen, whereas hPHT2 is found

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