Molecular and Structural Features of the Proton-Coupled Oligopeptide Transporter Superfamily

doi:10.1016/S0079-6603(08)60038-0

Progress in Nucleic Acid Research and Molecular Biology

Volume 58, 1997, Pages 239-261

https://doi.org/10.1016/S0079-6603(08)60038-0 Get rights and content

Work in the area of molecular biology of transport proteins has unveiled the presence of a distinct peptide transporter superfamily whose members extend from the prokaryotic to the eukaryotic kingdom. There are two subgroups within this superfamily, one subgroup harnessing the energy necessary for active transport from a transmembrane H⁺ gradient and the other subgroup relying directly on ATP hydrolysis. In addition to the use of different driving forces, the two subgroups are also distinguishable with regard to molecular structure and operational mechanism. This review is intended to analyze critically the molecular nature of the members of the H⁺ gradient-dependent peptide transporter subgroup, with emphasis on the cloning strategies utilized in the isolation of the individual transporter cDNAs or genes; on the structural patterns, motifs, and conserved amino add residues common to constituent members of the subgroup; and on the characteristic topological features of the individual members. © 1998 Academic Press

Section snippets

Two Different Peptide Transporter Subfamilies: A Comparison between the Members of the ABC Peptide Transporter Subfamily and the POT Subfamily

The ABC (for ATP-binding cassette) transporter superfamily is ubiquitous, present in both prokaryotic and eukaryotic kingdoms, and is involved in many important biological processes, such as transport of nutrients (e.g., amino acids, sugars, and oligopeptides) and ions (e.g., Cl^‒, K⁺), secretion of metabolic waste products, antigen presentation and development of multiple drug resistance in cancer cells 25., 26.. The members of the ABC transporter superfamily are operated by directly “burning”

Molecular Cloning Procedures Employed for Identification of the POT Family Members

cDNA cloning of functional cellular proteins remains one of the most arduous challenges in the contemporary molecular biology field, especially when the transcripts from the genes are present in low abundance. In recent years, several investigators have used different molecular biological techniques to clone more than a dozen members of the proton-dependent oligopeptide transporter (POT) family. Here we adapt the nomenclature of Paulsen and Skurray (27), who initially coined the term “POT” for

Comparison of Amino Acid Sequences of the Members of the POT Family

General structural features of individual members of the POT subfamily, the size and distinct structural profile of the transporter genes or cDNAs, predicted amino acid composition, molecular mass of the protein core, isoelectric point (pI), consensus glycosylation sites, and the potential phosphorylation sites (Ser or Thr) for protein kinase C and protein kinase A (for the mammalian POT members only) of the transporter proteins are listed in Table I. The gene symbols and generic names are kept

Topological Features of the POT Subfamily

The POT subfamily members are present in prokaryotic as well as in eukaryotic cells and share certain common structural and functional characteristics. It is believed that all of the carrier-mediated transport systems, including facilitative transporters, antiporters, and ion-coupled symporters, operate by similar mechanisms. This solute transport family, called the major facilitator superfamily (MFS), has an ancient evolutionary history and probably dates back more than 3.5 billion years (57).

Conclusion

There is clear evidence from structural and functional studies that the peptide transporters that are driven by a transmembrane H⁺ gradient (POT family) are distinct from the peptide transporters that are directly energized by ATP (ABC peptide transporter family). The occurrence of H⁺-coupled peptide transport is much more widespread in nature than the occurrence of ATP-driven peptide transport. Several members of the POT family have been cloned from bacteria, yeast, plants, and animal tissues.

ACKNOWLEDGMENTS

This work was supported by NIH grant DK28389. We thank Ms. Lisa Young for excellent secretarial assistance.

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As mentioned earlier, javamide-I/-II contain a dipeptide-like structure, consisting of phenylpropenoic acid and tryptophan. Therefore, it may be possible for javamide-I/-II and esters to be transported via proton-coupled peptide transporters (PepT1/2) which are known to mediate the uptake of di/tripeptide-like compounds via utilizing an inwardly directed proton gradient and negative membrane potential [16–29]. Particularly, PepT2 transporters are known to be expressed on monocytes/macrophage-like cells including THP-1 cells [20–22].
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Javamide-I/-II esters can be transported, biotransformed and inhibit inflammatory cytokines significantly in monocyte/macrophage-like cells, suggesting that they may be utilized as a potent cell-permeable carrier to inhibit inflammatory cytokines in the cells.
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PEPT1 is a low-affinity and high-capacity peptide transporter energized by a transmembrane H+ gradient. The physiological substrates for PEPT1 are dipeptides and tripeptides and the transport process is coupled to H+ co-transport in an electrogenic manner (Fei, Ganapathy, & Leibach, 1998). In addition, PEPT1 is also capable of transporting peptidomimetic drugs such as β-lactam antibiotics and prodrugs such as valyl esters of nucleoside analogs (Brandsch, 2013; Ganapathy, Brandsch, Prasad, Ganapathy, & Leibach, 1995; Ganapathy, Huang, Wang, Ganapathy, & Leibach, 1998; Terada & Inui, 2004).
Reprogramming of biochemical pathways is a hallmark of cancer cells, and generation of lactic acid from glucose/glutamine represents one of the consequences of such metabolic alterations. Cancer cells export lactic acid out to prevent intracellular acidification, not only increasing lactate levels but also creating an acidic pH in extracellular milieu. Lactate and protons in tumor microenvironment are not innocuous bystander metabolites but have special roles in promoting tumor-cell proliferation and growth. Lactate functions as a signaling molecule by serving as an agonist for the G-protein-coupled receptor GPR81, involving both autocrine and paracrine mechanisms. In the autocrine pathway, cancer cell-generated lactate activates GPR81 on cancer cells; in the paracrine pathway, cancer cell-generated lactate activates GPR81 on immune cells, endothelial cells, and adipocytes present in tumor stroma. The end result of GPR81 activation is promotion of angiogenesis, immune evasion, and chemoresistance. The acidic pH creates an inwardly directed proton gradient across the cancer-cell plasma membrane, which provides driving force for proton-coupled transporters in cancer cells to enhance supply of selective nutrients. There are several molecular targets in the pathways involved in the generation of lactic acid by cancer cells and its role in tumor promotion for potential development of novel anticancer therapeutics.
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Molecular and Structural Features of the Proton-Coupled Oligopeptide Transporter Superfamily

Section snippets

Two Different Peptide Transporter Subfamilies: A Comparison between the Members of the ABC Peptide Transporter Subfamily and the POT Subfamily

Molecular Cloning Procedures Employed for Identification of the POT Family Members

Comparison of Amino Acid Sequences of the Members of the POT Family

Topological Features of the POT Subfamily

Conclusion

ACKNOWLEDGMENTS

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Physiol. Rev.

Contrib. Infusion Ther. Clin. Nutr.

Trends Biochem. Sci.

Nature (London)

Biochim. Biophys. Acta

Biochim. Biophys. Acta

J. Pharmacol. Exp. Ther.

Annu. Rev. Neurosci.

J. Biol. Chem.

FEBS Lett.

FEBS Lett

Mol. Cell. Biol.

Plant Physiol.

J. Pharmacol. Exp. Ther.

BioEssays

BioEssays

Peptide Absorption: Development and Present State of the Subject

J. Membr. Biol.

Annu. Rev. Nutr.

Mol. Microbiol.

J. Bacteriol.

Mol. Microbiol.

Nature (London)

Mol. Microbiol

Life Sci.