2.A.17 The Proton-dependent Oligopeptide Transporter (POT/PTR) Family

Proteins of the POT family (also called the PTR (peptide transport) family) are from animals, plants, yeast, archaea and both Gram-negative and Gram-positive bacteria. Several of these organisms possess multiple POT family paralogues. The proteins are of about 450-600 amino acyl residues in length with the eukaryotic proteins in general being longer than the bacterial proteins. They exhibit 12 putative or established transmembrane α-helical spanners.  The plant homologues have been examined from phylogenetic standpoints (von Wittgenstein et al. 2014). This family is also called the solute carrier 15 family (Slc15). They utilize the proton gradient and negative membrane protential to drive the transmembrane transporter of di-/tripeptide and peptide-mimetic molecules, and they also play important roles in immunoreaction (Dong et al. 2020). Rehmannia glutinosa (Chinese foxglove) has 18 ntr1 genes encoding proteins of 419 to 601 aas with 7 - 12 TMSs. They are found primarily in the plasma membranes of various plant tissues (Gu et al. 2021). 109 members of the PTR family are encoded within the genome of tea plants (Camellia sinensis) (Wang et al. 2022).

Pairs of salt bridge interactions between transmembrane helices work in tandem to orchestrate alternating access transport within the PTR family (Newstead 2014). Key roles for residues conserved between bacterial and eukaryotic homologues suggest a conserved mechanism of peptide recognition and transport that in some cases has been subtly modified in individual species.  PepT1 and PepT2, mammalian members of this family, are responsible for the uptake of many pharmaceutically important drug molecules, including antibiotics and antiviral medications.  Thus, their promiscuity can be used for improving the oral bioavailability of poorly absorbed compounds (Newstead 2014). Signature peptides enabled the reliable quantification of human hepatic transporters (Mori et al. 2022). The QTAP protocol using these optimal signature peptides provides quantitative data on hepatic transporters, usable for integrated pharmacokinetic studies.

While most members of the POT family catalyze peptide transport, one is a nitrate permease and one can transport histidine as well as peptides. A nitrate permease of Arabidopsis, Chl1 (TC #2.A.17.3.1), exhibits dual affinity. When phosphorylated at threonine-101, it exhibits high affinity (50 μM) for nitrate, but when not phosphorylated, it exhibits low affinity (~5 mM) (Liu and Tsay, 2003). Some of the peptide transporters can also transport antibiotics. They function by proton symport, but the substrate:H+ stoichiometry is variable: the high affinity rat PepT2 carrier catalyzes uptake of 2 and 3H+ with neutral and anionic dipeptides, respectively, while the low affinity PepT1 carrier catalyzes uptake of one H+ per neutral peptide. In eukaryotes, some of these transporters may be in organellar membranes such as the lysosomes.  Candida tropicalis oligopeptide transporters assist in the transmembrane transport of the antimicrobial peptide, CGA-N9 (Wu et al. 2023).

Di- and tripeptide transporters of the POT/PTR/NRT1 family are localized either to the tonoplast (TP) or plasma membrane (PM). A 7 amino acid fragment of the hydrophilic N-terminal region of Arabidopsis PTR2, PTR4 and PTR6 is required for TP localization and sufficient to redirect not only PM-localized PTR1 or PTR5, but also sucrose transporter SUC2 to the tonopolast (Komarova et al., 2012). L(11) and I(12) of PTR2 are essential for TP targeting, while only one acidic amino acid at position 5, 6 or 7 is required, revealing a dileucine (LL or LI) motif with at least one upstream acidic residue. Similar dileucine motifs could be identified in other plant TP transporters. Targeting to the PM required the loop between transmembrane domains 6 and 7 of PTR1 or PTR5. Deletion of either PM or TP targeting signals resulted in retention in internal membranes, indicating that PTR trafficking to these destination membranes requires distinct signals and is in both cases not by default (Komarova et al., 2012). A comprehensive genome-wide systematic characterization of the PTR family led to the identification of 193 PTR genes in the genome of Brassica napus (Rapeseed) (Zhang et al. 2020). The  family exhibited high levels of genetic diversity among sub-families, and expression analyses identified a broad range of expression patterns for individual gene in response to multiple nutrient stresses, including nitrogen (N), phosphorus (P) and potassium (K+) deficiencies, as well as ammonium toxicity. Ten core genes induced in response to N stress (Zhang et al. 2020).

Both proton and ligand significantly change the conformational free-energy landscape of PepT (Batista et al. 2019). In the absence of ligand and protonation, only transitions involving inner facing (IF) and occluded (OC) states are allowed. After protonation of residue Glu300, the wider free-energy well  indicates a greater conformational variability relative to the apo system, and outward facing (OF) conformations become accessible. For the Glu300 protonated holo-PepT, the presence of a second free-energy minimum suggests that OF conformations are accessible and stable. Thus, the differences in the free-energy profiles suggest that transitions toward outward-facing conformations occur only after protonation, which is likely the first step in the mechanism of peptide transport. The transmembrane proton flux drives essential steps of the full functional cycle: 1) protonation of a glutamate in TMS 7 opens the extracellular gate, allowing ligand entry; 2) inward proton flow induces the cytosolic release of the ligand by varying the protonation state of a second conserved glutamate in TMS 10; 3) proton movement between TMS 7 and TMS 10 is thermodynamically driven and kinetically permissible via water proton shuttling without the participation of ligand (Li et al. 2022).

The generalized transport reaction catalyzed by the proteins of the POT family is:

Substrate (out) +  nH+ (out) → substrate (in) + nH+ (in)

This family belongs to the Major Facilitator (MFS) Superfamily.



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Wu, C., Y. Xiang, P. Huang, M. Zhang, M. Fang, W. Yang, W. Li, F. Cao, L.H. Liu, W. Pu, and S. Duan. (2023). Molecular identification and physiological functional analysis of NtNRT1.1B that mediated nitrate long-distance transport and improved plant growth when overexpressed in tobacco. Front Plant Sci 14: 1078978.

Wu, J., R. Li, Y. Shen, X. Zhang, X. Wang, Z. Wang, Y. Zhao, L. Huang, L. Zhang, and B. Zhang. (2023). Candida tropicalis oligopeptide transporters assist in the transmembrane transport of the antimicrobial peptide CGA-N9. Biochem. Biophys. Res. Commun. 649: 101-109.

Wu, M., S. Tong, S. Waltersperger, K. Diederichs, M. Wang, and L. Zheng. (2013). Crystal structure of Ca2+/H+ antiporter protein YfkE reveals the mechanisms of Ca2+ efflux and its pH regulation. Proc. Natl. Acad. Sci. USA 110: 11367-11372.

Xu, L., Y. Li, I.S. Haworth, and D.L. Davies. (2010). Functional role of the intracellular loop linking transmembrane domains 6 and 7 of the human dipeptide transporter hPEPT1. J. Membr. Biol. 238: 43-49.

Zhang, H., S. Li, M. Shi, S. Wang, L. Shi, F. Xu, and G. Ding. (2020). Genome-Wide Systematic Characterization of the Family Genes and Their Transcriptional Responses to Multiple Nutrient Stresses in Allotetraploid Rapeseed. Int J Mol Sci 21:.

Zhao, Y., G. Mao, M. Liu, L. Zhang, X. Wang, and X.C. Zhang. (2014). Crystal structure of the E. coli peptide transporter YbgH. Structure 22: 1152-1160.

Zhou, J.-J., F.L. Theodoulou, I. Muldin, B. Ingemarsson, and A.J. Miller. (1998). Cloning and functional characterization of a Brassica napus transporter that is able to transport nitrate and histidine. J. Biol.Chem. 273: 12017-12023.

Zhou, X., M. Thamotharan, A. Gangopadhyay, C. Serdikoff, and S.A. Adibi. (2000). Characterization of an oligopeptide transporter in renal lysosomes. Biochim. Biophys. Acta 1466: 372-378.


TC#NameOrganismal TypeExample

Di- or tripeptide:H+ symporter of 497 aas and 13 or 14 TMSs. DtpT is specific for di- and tripeptides, with the highest affinities for peptides with at least one hydrophobic residue. The effect of the hydrophobicity, size, or charge of the amino acid was different for the amino- and carboxyl-terminal positions of dipeptides. Free amino acids, omega-amino fatty acidss, and peptides with more than three amino acid residues do not interact with DtpT. For high-affinity interaction, the peptides need to have free amino and carboxyl termini, amino acids in the L configuration, and trans-peptide bonds. Comparison of the specificity of DtpT with those of the eukaryotic homologues PepT(1) and PepT(2) showed that the bacterial transporter is more restrictive in its substrate recognition. (Fang et al. 2000). 


DtpT of Lactococcus lactis (P0C2U2)


The di/tripeptide:H+ symport permease, TppB (DtpA or YdgR) (transports di and tripeptides and peptidomimetics such as aminocephalosporins (Weitz et al., 2007).  The transporter has two alternate conformations, one of which is promoted by inhbitor binding (Bippes et al. 2013).


TppB of E. coli (P77304)

2.A.17.1.3The dipeptide/tripeptide:H+ symport permease, DtpB (YhiP) (transports glycyl-sarcosine (Gly-Sar) with low affinity (6mM) and the toxic dipeptide, alafosfalin (Harder et al., 2008)BacteriaDtpB of E. coli (P36837)

DtpD (YbgH) peptide transporter.  A projection structure at 19 Å resolution and a high resolution x-ray structure are available; Casagrande et al., 2009; Zhao et al. 2014). Glu21 is the only conserved proton-titratable amino acyl residue (among POTs) that is located in the central cavity, and it is critical for in vivo transport (Zhao et al. 2014).


DtpD of E. coli (P75742)


Peptide transporter, YjdL (preference for di-peptides) (Ernst et al., 2009; Gabrielsen et al., 2011; Jensen et al., 2011).  The motif, ExxERFxxYY has been shown to be involved in proton translocation, and the nearby K117 may play a dual role in protonation and substrate binding (Jensen et al. 2014).


YjdL of E. coli (P39276)


POT famiy di- and tri-peptide porter, DtpT. 3-d structures (PDB:24APS; 5MMT: 5D58' 5D59) are available for an inward open conformation. A hinge-like movement in the C-terminal half facilitates opening of an intracellular gate controlling access to a central peptide binding site. Salt bridges may orchestrate alternating access (Solcan et al., 2012; Quistgaard et al. 2017).


Peptide porter, DtpT of Streptococcus thermophilus (Q5M4H8)


Peptide uptake transporter of 496 aas, POT.  The 3-d structure has been determined to 1.9Å resolution leading to a proposed mechanism (Doki et al. 2013).  Glu310 first may bind the carboxyl group of the peptide substrate. Then deprotonation of Glu310 in the inward open state triggers the release of the bound peptide toward the intracellular space, and salt bridge formation between Glu310 and Arg43 induces the transition state to the occluded conformation.


POT of Geobacillus kaustophilus


Proton-coupled oligopeptide uptake transporter of 485 aas and 14 TMSs, DtpT or Pot.  Expression of the encoded gene is upregulated upon infection. Transports di- and tripeptides but can not accumulate peptides with a positively charged residue in the C-terminal position.  An aromatic residue patch in the active site of the transporter may be responsible for it's unusual specificity (Sharma et al. 2016).

DtpT of Neisseria meningitidis


TC#NameOrganismal TypeExample
2.A.17.2.1Peptide:H+ symporter Plants PTR2-A of Arabidopsis thaliana

Peptide:H+ symporter (dipeptides and tripeptides preferred (Cai et al., 2007). Substrate preference is altered by mutations in the fifth TMS of Ptr2p (Hauser et al. 2013).


PTR2 of Saccharomyces cerevisiae


Dipeptide uptake porter, Ptr2.  Transports dipeptides such as Ala-Leu, Ala-Tyr and Tyr-Ala (Belmondo et al. 2014).


Ptr2 of Rhizophagus irregularis (Arbuscular mycorrhizal fungus) (Glomus intraradices)


Di- and tripeptide uptake transporter, Ptr2 of 577 aas and 12 TMSs. The ptr2 gene showed increased expression upon interaction with the plant-pathogenic fungus Botrytis cinerea, suggesting that it is involved in the mycoparasitic process. Its expression was triggered by nitrogen starvation (Vizcaíno et al. 2006).

Ptr2 of Trichoderma harzianum (Hypocrea lixii)


Oligopeptide transporter of 576 aas and 12 TMSs, PTR22. Transports a variety of peptides as well as derivatives of antifungal agents, such as chlorotetaine and lysyl-cholortetaine (Liu et al. 2018).

PTR22 of Candida albicans (Yeast)


TC#NameOrganismal TypeExample

Dual affinity Nitrate/Chlorate symporter, Nrt1.1; CHL1 of 590 aas and 12 TMSs in a 6 + 6 TMS arrangement (Martin et al., 2008).  The low affinity form is a homo-dimer and has Thr101 in the non-phosphorylated form; the high affinty form (0.1 micromolar Km) is a monomer and has Thr101 phosphorylated (Sun and Zheng 2015). Rehmannia glutinosa (Chinese foxglove) has 18 ntr1 genes encoding proteins of 419 to 601 aas with 7 - 12 TMSs. They are found primarily in the plasma membranes of various plant tissues (Gu et al. 2021).


Ntr1.1/CHL1 of Arabidopsis thaliana


solute carrier family 15, member 5.  Function unknown as of 1/17, but probably a di- and tri-peptide uptake porter (Verri et al. 2016). The tissue expression profile has been reported (Sreedharan et al. 2011).



SLC15A5 of Homo sapiens


Solute carrier family 15 member 4 (Peptide transporter 4) (Peptide/histidine transporter 1) (hPHT1; SLC15A4; ) present in immune cells (Verri et al. 2016). It is a proton-coupled amino-acid transporter that mediates the transmembrane transport of L-histidine and some di- and tripeptides from inside the lysosome to the cytosol, and plays a key role in innate immune responses (Bhardwaj et al. 2006, Kobayashi et al. 2014, Song et al. 2018). It is able to transport a variety of di- and tripeptides, including carnosine and some peptidoglycans (Song et al. 2018; Oppermann et al. 2019). Transport activity is pH-dependent and maximized in the acidic lysosomal environment. It is involved in the detection of microbial pathogens by toll-like receptors (TLRs) and NOD-like receptors (NLRs), probably by mediating transport of bacterial peptidoglycans across the endolysosomal membrane: it catalyzes the transport of certain bacterial peptidoglycans, such as muramyl dipeptide, the NOD2 ligand, and L-alanyl-gamma-D-glutamyl-meso-2,6-diaminoheptanedioate (tri-DAP), the NOD1 ligand (Kobayashi et al. 2014, Song et al. 2018). Byrnes et al. 2022 reviewed three specific mechanisms by which autophagy can regulate metabolism: A) nutrient regeneration, B) quality control of organelles, and C) signaling protein regulation. PHT1 is a histidine/oligopeptide transporter with an essential role in Toll-like receptor innate immune responses. It can act as a receptor by recruiting the adaptor protein TASL which leads to type I interferon production via IRF5 (Custódio et al. 2023). Persistent stimulation of this signalling pathway is involved in the pathogenesis of systemic lupus erythematosus (SLE). The authors presented the Cryo-EM structure of PHT1 stabilized in the outward-open conformation and proposed a model of the PHT1-TASL complex, in which the first 16 N-terminal TASL residues fold into a helical structure that bind in the central cavity of the inward-open conformation of PHT1. This work suggests the molecular basis of PHT1/TASL mediated type I interferon production (Custódio et al. 2023).




SLC15A4 of Homo sapiens


Putative peptide/nitrate transporter At3g25280


At3g25280 of Arabidopsis thaliana

2.A.17.3.13Probable peptide transporter At1g52190PlantsAt1g52190 of Arabidopsis thaliana

Nitrate transporter 1.6


NRT1.6 of Arabidopsis thaliana


Nitrate transporter 1.7


NRT1.7 of Arabidopsis thaliana


Nitrate transporter 1.2 (Nitrate transporter NTL1; NRT1:2; PRT family protein 4.6; NPF4.6; AIT1).  Low-affinity proton-dependent nitrate transporter involved in constitutive nitrate uptake but not histidine or dipeptides transport. Involved in (+)-abscisic acid (ABA) transport, but not in gibberellin, indole-3-acetic acid or jasmonic acid import; ABA and nitrate do not compete for substrate uptake (Kanno et al. 2013). Arabidopsis NPF4.6 and NPF5.1 control the leaf stomatal aperture by regulating abscisic acid transport (Shimizu et al. 2021). NPF4.6 is expressed in vascular tissues and guard cells, and it positively regulates stomatal closure in leaves (Shimizu et al. 2021).


NRT1.2 of Arabidopsis thaliana


Transporter for glucosinolates, GTR1, (aliphatic but not indole glucosinolates such as 4-methylthiobutyl glucosinolate, major defence compounds, translocated to seeds on maturation) as well as gibberellic acid and jasmonoyl-L-isoleucine, GTR1 or NPF2.10, of 636 aas and 12 TMSs (Nour-Eldin et al. 2012; Ishimaru et al. 2017). Regulated at the transcriptional level, but also postranslationally.  Dimerization of GTR1, possibly induced by dephosphorylation of a Thr residue, regulates its plasma membrane localization, leading to increased transport of glucosinolates and gibberellic acid (Ishimaru et al. 2017). Homologues have been found and characterized in Chinese kale (Jiang et al. 2019).


GTR1 of Arabidopsis thaliana


Nitrate transporter 1.4


NRT1.4 of Arabidopsis thaliana


Nitrate transporter 1.5


NRT1.5 of Arabidopsis thaliana


Histidine or peptide:H+ symporter of 585 aas and 12 TMSs. It mediates the transport of di- and tripeptides with high affinity, low capacity (Rentsch et al. 1995).


PTR2-B (NTR1) of Arabidopsis thaliana


High-affinity, proton-dependent glucosinolate-specific transporter-2, GTP2 or NPF2.11. Involved in apoplasmic phloem-loading of glucosinolates and in bidirectional long-distance transport of aliphatic but not indole glucosinolates. May be involved in removal of glucosinolates from the xylem in roots (Nour-Eldin et al. 2012; Andersen et al. 2013).


GTR2 of Arabidopsis thaliana


Low affinity nitrate transporter, Nrt1, of 584 aas and 13 putative TMSs.  Two splice variants, Ntr1.1a and Ntr1.1b, have been identified.  Under low nitrogen condition, Nrt1.1b accumulates more nitrogen in plants and improves rice growth, but Ntr1.1a had no such effect (Fan et al. 2015). The absorption and long-distance distribution/transport of nitrate is mediated by NRT1.1B in tabacco (Wu et al. 2023).

Ntr1 of Oryza sativa (Rice)


Uncharacterized peptide transport protein of 609 aas and 12 TMSs, PTR3-A.


aof Aegilops tauschii (Tausch's goatgrass) (Aegilops squarrosa)


Protein NRT1, PTR FAMILY 5.1, NPF5.1 of 583 aas and 12 TMSs in a 6 + 6 TMS arrangement. Mutants defective in NPF5.1 had a higher leaf surface temperature compared to the wild type and, NPF5.1 mediated cellular abscisic acid (ABA) uptake when expressed in a heterologous yeast system (Shimizu et al. 2021). An NRT  protein (NPF) from wheat (Triticum aestivum) has been identified and partiallly characterized (Kumar et al. 2022).

NPF5.1 of Arabidopsis thaliana (Mouse-ear cress)


NRT1.13; NPF4.4; PTR family 4.4 of 601 aas and 12 or 13 TMSs in a 6 or 7 + 6 TMS arrangement. In contrast to most NRT1 transporters, NRT1.13 does not have the conserved proline residue between TMSs 10 and 11, an essential residue for nitrate transport activity in CHL1/NRT1.1/NPF6.3 ( TC# 2.A.17.3.1). NRT1.13 showed no nitrate transport activity, but when Ser487 at the corresponding position was converted to proline, NRT1.13 S487P gained nitrate uptake activity, suggesting that wild-type NRT1.13 cannot transport nitrate but can bind it (Chen et al. 2021). Subcellular localization  indicated that NRT1.13 is a plasma membrane protein expressed at the parenchyma cells next to xylem in the petioles and the stem nodes. When plants were grown with a normal concentration of nitrate, the nrt1.13 mutant showed no severe growth phenotype, but when grown under low-nitrate conditions, nrt1.13 showed delayed flowering, increased node number, retarded branch outgrowth, and reduced lateral nitrate allocation to nodes. This suggested that NRT1.13 is required for low-nitrate acclimation and that internal nitrate is monitored near the xylem by NRT1.13 to regulate shoot architecture and flowering time.

Ntr1.13 of Arabidopsis thaliana


Protein NRT1/ PTR FAMILY 5.8, NPF5.8 or NAEZT1, of 552 aas and 11 or 12 TMSs.  NAET1 and NAET2, function as nicotianamide (nicotinamide NA) transporters required for translocation of both iron and copper to seeds. Chao et al. 2021 showed that NAET1 and NAET2 are predominantly expressed in the shoot and root vascular tissues and mediate secretion of NA out of the cells, resembling the release of neurotransmitters from animal synaptic vesicles (Chao et al. 2021).

NPF5.8 of Arabidopsis thaliana (Mouse-ear cress)


Nicotinamine efflux porter of 563 aas and 11 or 12 TMSs. NAET1 and NAET2 function as a nicotianamide (nicotinamide NA) transporters required for translocation of both iron and copper to seeds. Chao et al. 2021 showed that NAET1 and NAET2 are predominantly expressed in the shoot and root vascular tissues and mediate secretion of NA out of cells, resembling the release of neurotransmitters from animal synaptic vesicles.

NAET2 of Arabidopsis thaliana (Mouse-ear cress)


Nitrate (chlorate) or histidine:H+ symporter of 589 aas and 12 TMSs, Ntr1. Its structural gene is specifically expressed in the floral nectaries of Brassica species (Song et al. 2000).


RCH2 of Brassica napus

2.A.17.3.4Peptide transporter, PTR3-A (induced by histidine, leucine and phenylalanine in cotyledons and lower leaves; involved in stress tolerance in seeds during germination and in defense against virulent bacterial pathogens) (Karim et al., 2007; Karim et al., 2005)PlantsPTR3-A of Arabidopsis thaliana (Q9FNL7)

The nitrate excretion transporter1, NaxT1, og 558 aas and 13 TMSs in a 7 + 6 TMS arrangement (in the plasma membranes of plant cells).


NaxT1 of Arabidopsis thaliana (Q9M1E2)

2.A.17.3.6Chloroplast nitrite uptake system, Nitr1-L (Sugiura et al., 2007)PlantsNitr1-L of Arabidopsis thaliana (Q9SX20)

The root dipeptide/tripeptide transporter, PTRI (Komarova et al., 2008). Transport is electrogenic and dependent on protons. Leak currents are inhibited by Phe-Ala when this peptide binds at the active site with high affinity (Hammes et al., 2010).


PTR1 of Arabidopsis thaliana (Q9M390)


The germinating pollen dipeptide/tripeptide transporter, PTR5 (Komarova et al., 2008). Transport is electrogenic and dependent on protons. Leak currents are inhibited by Phe-Ala when this peptide binds at the active site with high affinity (Hammes et al., 2010).


PTR5 of Arabidopsis thaliana (Q0WR84)


solute carrier family 15, member 3, SLC15A3, OCTP, PHT2, PHT3, of 581 aas and 12 TMSs in a 3 + 3 + 3 + 3 TMS arrangement. It is a histidine + di- and tri-peptide uptake transporter in immune cells (Verri et al. 2016). The functions, regulation and pathophysiology of PHT2 and PHT1, specifically in immunoregulation have been reviewed (Dong et al. 2023). 


SLC15A3 of Homo sapiens


TC#NameOrganismal TypeExample

Peptide:H+ symporter (transports cationic, neutral and anionic dipeptides including glycylsarcosine (gly-sar) (Søndergaard et al., 2008) as well as anserine (β-alanyl-1-N-methyl-L-histidine) and carnosine (β-alanyl-L-histidine) (Geissler et al., 2010); also transports β-lactam antibiotics, the antitumor agent, bestatin, and various protease inhibitors). It is competitively inhibited by L-4,4'-biphenylalanyl-L-proline (Bip-Pro) with ~10-20µM affinity. Inhibitors/substrates include cefadroxil, Ala-4-nitroanilide and δ-aminolevulinic acid (Knutter et al., 2007). The intracellular loop linking transmembrane domains 6 and 7 of the human dipeptide transporter hPEPT1 includes two amphipathic alpha-helices, with net positive and negative charges which interact and influence conformational changes of hPEPT1 during and after glycylsarcosine transport (Xu et al., 2010).  The rabbit orthologue provides the main pathway for dietary nitrogen uptake. Five tyrosyl residues are important for function and/or substrate binding (Pieri et al. 2009).  Human PepT1 is modified by N-glycosylation, and all six asparagine residues in the large extracellular loop between transmembrane domains 9 and 10 are subject to N-glycosylation (Chan et al. 2016). Lat1 transports 26 biologically active ultrashort peptides (USPs) into cells as is also true of LAT2 and PEPT1 (Khavinson et al. 2023). The sizes and structures of ligand-binding sites of the amino acid transporters LAT1, LAT2, and of the peptide transporter PEPT1 are sufficient for the transport of the 26 biologically active di-, tri-, and tetra-peptides. Comparative analyses of the binding of all possible di- and tri-peptides (8400 compounds) at the binding sites of the LAT and PEPT family transporters was considered (Khavinson et al. 2023). The 26 biologically active USPs systematically showed higher binding scores to LAT1, LAT2, and PEPT1, as compared with di- and tri-peptides. Most of the 26 studied USPs were found to bind to the LAT1, LAT2, and PEPT1 transporters more efficiently than the previously known substrates or inhibitors of these transporters. Peptides ED, DS, DR, EDR, EDG, AEDR, AEDL, KEDP, and KEDG, and peptoids DS7 and KE17 with negatively charged Asp- or Glu- amino acid residues at the N-terminus and neutral or positively charged residues at the C-terminus of the peptide were found to be the most effective ligands of the transporters under investigation. It can be assumed that the antitumor effect of the KE, EW, EDG, and AEDG peptides could be associated with their ability to inhibit the LAT1, LAT2, and PEPT1 amino acid transporters (Khavinson et al. 2023).


PepT1 of Rattus norvegicus

2.A.17.4.10Peptide transporter 3 (Oligopeptide transporter 3)WormPept-3 of Caenorhabditis elegans

Peptide transporter, Pep1, also called CptB, Opt-2 and Pep-2.  It is of 835 aas and 11 TMSs.  It transports di-, tri- and tetra-peptides including phenylalanylmethionylarginylphenylalaninamide (FMRFamide) and N-acetylaspartylglutamate, both neuropeptides found throughout the animal kingdom. In contrast to CptA (TC# 2.A.17.4.3), CptB has low-affinity for its substrates (Fei et al. 1998). 

CptB of Caenorhabditis elegans

2.A.17.4.2Oligopeptide transporter 1 Animals Oligopeptide transporter of Drosophila melanogaster

High affinity oligopeptide transporter, CPTA. It transports di-, tri- and tetra peptides with low specificity. Neuropeptides (FMRF-amide and N-acetyl-Asp-Glu) are also transported (Fei et al. 1998).


CPTA of Caenorhabditis elegans


The renal brush-border electrogenic, proton-coupled, broad specificity, high affinity, peptide transporter, PepT2 (Rubio-Aliaga et al., 2000). It is competitively inhibited by L-4,4'-Biphenylalanyl-L-Proline (Bip-Pro) with ~10-20µM affinity. Inhibitor/substrates includes cefadroxil, Ala-4-nitroanilide and delta-aminolevulinic acid (Knutter et al., 2007).  It transports the fluorescent tracer-dipeptide beta-Ala-Lys-Nepsilon-7-amino-4-methyl-coumarin-3-acetic acid (Ala-Lys-AMCA). Whole-mount preparations from mouse, rat, and guinea pig stomach and small and large intestine were incubated with Ala-Lys-AMCA in the presence or absence of the uptake-inhibitors L-histidine, D-phenylalanyl-L-alanine (D-Phe-Ala), glycyl-L-sarcosine (Gly-Sar), glycyl-L-glutamine (Gly-Gln), benzylpenicillin, and cefadroxil. Fluorescence microscopy revealed that Ala-Lys-AMCA specifically accumulated in both ganglionic layers of the enteric nervous system (ENS) in all regions and species studied (Rühl et al. 2005). This could be inhibited by Gly-Sar, D-Phe-Ala, Gly-Gln, and cefadroxil, but not by free histidine and benzylpenicillin, indicating uptake via PEPT2. Accordingly, dipeptide uptake was completely abolished in PEPT2-deficient mice.


PepT2 of Mus musculus (Q9ES07)

2.A.17.4.5The high affinity, low capacity, peptide transporter, PepT2 (SLC15A2) [affinity for glycyl-L-glutamine=18μM] (Romano et al., 2006)AnimalsPepT2 of Danio rerio (NP_0010349)

Oligopeptide transporter, PepT1 (Slc15A1b) (Bucking and Schulte, 2012) (expressed in freshwater acclimated fish)


PepT1b of Fundulus heteroclitus (H2DJV9)


Di-/Tri-peptide porter. 3-d structure (PDB: 2XUT) known revealing a probable alternating access mechanism of transport (Newstead et al., 2011).  A second structure shows the protein in an inward open conformation with the peptidommetic, alafosfalin, bound (Guettou et al. 2013). Appears to take up glutathione (Deutschbauer et al. 2011).


Di-/Tri-peptide permease of Shewanella oneidensis (Q8EKT7)


Solute carrier family 15 member 2 (Kidney H+:peptide cotransporter) (Oligopeptide transporter, kidney isoform) (Peptide transporter 2, PEPT2) (Verri et al. 2016). Transports opioid peptides (Ganapathy and Miyauchi 2005). It is an electrogenic uphill peptide and peptidomimetic drug transporter, coupling of substrate translocation to a transmembrane electrochemical proton gradient serving as the driving force. In human airways, PEPT2 is localized to bronchial epithelium and alveolar type II pneumocytes, and transport studies revealed that both peptides and peptidomimetic drugs such as antibiotic, antiviral, and antineoplastic drugs are carried by the system. PEPT2 is also responsible for the transport of delta-aminolevulinic acid, which is used for photodynamic therapy and the diagnostics of pulmonary neoplasms (Groneberg et al. 2004). PepT2 in mammals plays essential roles in the reabsorption and conservation of peptide-bound amino acids in the kidney and in maintaining neuropeptide homeostasis in the brain. It is also responsible for the absorption and disposing of peptide-like drugs, including angiotensin-converting enzyme inhibitors, β-lactam antibiotics and antiviral prodrugs. Understanding the structure, function and regulation of PepT2 is of emerging interest in nutrition, medical and pharmacological research. Wang et al. 2022 provided an overview of the structure, substrate preferences and localizations of PepT2 in mammals. As PepT2 is expressed in various organs, its functions in the liver, kidney, brain, heart, lung and mammary gland have been addressed. Regulatory factors that affect the expression and function of PepT2, such as transcriptional activation and posttranslational modification, are also discussed (Wang et al. 2022).


SLC15A2 of Homo sapiens


Solute carrier family 15 member 1 (Intestinal H+:peptide cotransporter) (Oligopeptide transporter, small intestine isoform) (Peptide transporter 1, PepT1).  Takes up oligopeptides of 2 to 4 amino acids with a preference for dipeptides, a major route for the absorption of protein digestion end-products. PepT1 is modified by N-glycosylation, and all six asparagine residues in the large extracellular loop between TMSs 9 and 10 are subject to N-glycosylation.  This allows proper association with the plasma membrane and/or stabilization (Chan et al. 2016). Transports opioid peptides (Ganapathy and Miyauchi 2005), can serve as a druh importer and plays a role in inflammatory bowel diseases (Viennois et al. 2018). PEPT1 is upregulated in kidney cancer cell lines, with little expression in normal pancreas. PEPT1 is essential for the growth of pancreatic cancer cells and is therefore a viable drug target (Schniers et al. 2021). Mutation of arginine 282 to glutamate uncouples the movement of peptides and protons by the rabbit PepT1 (Meredith 2004). The structure of the horse ortholog shows that the extracellular domain between TMSs 9 and 10 bridges the NTD and CTD by interacting with TMS1. Deletion of ECD or mutations to the ECD-TMS1 interface impairs transport activity (Shen et al. 2022).


PepT1 of Homo sapiens