Clinical activity of carfilzomib correlates with inhibition of multiple proteasome subunits: application of a novel pharmacodynamic assay.

While proteasome inhibition is a validated therapeutic approach for multiple myeloma (MM), inhibition of individual constitutive proteasome (c20S) and immunoproteasome (i20S) subunits has not been fully explored owing to a lack of effective tools. We utilized the novel proteasome constitutive/immunoproteasome subunit enzyme-linked immunosorbent (ProCISE) assay to quantify proteasome subunit occupancy in samples from five phase I/II and II trials before and after treatment with the proteasome inhibitor carfilzomib. Following the first carfilzomib dose (15-56 mg/m(2) ), dose-dependent inhibition of c20S and i20S chymotrypsin-like active sites was observed [whole blood: ≥67%; peripheral blood mononuclear cells (PBMCs): ≥75%]. A similar inhibition profile was observed in bone marrow-derived CD138(+) tumour cells. Carfilzomib-induced proteasome inhibition was durable, with minimal recovery in PBMCs after 24 h but near-complete recovery between cycles. Importantly, the ProCISE assay can be used to quantify occupancy of individual c20S and i20S subunits. We observed a relationship between MM patient response (n = 29), carfilzomib dose and occupancy of multiple i20S subunits, where greater occupancy was associated with an increased likelihood of achieving a clinical response at higher doses. ProCISE represents a new tool for measuring proteasome inhibitor activity in clinical trials and relating drug action to patient outcomes.

A first-in-human dose-escalation study of the oral proteasome inhibitor oprozomib in patients with advanced solid tumors.

PURPOSE: To determine the dose-limiting toxicities (DLTs), maximum tolerated dose (MTD), safety, and pharmacokinetic and pharmacodynamic profiles of the tripeptide epoxyketone proteasome inhibitor oprozomib in patients with advanced refractory or recurrent solid tumors. METHODS: Patients received escalating once daily (QD) or split doses of oprozomib on days 1-5 of 14-day cycles (C). The split-dose arm was implemented and compared in fasted (C1) and fed (C2) states. Pharmacokinetic samples were collected during C1 and C2. Proteasome inhibition was evaluated in red blood cells and peripheral blood mononuclear cells. RESULTS: Forty-four patients (QD, n = 25; split dose, n = 19) were enrolled. The most common primary tumor types were non-small cell lung cancer (18%) and colorectal cancer (16%). In the 180-mg QD cohort, two patients experienced DLTs: grade 3 vomiting and dehydration; grade 3 hypophosphatemia (n = 1 each). In the split-dose group, three DLTs were observed (180-mg cohort: grade 3 hypophosphatemia; 210-mg cohort: grade 5 gastrointestinal hemorrhage and grade 3 hallucinations (n = 1 each). In the QD and split-dose groups, the MTD was 150 and 180 mg, respectively. Common adverse events (all grades) included nausea (91%), vomiting (86%), and diarrhea (61%). Peak concentrations and total exposure of oprozomib generally increased with the increasing dose. Oprozomib induced dose-dependent proteasome inhibition. Best response was stable disease. CONCLUSIONS: While generally low-grade, clinically relevant gastrointestinal toxicities occurred frequently with this oprozomib formulation. Despite dose-dependent increases in pharmacokinetics and pharmacodynamics, single-agent oprozomib had minimal antitumor activity in this patient population with advanced solid tumors.

Ricolinostat (ACY-1215) induced inhibition of aggresome formation accelerates carfilzomib-induced multiple myeloma cell death.

Proteasome inhibition induces the accumulation of aggregated misfolded/ubiquitinated proteins in the aggresome; conversely, histone deacetylase 6 (HDAC6) inhibition blocks aggresome formation. Although this rationale has been the basis of proteasome inhibitor (PI) and HDAC6 inhibitor combination studies, the role of disruption of aggresome formation by HDAC6 inhibition has not yet been studied in multiple myeloma (MM). The present study aimed to evaluate the impact of carfilzomib (CFZ) in combination with a selective HDAC6 inhibitor (ricolinostat) in MM cells with respect to the aggresome-proteolysis pathway. We observed that combination treatment of CFZ with ricolinostat triggered synergistic anti-MM effects, even in bortezomib-resistant cells. Immunofluorescent staining showed that CFZ increased the accumulation of ubiquitinated proteins and protein aggregates in the cytoplasm, as well as the engulfment of aggregated ubiquitinated proteins by autophagosomes, which was blocked by ricolinostat. Electron microscopy imaging showed increased autophagy triggered by CFZ, which was inhibited by the addition of ACY-1215. Finally, an in vivo mouse xenograft study confirmed a decrease in tumour volume, associated with apoptosis, following treatment with CFZ in combination with ricolinostat. Our results suggest that ricolinostat inhibits aggresome formation, caused by CFZ-induced inhibition of the proteasome pathway, resulting in enhanced apoptosis in MM cells.

Identification of proteases that regulate erythrocyte rupture by the malaria parasite Plasmodium falciparum.

Newly replicated Plasmodium falciparum parasites escape from host erythrocytes through a tightly regulated process that is mediated by multiple classes of proteolytic enzymes. However, the identification of specific proteases has been challenging. We describe here a forward chemical genetic screen using a highly focused library of more than 1,200 covalent serine and cysteine protease inhibitors to identify compounds that block host cell rupture by P. falciparum. Using hits from the library screen, we identified the subtilisin-family serine protease PfSU B1 and the cysteine protease dipeptidyl peptidase 3 (DPAP3) as primary regulators of this process. Inhibition of both DPAP3 and PfSUB1 caused a block in proteolytic processing of the serine repeat antigen (SERA) protein SERA5 that correlated with the observed block in rupture. Furthermore, DPAP3 inhibition reduced the levels of mature PfSUB1. These results suggest that two mechanistically distinct proteases function to regulate processing of downstream substrates required for efficient release of parasites from host red blood cells.

Alzheimer's Disease-Like Neurodegeneration in Porphyromonas gingivalis Infected Neurons with Persistent Expression of Active Gingipains.

BACKGROUND: Porphyromonas gingivalis (P. gingivalis) and its gingipain virulence factors have been identified as pathogenic effectors in Alzheimer's disease (AD). In a recent study we demonstrated the presence of gingipains in over 90% of postmortem AD brains, with gingipains localizing to the cytoplasm of neurons. However, infection of neurons by P. gingivalis has not been previously reported. OBJECTIVE: To demonstrate intraneuronal P. gingivalis and gingipain expression in vitro after infecting neurons derived from human inducible pluripotent stem cells (iPSC) with P. gingivalis for 24, 48, and 72 h. METHODS: Infection was characterized by transmission electron microscopy, confocal microscopy, and bacterial colony forming unit assays. Gingipain expression was monitored by immunofluorescence and RT-qPCR, and protease activity monitored with activity-based probes. Neurodegenerative endpoints were assessed by immunofluorescence, western blot, and ELISA. RESULTS: Neurons survived the initial infection and showed time dependent, infection induced cell death. P. gingivalis was found free in the cytoplasm or in lysosomes. Infected neurons displayed an accumulation of autophagic vacuoles and multivesicular bodies. Tau protein was strongly degraded, and phosphorylation increased at T231. Over time, the density of presynaptic boutons was decreased. CONCLUSION: P. gingivalis can invade and persist in mature neurons. Infected neurons display signs of AD-like neuropathology including the accumulation of autophagic vacuoles and multivesicular bodies, cytoskeleton disruption, an increase in phospho-tau/tau ratio, and synapse loss. Infection of iPSC-derived mature neurons by P. gingivalis provides a novel model system to study the cellular mechanisms leading to AD and to investigate the potential of new therapeutic approaches.

Treatment of Porphyromonas gulae infection and downstream pathology in the aged dog by lysine-gingipain inhibitor COR388.

COR388, a small-molecule lysine-gingipain inhibitor, is currently being investigated in a Phase 2/3 clinical trial for Alzheimer's disease (AD) with exploratory endpoints in periodontal disease. Gingipains are produced by two species of bacteria, Porphyromonas gingivalis and Porphyromonas gulae, typically associated with periodontal disease and systemic infections in humans and dogs, respectively. P. gulae infection in dogs is associated with periodontal disease, which provides a physiologically relevant model to investigate the pharmacology of COR388. In the current study, aged dogs with a natural oral infection of P. gulae and periodontal disease were treated with COR388 by oral administration for up to 90 days to assess lysine-gingipain target engagement and reduction of bacterial load and downstream pathology. In a 28-day dose-response study, COR388 inhibited the lysine-gingipain target and reduced P. gulae load in saliva, buccal cells, and gingival crevicular fluid. The lowest effective dose was continued for 90 days and was efficacious in continuous reduction of bacterial load and downstream periodontal disease pathology. In a separate histology study, dog brain tissue showed evidence of P. gulae DNA and neuronal lysine-gingipain, demonstrating that P. gulae infection is systemic and spreads beyond its oral reservoir, similar to recent observations of P. gingivalis in humans. Together, the pharmacokinetics and pharmacodynamics of COR388 lysine-gingipain inhibition, along with reduction of bacterial load and periodontal disease in naturally occurring P. gulae infection in the dog, support the use of COR388 in targeting lysine-gingipain and eliminating P. gingivalis infection in humans.

Characterization of P. falciparum dipeptidyl aminopeptidase 3 specificity identifies differences in amino acid preferences between peptide-based substrates and covalent inhibitors.

Malarial dipeptidyl aminopeptidases (DPAPs) are cysteine proteases important for parasite development thus making them attractive drug targets. In order to develop inhibitors specific to the parasite enzymes, it is necessary to map the determinants of substrate specificity of the parasite enzymes and its mammalian homologue cathepsin C (CatC). Here, we screened peptide-based libraries of substrates and covalent inhibitors to characterize the differences in specificity between parasite DPAPs and CatC, and used this information to develop highly selective DPAP1 and DPAP3 inhibitors. Interestingly, while the primary amino acid specificity of a protease is often used to develop potent inhibitors, we show that equally potent and highly specific inhibitors can be developed based on the sequences of nonoptimal peptide substrates. Finally, our homology modelling and docking studies provide potential structural explanations of the differences in specificity between DPAP1, DPAP3, and CatC, and between substrates and inhibitors in the case of DPAP3. Overall, this study illustrates that focusing the development of protease inhibitors solely on substrate specificity might overlook important structural features that can be exploited to develop highly potent and selective compounds.

Porphyromonas gingivalis in Alzheimer's disease brains: Evidence for disease causation and treatment with small-molecule inhibitors.

Porphyromonas gingivalis, the keystone pathogen in chronic periodontitis, was identified in the brain of Alzheimer's disease patients. Toxic proteases from the bacterium called gingipains were also identified in the brain of Alzheimer's patients, and levels correlated with tau and ubiquitin pathology. Oral P. gingivalis infection in mice resulted in brain colonization and increased production of Aß1-42, a component of amyloid plaques. Further, gingipains were neurotoxic in vivo and in vitro, exerting detrimental effects on tau, a protein needed for normal neuronal function. To block this neurotoxicity, we designed and synthesized small-molecule inhibitors targeting gingipains. Gingipain inhibition reduced the bacterial load of an established P. gingivalis brain infection, blocked Aß1-42 production, reduced neuroinflammation, and rescued neurons in the hippocampus. These data suggest that gingipain inhibitors could be valuable for treating P. gingivalis brain colonization and neurodegeneration in Alzheimer's disease.

Falstatin, a cysteine protease inhibitor of Plasmodium falciparum, facilitates erythrocyte invasion.

Erythrocytic malaria parasites utilize proteases for a number of cellular processes, including hydrolysis of hemoglobin, rupture of erythrocytes by mature schizonts, and subsequent invasion of erythrocytes by free merozoites. However, mechanisms used by malaria parasites to control protease activity have not been established. We report here the identification of an endogenous cysteine protease inhibitor of Plasmodium falciparum, falstatin, based on modest homology with the Trypanosoma cruzi cysteine protease inhibitor chagasin. Falstatin, expressed in Escherichia coli, was a potent reversible inhibitor of the P. falciparum cysteine proteases falcipain-2 and falcipain-3, as well as other parasite- and nonparasite-derived cysteine proteases, but it was a relatively weak inhibitor of the P. falciparum cysteine proteases falcipain-1 and dipeptidyl aminopeptidase 1. Falstatin is present in schizonts, merozoites, and rings, but not in trophozoites, the stage at which the cysteine protease activity of P. falciparum is maximal. Falstatin localizes to the periphery of rings and early schizonts, is diffusely expressed in late schizonts and merozoites, and is released upon the rupture of mature schizonts. Treatment of late schizionts with antibodies that blocked the inhibitory activity of falstatin against native and recombinant falcipain-2 and falcipain-3 dose-dependently decreased the subsequent invasion of erythrocytes by merozoites. These results suggest that P. falciparum requires expression of falstatin to limit proteolysis by certain host or parasite cysteine proteases during erythrocyte invasion. This mechanism of regulation of proteolysis suggests new strategies for the development of antimalarial agents that specifically disrupt erythrocyte invasion.

Immunoproteasome functions explained by divergence in cleavage specificity and regulation.

The immunoproteasome (iP) has been proposed to perform specialized roles in MHC class I antigen presentation, cytokine modulation, and T cell differentiation and has emerged as a promising therapeutic target for autoimmune disorders and cancer. However, divergence in function between the iP and the constitutive proteasome (cP) has been unclear. A global peptide library-based screening strategy revealed that the proteasomes have overlapping but distinct substrate specificities. Differing iP specificity alters the quantity of production of certain MHC I epitopes but does not appear to be preferentially suited for antigen presentation. Furthermore, iP specificity was found to have likely arisen through genetic drift from the ancestral cP. Specificity differences were exploited to develop isoform-selective substrates. Cellular profiling using these substrates revealed that divergence in regulation of the iP balances its relative contribution to proteasome capacity in immune cells, resulting in selective recovery from inhibition. These findings have implications for iP-targeted therapeutic development.

Discovery of Highly Selective Inhibitors of the Immunoproteasome Low Molecular Mass Polypeptide 2 (LMP2) Subunit.

Building upon the success of bortezomib (VELCADE) and carfilzomib (KYPROLIS), the design of a next generation of inhibitors targeting specific subunits within the immunoproteasome is of interest for the treatment of autoimmune disease. There are three catalytic subunits within the immunoproteasome (low molecular mass polypeptide-7, -2, and multicatalytic endopeptidase complex subunit-1; LMP7, LMP2, and MECL-1), and a campaign was undertaken to design a potent and selective LMP2 inhibitor with sufficient properties to allow for sustained inhibition in vivo. Screening a focused library of epoxyketones revealed a series of potent dipeptides that were optimized to provide the highly selective inhibitor KZR-504 (12).

Point mutations within the LdNT2 nucleoside transporter gene from Leishmania donovani confer drug resistance and transport deficiency.

Leishmania donovani, a protozoan parasite, expresses an unusual inosine/guanosine-specific transporter, LdNT2, the gene for which was cloned by functional rescue of a drug-resistant, LdNT2-deficient (FBD5) strain. In this investigation, we have uncovered and characterized the mutations within the LdNT2 open reading frame that are the basis for the drug-resistance and transport-incompetent phenotype of the FBD5 line. The FBD5 cells were shown to be compound heterozygotes in which both mutant ldnt2 alleles harbor discrete point mutations, each of which impaired transport function and conferred resistance to formycin B, the drug to which the clonal FBD5 line was selected. One of the mutant ldnt2 alleles encoded an S189L alteration in predicted transmembrane domain 5, while the second allele accommodated a null mutation at codon 376, which truncated the transporter just prior to transmembrane domain 8. In addition to the null transport phenotype, very little S189L ldnt2 mutant transporter targeted to the surface of the parasite. The bulk of the truncated ldnt2 appeared to be sequestered internally, possibly within the endoplasmic reticulum, but some of the truncated transporter seemed to be cell surface exposed. The ability to dissect mutations within a viable parasite offers LdNT2 as an attractive model for implementing a thorough forward genetic dissection of transporter function in a eukaryotic cell.

Molecular mechanisms of bortezomib resistant adenocarcinoma cells.

Bortezomib (Velcade™) is a reversible proteasome inhibitor that is approved for the treatment of multiple myeloma (MM). Despite its demonstrated clinical success, some patients are deprived of treatment due to primary refractoriness or development of resistance during therapy. To investigate the role of the duration of proteasome inhibition in the anti-tumor response of bortezomib, we established clonal isolates of HT-29 adenocarcinoma cells adapted to continuous exposure of bortezomib. These cells were ~30-fold resistant to bortezomib. Two novel and distinct mutations in the ß5 subunit, Cys63Phe, located distal to the binding site in a helix critical for drug binding, and Arg24Cys, found in the propeptide region were found in all resistant clones. The latter mutation is a natural variant found to be elevated in frequency in patients with MM. Proteasome activity and levels of both the constitutive and immunoproteasome were increased in resistant cells, which correlated to an increase in subunit gene expression. These changes correlated with a more rapid recovery of proteasome activity following brief exposure to bortezomib. Increased recovery rate was not due to increased proteasome turnover as similar findings were seen in cells co-treated with cycloheximide. When we exposed resistant cells to the irreversible proteasome inhibitor carfilzomib we noted a slower rate of recovery of proteasome activity as compared to bortezomib in both parental and resistant cells. Importantly, carfilzomib maintained its cytotoxic potential in the bortezomib resistant cell lines. Therefore, resistance to bortezomib, can be overcome with irreversible inhibitors, suggesting prolonged proteasome inhibition induces a more potent anti-tumor response.

Nonproteasomal targets of the proteasome inhibitors bortezomib and carfilzomib: a link to clinical adverse events.

PURPOSE: Bortezomib (Velcade), a dipeptide boronate 20S proteasome inhibitor and an approved treatment option for multiple myeloma, is associated with a treatment-emergent, painful peripheral neuropathy (PN) in more than 30% of patients. Carfilzomib, a tetrapeptide epoxyketone proteasome inhibitor, currently in clinical investigation in myeloma, is associated with low rates of PN. We sought to determine whether PN represents a target-mediated adverse drug reaction (ADR). EXPERIMENTAL DESIGN: Neurodegenerative effects of proteasome inhibitors were assessed in an in vitro model utilizing a differentiated neuronal cell line. Secondary targets of both inhibitors were identified by a multifaceted approach involving candidate screening, profiling with an activity-based probe, and database mining. Secondary target activity was measured in rats and patients receiving both inhibitors. RESULTS: Despite equivalent levels of proteasome inhibition, only bortezomib reduced neurite length, suggesting a nonproteasomal mechanism. In cell lysates, bortezomib, but not carfilzomib, significantly inhibited the serine proteases cathepsin G (CatG), cathepsin A, chymase, dipeptidyl peptidase II, and HtrA2/Omi at potencies near or equivalent to that for the proteasome. Inhibition of CatG was detected in splenocytes of rats receiving bortezomib and in peripheral blood mononuclear cells derived from bortezomib-treated patients. Levels of HtrA2/Omi, which is known to be involved in neuronal survival, were upregulated in neuronal cells exposed to both proteasome inhibitors but was inhibited only by bortezomib exposure. CONCLUSION: These data show that bortezomib-induced neurodegeneration in vitro occurs via a proteasome-independent mechanism and that bortezomib inhibits several nonproteasomal targets in vitro and in vivo, which may play a role in its clinical ADR profile.

An ab Initio structural model of a nucleoside permease predicts functionally important residues.

Permeases belonging to the equilibrative nucleoside transporter family promote uptake of nucleosides and/or nucleobases into a wide range of eukaryotes and mediate the uptake of a variety of drugs used in the treatment of cancer, heart disease, AIDS, and parasitic infections. No experimental three-dimensional structure exists for any of these permeases, and they are not present in prokaryotes, the source of many membrane proteins used in crystal structure determination. To generate a structural model for such a transporter, the LdNT1.1 nucleoside permease from the parasitic protozoan Leishmania donovani was modeled using ab initio computation. Site-directed mutations that strongly impair transport or that alter substrate specificity map to the central pore of the ab initio model, whereas mutations that have less pronounced phenotypes map to peripheral positions. The model suggests that aromatic residues present in transmembrane helices 1, 2, and 7 may interact to form an extracellular gate that closes the permeation pathway in the inward oriented conformation. Mutation of two of these three residues abrogated transport activity, consistent with the prediction of the model. The ab initio model is similar to one derived previously using threading analysis, a distinct computational approach, supporting the overall accuracy of both models. However, significant differences in helix orientation and residue position between the two models are apparent, and the mutagenesis data suggest that the ab initio model represents an improvement regarding structural details over the threading model. The putative gating interaction may also help explain differences in substrate specificity between members of this family.

Novel aza peptide inhibitors and active-site probes of papain-family cysteine proteases.

Recent characterization of multiple classes of functionalized azapeptides as effective covalent inhibitors of cysteine proteases prompted us to investigate O-acyl hydroxamates and their azapeptide analogues for use as activity-based probes (ABPs). We report here a new class of azaglycine-containing O-acylhydroxamates that form stable covalent adducts with target proteases. This allows them to be used as ABPs for papain family cysteine proteases. A second class of related analogues containing a novel O-acyl hydroxyurea warhead was found to function as covalent inhibitors of papain-like proteases. These inhibitors can be easily synthesized on solid support, which allows rapid optimization of compounds with improved selectivity and potency for a given target enzyme. We present here one such optimized inhibitor that showed selective inhibition of falcipain 1, a protease of the malaria-causing parasite, Plasmodium falciparum.

Plasmodium falciparum AMA1 binds a rhoptry neck protein homologous to TgRON4, a component of the moving junction in Toxoplasma gondii.

Plasmodium falciparum apical membrane antigen 1 (PfAMA1) coimmunoprecipitates with the Plasmodium homologue of TgRON4, a secreted rhoptry neck protein of Toxoplasma gondii that migrates at the moving junction in association with TgAMA1 during invasion. PfRON4 also originates in the rhoptry necks, suggesting that this unusual collaboration of micronemes and rhoptries is a conserved feature of Apicomplexa.

Second-site suppression of a nonfunctional mutation within the Leishmania donovani inosine-guanosine transporter.

LdNT2 is a member of the equilibrative nucleoside transporter family, which possesses several conserved residues located mainly within transmembrane domains. One of these residues, Asp(389) within LdNT2, was shown previously to be critical for transporter function without affecting ligand affinity or plasma membrane targeting. To further delineate the role of Asp(389) in LdNT2 function, second-site suppressors of the ldnt2-D389N null mutation were selected in yeast deficient in purine nucleoside transport and incapable of purine biosynthesis. A library of random mutants within the ldnt2-D389N background was screened in yeast for restoration of growth on inosine. Twelve different clones were obtained, each containing secondary mutations enabling inosine transport. One mutation, N175I, occurred in four clones and conferred augmented inosine transport capability compared with LdNT2 in yeast. N175I was subsequently introduced into an ldnt2-D389N construct tagged with green fluorescent protein and transfected into a Deltaldnt1/Deltaldnt2 Leishmania donovani knockout. GFP-N175I/D389N significantly suppressed the D389N phenotype and targeted properly to the plasma membrane and flagellum. Most interestingly, N175I increased the inosine K(m) by 10-fold within the D389N background relative to wild type GFP-LdNT2. Additional substitutions introduced at Asn(175) established that only large, nonpolar amino acids suppressed the D389N phenotype, indicating that suppression by Asn(175) has a specific size and charge requirement. Because multiple suppressor mutations alleviate the constraint imparted by the D389N mutation, these data suggest that Asp(389) is a conformationally sensitive residue. To impart spatial information to the clustering of second-site mutations, a three-dimensional model was constructed based upon members of the major facilitator superfamily using threading analysis. The model indicates that Asn(175) and Asp(389) lie in close proximity and that the second-site suppressor mutations cluster to one region of the transporter.

Functional analysis of an inosine-guanosine transporter from Leishmania donovani. The role of conserved residues, aspartate 389 and arginine 393.

Equilibrative nucleoside transporters encompass two conserved, charged residues that occur within predicted transmembrane domain 8. To assess the role of these "signature" residues in transporter function, the Asp389 and Arg393 residues within the LdNT2 nucleoside transporter from Leishmania donovani were mutated and the resultant phenotypes evaluated after transfection into Delta ldnt2 parasites. Whereas an R393K mutant retained transporter activity similar to that of wild type LdNT2, the R393L, D389E, and D389N mutations resulted in dramatic losses of transport capability. Tagging the wild type and mutant ldnt2 proteins with green fluorescent protein demonstrated that the D389N and D389E mutants targeted properly to the parasite cell surface and flagellum, whereas the expression of R393L at the cell surface was profoundly compromised. To test whether Asp389 and Arg393 interact, a series of mutants was generated, D389R/R393R, D389D/R393D, and D389R/R393D, within the green fluorescent protein-tagged LdNT2 construct. Although all of these ldnt2 mutants were transport-deficient, D389R/R393D localized properly to the plasma membrane, while neither D389R/R393R nor D389D/R393D could be detected. Moreover, a transport-incompetent D389N/R393N double ldnt2 mutant also localized to the parasite membrane, whereas a D389L/R393L ldnt2 mutant did not, suggesting that an interaction between residues 389 and 393 may be involved in LdNT2 membrane targeting. These studies establish genetically that Asp389 is critical for optimal transporter function and that a positively charged or polar residue at Arg393 is essential for proper expression of LdNT2 at the plasma membrane.