A Novel-type Substrate-selectivity Filter and ER-exit Determinants in the UapA Purine Transporter

Dissecting key residues of a C4-dicarboxylic acid transporter to accelerate malate export in Myceliophthora

Efficient transporters are necessary for high concentration and purity of desired products during industrial production. In this study, we explored the mechanism of substrate transport and preference of the C4-dicarboxylic acid transporter AoMAE in the fungus Myceliophthora thermophila, and investigated the roles of 18 critical amino acid residues within this process. Among them, the residue Arg78, forming a hydrogen bond network with Arg23, Phe25, Thr74, Leu81, His82, and Glu94 to stabilize the protein conformation, is irreplaceable for the export function of AoMAE. Furthermore, varying the residue at position 100 resulted in changes to the size and shape of the substrate binding pocket, leading to alterations in transport efficiencies of both malic acid and succinic acid. We found that the mutation T100S increased malate production by 68%. Using these insights, we successfully generated an AoMAE variant with mutation T100S and deubiquitination, exhibiting an 81% increase in the selective export activity of malic acid. Simply introducing this version of AoMAE into M. thermophila wild-type strain increased production of malic acid from 1.22 to 54.88 g/L. These findings increase our understanding of the structure-function relationships of organic acid transporters and may accelerate the process of engineering dicarboxylic acid transporters with high efficiency. KEY POINTS: • This is the first systematical analysis of key residues of a malate transporter in fungi. • Protein engineering of AoMAE led to 81% increase of malate export activity. • Arg78 was essential for the normal function of AoMAE in M. thermophila. • Substitution of Thr100 affected export efficiency and substrate selectivity of AoMAE.

Transmembrane helices 5 and 12 control transport dynamics, substrate affinity, and specificity in the elevator-type UapA transporter

An increasing number of solute transporters have been shown to function with the so-called sliding-elevator mechanism. Despite structural and functional differences, all elevator-type transporters use a common mechanism of substrate translocation via reversible movements of a mobile core domain (the elevator) hosting the substrate binding site along a rigid scaffold domain stably anchored in the plasma membrane via homodimerization. One of the best-studied elevator transporters is the UapA uric acid-xanthine/H+ symporter of the filamentous fungus Aspergillus nidulans. Here, we present a genetic analysis for deciphering the role of transmembrane segments (TMS) 5 and 12 in UapA transport function. We show that specific residues in both TMS5 and TMS12 control, negatively or positively, the dynamics of transport, but also substrate binding affinity and specificity. More specifically, mutations in TMS5 can lead not only to increased rate of transport but also to an inactive transporter due to high-affinity substrate-trapping, whereas mutations in TMS12 lead to apparently uncontrolled sliding and broadened specificity, leading in specific cases to UapA-mediated purine toxicity. Our findings shed new light on how elevator transporters function and how this knowledge can be applied to genetically modify their transport characteristics.

Optimizing the Substrate Uptake Rate of Solute Carriers

The diversity in solute carriers arose from evolutionary pressure. Here, we surmised that the adaptive search for optimizing the rate of substrate translocation was also shaped by the ambient extracellular and intracellular concentrations of substrate and co-substrate(s). We explored possible solutions by employing kinetic models, which were based on analytical expressions of the substrate uptake rate, that is, as a function of the microscopic rate constants used to parameterize the transport cycle. We obtained the defining terms for five reaction schemes with identical transport stoichiometry (i.e., Na+: substrate = 2:1). We then utilized an optimization algorithm to find the set of numeric values for the microscopic rate constants, which provided the largest value for the substrate uptake rate: The same optimized rate was achieved by different sets of numerical values for the microscopic rate constants. An in-depth analysis of these sets provided the following insights: (i) In the presence of a low extracellular substrate concentration, a transporter can only cycle at a high rate, if it has low values for both, the Michaelis-Menten constant (KM) for substrate and the maximal substrate uptake rate (Vmax). (ii) The opposite is true for a transporter operating at high extracellular substrate concentrations. (iii) Random order of substrate and co-substrate binding is superior to sequential order, if a transporter is to maintain a high rate of substrate uptake in the presence of accumulating intracellular substrate. Our kinetic models provide a framework to understand how and why the transport cycles of closely related transporters differ.

Transporter Specificity: A Tale of Loosened Elevator-Sliding

Elevator-type transporters are a group of proteins translocating nutrients and metabolites across cell membranes. Despite structural and functional differences, elevator-type transporters use a common mechanism of substrate translocation via reversible movements of a mobile core domain (the elevator), which includes the substrate binding site, along a rigid scaffold domain, stably anchored in the plasma membrane. How substrate specificity is determined in elevator transporters remains elusive. Here, I discuss how a recent report on the sliding elevator mechanism, seen under the context of genetic analysis of a prototype fungal transporter, sheds light on how specificity might be genetically modified. I propose that flexible specificity alterations might occur by 'loosening' of the sliding mechanism from tight coupling to substrate binding.

Context-dependent Cryptic Roles of Specific Residues in Substrate Selectivity of the UapA Purine Transporter

Members of the ubiquitous Nucleobase Ascorbate Transporter (NAT) family are H⁺ or Na⁺ symporters specific for the cellular uptake of either purines and pyrimidines or L-ascorbic acid. Despite the fact that several bacterial and fungal members have been extensively characterised at a genetic, biochemical or cellular level, and crystal structures of NAT members from Escherichia coli and Aspergillus nidulans have been determined pointing to a mechanism of transport, we have little insight on how substrate selectivity is determined. Here, we present systematic mutational analyses, rational combination of mutations, and novel genetic screens that reveal cryptic context-dependent roles of partially conserved residues in the so-called NAT signature motif in determining the specificity of the UapA transporter of A. nidulans. We show that specific NAT signature motif substitutions, alone and in combinations with each other or with distant mutations in residues known to affect substrate selectivity, lead to novel UapA versions possessing variable transport capacities and specificities for nucleobases. In particular, we show that a UapA version including the quadruple mutation T405S/F406Y/A407S/Q408E in the NAT signature motif (UapA-SYSE) becomes incapable of purine transport, but gains a novel pyrimidine-related profile, which can be further altered to a more promiscuous purine/pyrimidine profile when combined with replacements at distantly located residues, especially at F528. Our results reveal that UapA specificity is genetically highly modifiable and allow us to speculate on how the elevator-type mechanism of transport might account for this flexibility.

Tales of tails in transporters

Cell nutrition, detoxification, signalling, homeostasis and response to drugs, processes related to cell growth, differentiation and survival are all mediated by plasma membrane (PM) proteins called transporters. Despite their distinct fine structures, mechanism of function, energetic requirements, kinetics and substrate specificities, all transporters are characterized by a main hydrophobic body embedded in the PM as a series of tightly packed, often intertwined, α-helices that traverse the lipid bilayer in a zigzag mode, connected with intracellular or extracellular loops and hydrophilic N- and C-termini. Whereas longstanding genetic, biochemical and biophysical evidence suggests that specific transmembrane segments, and also their connecting loops, are responsible for substrate recognition and transport dynamics, emerging evidence also reveals the functional importance of transporter N- and C-termini, in respect to transport catalysis, substrate specificity, subcellular expression, stability and signalling. This review highlights selected prototypic examples of transporters in which their termini play important roles in their functioning.

Evolution of substrate specificity in the Nucleobase-Ascorbate Transporter (NAT) protein family

L-ascorbic acid (vitamin C) is an essential metabolite in animals and plants due to its role as an enzyme co-factor and antioxidant activity. In most eukaryotic organisms, L-ascorbate is biosynthesized enzymatically, but in several major groups, including the primate suborder Haplorhini, this ability is lost due to gene truncations in the gene coding for L-gulonolactone oxidase. Specific ascorbate transporters (SVCTs) have been characterized only in mammals and shown to be essential for life. These belong to an extensively studied transporter family, called Nucleobase-Ascorbate Transporters (NAT). The prototypic member of this family, and one of the most extensively studied eukaryotic transporters, is UapA, a uric acid-xanthine/H⁺ symporter in the fungus Aspergillus nidulans. Here, we investigate molecular aspects of NAT substrate specificity and address the evolution of ascorbate transporters apparently from ancestral nucleobase transporters. We present a phylogenetic analysis, identifying a distinct NAT clade that includes all known L-ascorbate transporters. This clade includes homologues only from vertebrates, and has no members in non-vertebrate or microbial eukaryotes, plants or prokaryotes. Additionally, we identify within the substrate-binding site of NATs a differentially conserved motif, which we propose is critical for nucleobase versus ascorbate recognition. This conclusion is supported by the amino acid composition of this motif in distinct phylogenetic clades and mutational analysis in the UapA transporter. Together with evidence obtained herein that UapA can recognize with extremely low affinity L-ascorbate, our results support that ascorbate-specific NATs evolved by optimization of a sub-function of ancestral nucleobase transporters.

The novel class of seven transmembrane segment inverted repeat carriers

Solute carriers from the SLC4, SLC23, and SLC26 families are involved in pH regulation, vitamin C transport and ion homeostasis. While these families do not share any obvious sequence relationship, they are united by their unique and novel architecture. Each member of this structural class is organized into two structurally related halves of seven transmembrane segments each. These halves span the membrane with opposite orientations and form an intricately intertwined structure of two inverted repeats. This review highlights the general design principles of this fold and reveals the diversity between the different families. We discuss their domain architecture, structural framework and transport mode and detail an initial transport mechanism for this fold inferred from the recently solved structures of different members.

BsdA(Bsd2) -dependent vacuolar turnover of a misfolded version of the UapA transporter along the secretory pathway: prominent role of selective autophagy

Transmembrane proteins translocate cotranslationally in the endoplasmic reticulum (ER) membrane and traffic as vesicular cargoes, via the Golgi, in their final membrane destination. Misfolding in the ER leads to protein degradation basically through the ERAD/proteasome system. Here, we use a mutant version of the purine transporter UapA (ΔR481) to show that specific misfolded versions of plasma membrane cargoes undergo vacuolar turnover prior to localization in the plasma membrane. We show that non-endocytic vacuolar turnover of ΔR481 is dependent on BsdA(Bsd2) , an ER transmembrane adaptor of HulA(Rsp5) ubiquitin ligase. We obtain in vivo evidence that BsdA(Bsd2) interacts with HulA(Rsp5) and ΔR481, primarily in the ER. Importantly, accumulation of ΔR481 in the ER triggers delivery of the selective autophagy marker Atg8 in vacuoles along with ΔR481. Genetic block of autophagy (atg9Δ, rabO(ts) ) reduces, but does not abolish, sorting of ΔR481 in the vacuoles, suggesting that a fraction of the misfolded transporter might be redirected for vacuolar degradation via the Golgi. Our results support that multiple routes along the secretory pathway operate for the detoxification of Aspergillus nidulans cells from misfolded membrane proteins and that BsdA is a key factor for marking specific misfolded cargoes.

Functional characterization of NAT/NCS2 proteins of Aspergillus brasiliensis reveals a genuine xanthine-uric acid transporter and an intrinsically misfolded polypeptide

The Nucleobase-Ascorbate Transporter (NAT) family includes members in nearly all domains of life. Functionally characterized NAT transporters from bacteria, fungi, plants and mammals are ion-coupled symporters specific for the uptake of purines, pyrimidines and related analogues. The characterized mammalian NATs are specific for the uptake of L-ascorbic acid. In this work we identify in silico a group of fungal putative transporters, named UapD-like proteins, which represent a novel NAT subfamily. To understand the function and specificity of UapD proteins, we cloned and functionally characterized the two Aspergillus brasiliensis NAT members (named AbUapC and AbUapD) by heterologous expression in Aspergillus nidulans. AbUapC represents canonical NATs (UapC or UapA), while AbUapD represents the new subfamily. AbUapC is a high-affinity, high-capacity, H(+)/xanthine-uric acid transporter, which can also recognize other purines with very low affinity. No apparent transport function could be detected for AbUapD. GFP-tagging showed that, unlike AbUapC which is localized in the plasma membrane, AbUapD is ER-retained and degraded in the vacuoles, a characteristic of misfolded proteins. Chimeric UapA/AbUapD molecules are also turned-over in the vacuole, suggesting that UapD includes intrinsic peptidic sequences leading to misfolding. The possible evolutionary implication of such conserved, but inactive proteins is discussed.

Understanding transporter specificity and the discrete appearance of channel-like gating domains in transporters

Transporters are ubiquitous proteins mediating the translocation of solutes across cell membranes, a biological process involved in nutrition, signaling, neurotransmission, cell communication and drug uptake or efflux. Similarly to enzymes, most transporters have a single substrate binding-site and thus their activity follows Michaelis-Menten kinetics. Substrate binding elicits a series of structural changes, which produce a transporter conformer open toward the side opposite to the one from where the substrate was originally bound. This mechanism, involving alternate outward- and inward-facing transporter conformers, has gained significant support from structural, genetic, biochemical and biophysical approaches. Most transporters are specific for a given substrate or a group of substrates with similar chemical structure, but substrate specificity and/or affinity can vary dramatically, even among members of a transporter family that show high overall amino acid sequence and structural similarity. The current view is that transporter substrate affinity or specificity is determined by a small number of interactions a given solute can make within a specific binding site. However, genetic, biochemical and in silico modeling studies with the purine transporter UapA of the filamentous ascomycete Aspergillus nidulans have challenged this dogma. This review highlights results leading to a novel concept, stating that substrate specificity, but also transport kinetics and transporter turnover, are determined by subtle intramolecular interactions between a major substrate binding site and independent outward- or cytoplasmically-facing gating domains, analogous to those present in channels. This concept is supported by recent structural evidence from several, phylogenetically and functionally distinct transporter families. The significance of this concept is discussed in relationship to the role and potential exploitation of transporters in drug action.

Allopurinol and xanthine use different translocation mechanisms and trajectories in the fungal UapA transporter

In Aspergillus nidulans UapA is a H(+)-driven transporter specific for xanthine, uric acid and several analogues. Here, genetic and physiological evidence is provided showing that allopurinol is a high-affinity, low-capacity, substrate for UapA. Surprisingly however, transport kinetic measurements showed that, uniquely among all recognized UapA substrates, allopurinol is transported by apparent facilitated diffusion and exhibits a paradoxical effect on the transport of physiological substrates. Specifically, excess xanthine or other UapA substrates inhibit allopurinol uptake, as expected, but the presence of excess allopurinol results in a concentration-dependent enhancement of xanthine binding and transport. Flexible docking approaches failed to detect allopurinol binding in the major UapA substrate binding site, which was recently identified by mutational analysis and substrate docking using all other UapA substrates. These results and genetic evidence suggest that the allopurinol translocation pathway is distinct from, but probably overlapping with, that of physiological UapA substrates. Furthermore, although the stimulating effect of allopurinol on xanthine transport could, in principle, be rationalized by a cryptic allopurinol-specific allosteric site, evidence was obtained supporting that accelerated influx of xanthine is triggered through exchange with cytoplasmically accumulated allopurinol. Our results are in line with recently accumulating evidence revealing atypical and complex mechanisms underlying transport systems.

Modeling, substrate docking, and mutational analysis identify residues essential for the function and specificity of a eukaryotic purine-cytosine NCS1 transporter

The recent elucidation of crystal structures of a bacterial member of the NCS1 family, the Mhp1 benzyl-hydantoin permease from Microbacterium liquefaciens, allowed us to construct and validate a three-dimensional model of the Aspergillus nidulans purine-cytosine/H(+) FcyB symporter. The model consists of 12 transmembrane α-helical, segments (TMSs) and cytoplasmic N- and C-tails. A distinct core of 10 TMSs is made of two intertwined inverted repeats (TMS1-5 and TMS6-10) that are followed by two additional TMSs. TMS1, TMS3, TMS6, and TMS8 form an open cavity that is predicted to host the substrate binding site. Based on primary sequence alignment, three-dimensional topology, and substrate docking, we identified five residues as potentially essential for substrate binding in FcyB; Ser-85 (TMS1), Trp-159, Asn-163 (TMS3), Trp-259 (TMS6), and Asn-354 (TMS8). To validate the role of these and other putatively critical residues, we performed a systematic functional analysis of relevant mutants. We show that the proposed substrate binding residues, plus Asn-350, Asn-351, and Pro-353 are irreplaceable for FcyB function. Among these residues, Ser-85, Asn-163, Asn-350, Asn-351, and Asn-354 are critical for determining the substrate binding affinity and/or the specificity of FcyB. Our results suggest that Ser-85, Asn-163, and Asn-354 directly interact with substrates, Trp-159 and Trp-259 stabilize binding through π-π stacking interactions, and Pro-353 affects the local architecture of substrate binding site, whereas Asn-350 and Asn-351 probably affect substrate binding indirectly. Our work is the first systematic approach to address structure-function-specificity relationships in a eukaryotic member of NCS1 family by combining genetic and computational approaches.

Identification of the substrate recognition and transport pathway in a eukaryotic member of the nucleobase-ascorbate transporter (NAT) family

Using the crystal structure of the uracil transporter UraA of Escherichia coli, we constructed a 3D model of the Aspergillus nidulans uric acid-xanthine/H(+) symporter UapA, which is a prototype member of the Nucleobase-Ascorbate Transporter (NAT) family. The model consists of 14 transmembrane segments (TMSs) divided into a core and a gate domain, the later being distinctly different from that of UraA. By implementing Molecular Mechanics (MM) simulations and quantitative structure-activity relationship (SAR) approaches, we propose a model for the xanthine-UapA complex where the substrate binding site is formed by the polar side chains of residues E356 (TMS8) and Q408 (TMS10) and the backbones of A407 (TMS10) and F155 (TMS3). In addition, our model shows several polar interactions between TMS1-TMS10, TMS1-TMS3, TMS8-TMS10, which seem critical for UapA transport activity. Using extensive docking calculations we identify a cytoplasm-facing substrate trajectory (D360, A363, G411, T416, R417, V463 and A469) connecting the proposed substrate binding site with the cytoplasm, as well as, a possible outward-facing gate leading towards the substrate major binding site. Most importantly, re-evaluation of the plethora of available and analysis of a number of herein constructed UapA mutations strongly supports the UapA structural model. Furthermore, modeling and docking approaches with mammalian NAT homologues provided a molecular rationale on how specificity in this family of carriers might be determined, and further support the importance of selectivity gates acting independently from the major central substrate binding site.

Comparative kinetic analysis of AzgA and Fcy21p, prototypes of the two major fungal hypoxanthine-adenine-guanine transporter families

In fungi, uptake of salvageable purines is carried out by members of two evolutionarily distinct protein families, the Purine-Related Transporters (PRT/NCS1) and the AzgA-like Transporters. We carried out a comparative kinetic analysis of two prototypes of these transporter families. The first was Fcy21p, a herein characterized protein of Candida albicans, and the second was AzgA, a transporter of Aspergillus nidulans. Our results showed that: (i) AzgA and Fcy21p are equally efficient high-affinity, high-capacity, purine transporters, (ii) Fcy21p, but not AzgA, is an efficient cytosine and 5-fluorocytosine transporter, interacting with =O2 and C4-NH2 of the pyrimidine ring, (iii) the major interactions of AzgA and Fcy21p with the purine ring are similar, but not identical, involving in all cases positions 6 and 7, and for some substrates, positions 1 and 9 as well, and (iv) in AzgA, bulky groups at position N3 have a detrimental steric effect on substrate binding, while similar substitutions at C2 or N9 are fully or partially tolerated. In contrast, in Fcy21p, C2 and N9 bulky substitutions abolish substrate binding, while similar substitutions in N3 are fully tolerated. These results suggest that all fungal purine transporters might have evolved from a single ancestral protein, and show that fungal transporters use different substrate interactions compared to the analogous protozoan or mammalian proteins. Finally, results are also discussed in respect of the possibility of using fungal purine transporters as specific gateways for the development of targeted antifungal pharmacological therapies.

Purine substrate recognition by the nucleobase-ascorbate transporter signature motif in the YgfO xanthine permease: ASN-325 binds and ALA-323 senses substrate

The nucleobase-ascorbate transporter (NAT) signature motif is a conserved 11-amino acid sequence of the ubiquitous NAT/NCS2 family, essential for function and selectivity of both a bacterial (YgfO) and a fungal (UapA) purine-transporting homolog. We examined the role of NAT motif in more detail, using Cys-scanning and site-directed alkylation analysis of the YgfO xanthine permease of Escherichia coli. Analysis of single-Cys mutants in the sequence 315-339 for sensitivity to inactivation by 2-sulfonatoethyl methanethiosulfonate (MTSES(-)) and N-ethylmaleimide (NEM) showed a similar pattern: highly sensitive mutants clustering at the motif sequence (323-329) and a short alpha-helical face downstream (332, 333, 336). In the presence of substrate, N325C is protected from alkylation with either MTSES(-) or NEM, whereas sensitivity of A323C to inactivation by NEM is enhanced, shifting IC(50) from 34 to 14 microM. Alkylation or sensitivity of the other mutants is unaffected by substrate; the lack of an effect on Q324C is attributed to gross inability of this mutant for high affinity binding. Site-directed mutants G333R and S336N at the alpha-helical face downstream the motif display specific changes in ligand recognition relative to wild type; G333R allows binding of 7-methyl and 8-methylxanthine, whereas S336N disrupts affinity for 6-thioxanthine. Finally, all assayable motif-mutants are highly accessible to MTSES(-) from the periplasmic side. The data suggest that the NAT motif region lines the solvent- and substrate-accessible inner cavity, Asn-325 is at the binding site, Ala-323 responds to binding with a specific conformational shift, and Gly-333 and Ser-336 form part of the purine permeation pathway.

Expression and purification of a functional uric acid-xanthine transporter (UapA)

The Nucleobase-Ascorbate Transporters (NATs) family includes carriers with fundamental functions in uptake of key cellular metabolites, such as uric acid or vitamin C. The best studied example of a NAT transporter is the uric acid-xanthine permease (UapA) from the model ascomycete Aspergillus nidulans. Detailed genetic and biochemical analyses have revealed much about the mechanism of action of this protein; however, the difficulties associated with handling eukaryotic membrane proteins have limited efforts to elucidate the precise structure-function relationships of UapA by structural analysis. In this manuscript, we describe the heterologous overexpression of functional UapA as a fusion with GFP in different strains of Saccharomyces cerevisiae. The UapA-GFP construct expressed to 2.3 mg/L in a pep4Delta deletion strain lacking a key vacuolar endopeptidase and 3.8 mg/L in an npi1-1 mutant strain with defective Rsp5 ubiquitin ligase activity. Epifluorescence microscopy revealed that the UapA-GFP was predominately localized to the plasma membrane in both strains, although a higher intensity of fluorescence was observed for the npi1-1 mutant strain plasma membrane. In agreement with these observations, the npi1-1 mutant strain demonstrated a approximately 5-fold increase in uptake of [(3)H]-xanthine compared to the pep4Delta deletion strain. Despite yielding the best results for functional expression, in-gel fluorescence of the UapA-GFP expressed in the npi1-1 mutant strain revealed that the protein was subject to significant proteolytic degradation. Large scale expression of the protein using the pep4Delta deletion strain followed by purification produced mg quantities of pure, monodispersed protein suitable for further structural and functional studies. In addition, this work has generated a yeast cell based system for performing reverse genetics and other targeted approaches, in order to further understand the mechanism of action of this important model protein.

Role of intramembrane polar residues in the YgfO xanthine permease: HIS-31 and ASN-93 are crucial for affinity and specificity, and ASP-304 and GLU-272 are irreplaceable

Using the YgfO xanthine permease of Escherichia coli as a bacterial model for the study of the evolutionarily ubiquitous nucleobase-ascorbate transporter (NAT/NCS2) family, we performed a systematic Cys-scanning and site-directed mutagenesis of 14 putatively charged (Asp, Glu, His, Lys, or Arg) and 7 highly polar (Gln or Asn) residues that are predicted to lie in transmembrane helices (TMs). Of 21 single-Cys mutants engineered in the background of a functional YgfO devoid of Cys residues (C-less), only four are inactive or have marginal activity (H31C, N93C, E272C, D304C). The 4 residues are conserved throughout the family in TM1 (His-31), TM3 (Asn-93/Ser/Thr), TM8 (Glu-272), and putative TM9a (Asp-304/Asn/Glu). Extensive site-directed mutagenesis in wild-type background showed that H31N and H31Q have high activity and affinity for xanthine but H31Q recognizes novel purine bases and analogues, whereas H31C and H31L have impaired affinity for xanthine and analogues, and H31K or H31R impairs expression in the membrane. N93S and N93A are highly active but more promiscuous for recognition of analogues at the imidazole moiety of substrate, N93D has low activity, N93T has low affinity for xanthine or analogues, and N93Q or N93C is inactive. All mutants replacing Glu-272 or Asp-304, including E272D, E272Q, D304E, and D304N, are inactive, although expressed to high levels in the membrane. Finally, one of the 17 assayable single-Cys mutants, Q258C, was sensitive to inactivation by N-ethylmaleimide. The findings suggest that polar residues important for the function of YgfO cluster in TMs 1, 3, 8 and 9a.

Characterization and kinetics of the major purine transporters in Aspergillus fumigatus

Three genes encoding putative purine transporters have been identified in silico in the genome of Aspergillus fumigatus by their very close similarity of their translation products to well-studied homologues in A. nidulans. Two of these transporters, called AfUapC and AfAzgA, were found responsible for bulk uptake of purines and studied in detail herein. Genetic knock-out analysis, regulation of transcription, direct purine uptake assays and heterologous expression in A. nidulans have unequivocally shown that AfUapC and AfAzgA are high-affinity, high-capacity, purine/H(+) symporters, the first being specific for xanthine, uric acid and oxypurinol, whereas the second for adenine, hypoxanthine, guanine and purine. The expression of these transporters is primarily controlled at the level of transcription. Transcription of both genes is purine-inducible, albeit with different efficiencies, whereas AfuapC is also ammonium-repressible. We characterised in detail the kinetics of the AfUapC and AfAzgA transporters, both in A. fumigatus and in A. nidulans, using a plethora of possible purine substrates. This analysis led us to propose kinetic models describing the molecular interactions of AfUapC and AfAzgA with purines. These models are discussed comparatively with analogous models from other purine transporters from fungi, bacteria and humans, and within the frame of a systematic development of novel purine-related antifungals.

Kinetic and mutational analysis of the Trypanosoma brucei NBT1 nucleobase transporter expressed in Saccharomyces cerevisiae reveals structural similarities between ENT and MFS transporters

Parasitic protozoa are unable to synthesise purines de novo and thus depend on the uptake of nucleosides and nucleobases across their plasma membrane through specific transporters. A number of nucleoside and nucleobase transporters from Trypanosoma brucei brucei and Leishmania major have recently been characterised and shown to belong to the equilibrative nucleoside transporter (ENT) family. A number of studies have demonstrated the functional importance of particular transmembrane segments (TMS) in nucleoside-specific ENT proteins. TbNBT1, one of only three bona fide nucleobase-selective members of the ENT family, has previously been shown to be a high-affinity transporter for purine nucleobases and guanosine. In this study, we use the Saccharomyces cerevisiae expression system to build a biochemical model of how TbNBT1 recognises nucleobases. We next performed random in vitro and site-directed mutagenesis to identify residues critical for TbNBT1 function. The identification of residues likely to contribute to permeant binding, when combined with a structural model of TbNBT1 obtained by homology threading, yield a tentative three-dimensional model of the transporter binding site that is consistent with the binding model emerging from the biochemical data. The model strongly suggests the involvement of TMS5, TMS7 and TMS8 in TbNBT1 function. This situation is very similar to that concerning transporters of the major facilitator superfamily (MFS), one of which was used as a template for the threading. This point raises the possibility that ENT and MFS carriers, despite being considered evolutionarily distinct, might in fact share similar topologies and substrate translocations pathways.

Fungal nucleobase transporters

Early genetic and physiological work in bacteria and fungi has suggested the presence of highly specific nucleobase transport systems. Similar transport systems are now known to exist in algae, plants, protozoa and metazoa. Within the last 15 years, a small number of microbial genes encoding nucleobase transporters have been cloned and studied in great detail. The sequences of several other putative proteins submitted to databases are homologous to the microbial nucleobase transporters but their physiological functions remain largely undetermined. In this review, genetic, biochemical and molecular data are described concerning mostly the nucleobase transporters of Aspergillus nidulans and Saccharomyces cerevisiae, the two model ascomycetes from which the great majority of data come from. It is also discussed as to what is known on the nucleobase transporters of the two most significant pathogenic fungi: Candida albicans and Aspergillus fumigatus. Apart from highlighting how a basic process such as nucleobase recognition and transport operates, this review intends to highlight features that might be applicable to antifungal pharmacology.

Cysteine-scanning Analysis of the Nucleobase-Ascorbate Transporter Signature Motif in YgfO Permease of Escherichia coli

The nucleobase-ascorbate transporter (NAT) signature motif is a conserved sequence motif of the ubiquitous NAT/NCS2 family implicated in defining the function and selectivity of purine translocation pathway in the major fungal homolog UapA. To analyze the role of NAT motif more systematically, we employed Cys-scanning mutagenesis of the Escherichia coli xanthine-specific homolog YgfO. Using a functional mutant devoid of Cys residues (C-less), each amino acid residue in sequence (315)GSIPITTFAQNNGVIQMTGVASRYVG(340) (motif underlined) was replaced individually with Cys. Of the 26 single-Cys mutants, 16 accumulate xanthine to > or =50% of the steady state observed with C-less YgfO, 4 accumulate to low levels (10-25% of C-less), F322C, N325C, and N326C accumulate marginally (5-8% of C-less), and P318C, Q324C, and G340C are inactive. When transferred to wild type, F322C(wt) and N326C(wt) are highly active, but P318G(wt), Q324C(wt), N325C(wt), and G340C(wt) are inactive, and G340A(wt) displays low activity. Immunoblot analysis shows that replacements at Pro-318 or Gly-340 are associated with low or negligible expression in the membrane. More extensive mutagenesis reveals that Gln-324 is critical for high affinity uptake and ligand recognition, and Asn-325 is irreplaceable for active xanthine transport, whereas Thr-332 and Gly-333 are important determinants of ligand specificity. All single-Cys mutants react with N-ethylmaleimide, but regarding sensitivity to inactivation, they fall to three regions; positions 315-322 are insensitive to N-ethylmaleimide, with IC(50) values > or =0.4 mM, positions 323-329 are highly sensitive, with IC(50) values of 15-80 microM, and sensitivity of positions 330-340 follows a periodicity, with mutants sensitive to inactivation clustering on one face of an alpha-helix.

Differential physiological and developmental expression of the UapA and AzgA purine transporters in Aspergillus nidulans

In this article we study the cellular expression of UapA and AzgA, the two major purine transporters of Aspergillus nidulans, by constructing strains expressing, from their native promoters, fully functional fluorescent (UapA-sGFP, AzgA-sGFP) or immunological (UapA-His) chimeric transporters. Epifluorescence microscopy and immunodetection showed that under different physiological conditions and during Aspergillus development: (i) UapA and AzgA expression in the plasma membrane becomes evident early during germination and remains at a significant basal level in mycelium, (ii) Neither of the two transporters is expressed in the stalk, the vesicle, the phialides and the conidiospores, but surprisingly, UapA is specifically and strongly expressed in the periphery of metulae, (iii) Both transporters are expressed in ascogenous hyphae and in hülle cells but not in cleistothecia or ascospores, (iv) Purine induction leads to approximately 4-fold increase in UapA and AzgA protein content in mycelium, compatible with an analogous increase at the transcriptional level, (v) Ammonium leads to removal of UapA, but not of AzgA, from the plasma membrane by sorting of the protein to the vacuole.

The first transmembrane segment (TMS1) of UapA contains determinants necessary for expression in the plasma membrane and purine transport

UapA, a member of the NAT/NCS2 family, is a high affinity, high capacity, uric acid-xanthine/H+ symporter in Aspergillus nidulans. Determinants critical for substrate binding and transport lie in a highly conserved signature motif downstream from TMS8 and within TMS12. Here we examine the role of TMS1 in UapA biogenesis and function. First, using a mutational analysis, we studied the role of a short motif (Q85H86), conserved in all NATs. Q85 mutants were cryosensitive, decreasing (Q85L, Q85N, Q85E) or abolishing (Q85T) the capacity for purine transport, without affecting physiological substrate binding or expression in the plasma membrane. All H86 mutants showed nearly normal substrate binding affinities but most (H86A, H86K, H86D) were cryosensitive, a phenotype associated with partial ER retention and/or targeting of UapA in small vacuoles. Only mutant H86N showed nearly wild-type function, suggesting that His or Asn residues might act as H donors in interactions affecting UapA topology. Thus, residues Q85 and H86 seem to affect the flexibility of UapA, in a way that affects either transport catalysis per se (Q85), or expression in the plasma membrane (H86). We then examined the role of a transmembrane Leu Repeat (LR) motif present in TMS1 of UapA, but not in other NATs. Mutations replacing Leu with Ala residues altered differentially the binding affinities of xanthine and uric acid, in a temperature-sensitive manner. This result strongly suggested that the presence of L77, L84 and L91 affects the flexibility of UapA substrate binding site, in a way that is necessary for high affinity uric acid transport. A possible role of the LR motif in intramolecular interactions or in UapA dimerization is discussed.