The proteolytic systems of lactic acid bacteria

Proteolysis in dairy lactic acid bacteria has been studied in great detail by genetic, biochemical and ultrastructural methods. From these studies the picture emerges that the proteolytic systems of lactococci and lactobacilli are remarkably similar in their components and mode of action. The proteolytic system consists of an extracellularly located serine-proteinase, transport systems specific for di-tripeptides and oligopeptides (> 3 residues), and a multitude of intracellular peptidases. This review describes the properties and regulation of individual components as well as studies that have led to identification of their cellular localization. Targeted mutational techniques developed in recent years have made it possible to investigate the role of individual and combinations of enzymes in vivo. Based on these results as well as in vitro studies of the enzymes and transporters, a model for the proteolytic pathway is proposed. The main features are: (i) proteinases have a broad specificity and are capable of releasing a large number of different oligopeptides, of which a large fraction falls in the range of 4 to 8 amino acid residues; (ii) oligopeptide transport is the main route for nitrogen entry into the cell; (iii) all peptidases are located intracellularly and concerted action of peptidases is required for complete degradation of accumulated peptides.

Genetic and biochemical characterization of the oligopeptide transport system of Lactococcus lactis

The nucleotide sequence of a chromosomal DNA fragment of Lactococcus lactis subsp. lactis SSL135, previously implicated in peptide utilization, has been determined. The genes oppDFBCA, encoding the oligopeptide transport system (Opp), and that encoding the endopeptidase PepO were located on this 8.9-kb DNA fragment. The oppDFBCA and pepO genes are probably organized in an operon. Analysis of the deduced amino acid sequences of the genes indicated that the oligopeptide transport system consists of two ATP-binding proteins OppD and OppF, two integral membrane proteins OppB and OppC, and a substrate-binding protein OppA. On the basis of the homology of OppF and OppD of L. lactis with other ABC (ATP-binding cassette) transporter proteins, the L. lactis Opp system can be classified as a member of this group. Two integration mutants, one defective in OppA and the other defective in PepO, were constructed. Growth of these mutants in a chemically defined medium with oligopeptides showed that the transport system, but not the endopeptidase, is essential for the utilization of peptides longer than three residues. Uptake of the pentapeptide Leu-enkephalin in glycolyzing lactococcal cells was followed by rapid hydrolysis of the peptide intracellularly. Importantly, extracellular hydrolysis of Leu-enkephalin is not observed. The OppA-deficient mutant was unable to transport Leu-enkephalin. Growth experiments with pasteurized milk revealed that transport of oligopeptides forms an essential part of the proteolytic system in lactococci.

Oligopeptides are the main source of nitrogen for Lactococcus lactis during growth in milk

The consumption of amino acids and peptides was monitored during growth in milk of proteinase-positive (Prt+) and -negative (Prt-) strains of Lactococcus lactis. The Prt- strains showed monophasic exponential growth, while the Prt+ strains grew in two phases. The first growth phases of the Prt+ and Prt- strains were in same, and no hydrolysis of casein was observed. Also, the levels of consumption of amino acids and peptides in the Prt+ and Prt- strains were similar. At the end of this growth phase, not all free amino acids and peptides were used, indicating that the remaining free amino acids and peptides were unable to sustain growth. The consumption of free amino acids was very low (about 5 mg/liter), suggesting that these nitrogen sources play only a minor role in growth. Oligopeptide transport-deficient strains (Opp-) of L. lactis were unable to utilize oligopeptides and grew poorly in milk. However, a di- and tripeptide transport-deficient strain (DtpT-) grew exactly like the wild type (Opp+ Dtpt+) did. These observations indicate that oligopeptides represent the main nitrogen source for growth in milk during the first growth phase. In the second phase of growth of Prt+ strains, milk proteins are hydrolyzed to peptides by the proteinase. Several of the oligopeptides formed are taken up and hydrolyzed internally by peptidases to amino acids, several of which are subsequently released into the medium (see also E.R.S. Kunji, A. Hagting, C.J. De Vries, V. Juillard, A.J. Haandrikman, B. Poolman, and W.N. Konings, J. Biol. Chem. 270:1569-1574, 1995).(ABSTRACT TRUNCATED AT 250 WORDS)

A scalable, GFP-based pipeline for membrane protein overexpression screening and purification

We describe a generic, GFP-based pipeline for membrane protein overexpression and purification in Escherichia coli. We exemplify the use of the pipeline by the identification and characterization of E. coli YedZ, a new, membrane-integral flavocytochrome. The approach is scalable and suitable for high-throughput applications. The GFP-based pipeline will facilitate the characterization of the E. coli membrane proteome and serves as an important reference for the characterization of other membrane proteomes.

The extracellular PI-type proteinase of Lactococcus lactis hydrolyzes beta-casein into more than one hundred different oligopeptides

The peptides released from beta-casein by the action of PI-type proteinase (PrtP) from Lactococcus lactis subsp. cremoris Wg2 have been identified by on-line coupling of liquid chromatography to mass spectrometry. After 24 h of incubation of beta-casein with purified PrtP, a stable mixture of peptides was obtained. The trifluoroacetic acid-soluble peptides of this beta-casein hydrolysate were fractionated by high-performance liquid chromatography and introduced into the liquid chromatography-ion spray mass spectrometry interface. Multiply charged ions were generated from trifluoroacetic acid-soluble peptides under low nozzle voltage conditions, yielding the MH+ mass of each eluted peptide. All peptides corresponding to each of the MH+ calculated masses were determined. In those cases in which different peptides were possible, further identification was achieved by collision-induced dissociation under higher nozzle voltage conditions. Hydrolysis of beta-casein by PrtP was observed to proceed much further than reported previously. More than 40% of the peptide bonds are cleaved by PrtP, resulting in the formation of more than 100 different oligopeptides. With the exception of Phe, significant release of amino acids or di- and tripeptides could not be observed. Interestingly, one-fifth of the identified oligopeptides are small enough to be taken up by the oligopeptide transport system. Uptake of these peptides could supply L. lactis with all amino acids, including the essential ones, indicating that growth of L. lactis might be possible on peptides released from beta-casein by proteinase only.

The insulin-like growth factor-I-mTOR signaling pathway induces the mitochondrial pyrimidine nucleotide carrier to promote cell growth

The insulin/insulin-like growth factor (IGF) signaling pathway to mTOR is essential for the survival and growth of normal cells and also contributes to the genesis and progression of cancer. This signaling pathway is linked with regulation of mitochondrial function, but how is incompletely understood. Here we show that IGF-I and insulin induce rapid transcription of the mitochondrial pyrimidine nucleotide carrier PNC1, which shares significant identity with the essential yeast mitochondrial carrier Rim2p. PNC1 expression is dependent on PI-3 kinase and mTOR activity and is higher in transformed fibroblasts, cancer cell lines, and primary prostate cancers than in normal tissues. Overexpression of PNC1 enhances cell size, whereas suppression of PNC1 expression causes reduced cell size and retarded cell cycle progression and proliferation. Cells with reduced PNC1 expression have reduced mitochondrial UTP levels, but while mitochondrial membrane potential and cellular ATP are not altered, cellular ROS levels are increased. Overall the data indicate that PNC1 is a target of the IGF-I/mTOR pathway that is essential for mitochondrial activity in regulating cell growth and proliferation.

The projection structure of EmrE, a proton-linked multidrug transporter from Escherichia coli, at 7 A resolution

EmrE belongs to a family of eubacterial multidrug transporters that confer resistance to a wide variety of toxins by coupling the influx of protons to toxin extrusion. EmrE was purified and crystallized in two dimensions by reconstitution with dimyristoylphosphatidylcholine into lipid bilayers. Images of frozen hydrated crystals were collected by cryo-electron microscopy and a projection structure of EmrE was calculated to 7 A resolution. The projection map shows an asymmetric EmrE dimer with overall dimensions approximately 31 x 40 A, comprising an arc of highly tilted helices separating two helices nearly perpendicular to the membrane from another two helices, one tilted and the other nearly perpendicular. There is no obvious 2-fold symmetry axis perpendicular to the membrane within the dimer, suggesting that the monomers may have different structures in the functional unit.

Conserved properties of hydrogenosomal and mitochondrial ADP/ATP carriers: a common origin for both organelles

Mitochondria are one of the hallmarks of eukaryotic cells, exporting ATP in exchange for cytosolic ADP using ADP/ATP carriers (AAC) located in the inner mitochondrial membrane. In contrast, several evolutionarily important anaerobic eukaryotes lack mitochondria but contain hydrogenosomes, peculiar organelles of controversial ancestry that also supply ATP but, like some fermentative bacteria, make molecular hydrogen in the process. We have now identified genes from two species of the hydrogenosome-containing fungus Neocallimastix that have three-fold sequence repeats and signature motifs that, along with phylogenetic analysis, identify them as AACs. When expressed in a mitochondrial AAC- deficient yeast strain, the hydrogenosomal protein was correctly targeted to the yeast mitochondria inner membrane and yielded mitochondria able to perform ADP/ATP exchange. Characteristic inhibitors of mitochondrial AACs blocked adenine nucleotide exchange by the Neocallimastix protein. Thus, our data demonstrate that fungal hydrogenosomes and yeast mitochondria use the same pathway for ADP/ATP exchange. These experiments provide some of the strongest evidence yet that yeast mitochondria and Neocallimastix hydrogenosomes are but two manifestations of the same fundamental organelle.

Multiple-peptidase mutants of Lactococcus lactis are severely impaired in their ability to grow in milk

To examine the contribution of peptidases to the growth of lactococcus lactis in milk, 16 single- and multiple-deletion mutants were constructed. In successive rounds of chromosomal gene replacement mutagenesis, up to all five of the following peptidase genes were inactivated (fivefold mutant): pepX, pepO, pepT, pepC, and pepN. Multiple mutations led to slower growth rates in milk, the general trend being that growth rates decreased when more peptidases were inactivated. The fivefold mutant grew more than 10 times more slowly in milk than the wild-type strain. In one of the fourfold mutants and in the fivefold mutant, the intracellular pools of amino acids were lower than those of the wild type, whereas peptides had accumulated inside the cell. No significant differences in the activities of the cell envelope-associated proteinase and of the oligopeptide transport system were observed. Also, the expression of the peptidases still present in the various mutants was not detectably affected. Thus, the lower growth rates can directly be attributed to the inability of the mutants to degrade casein-derived peptides. These results supply the first direct evidence for the functioning of lactococcal peptidases in the degradation of milk proteins. Furthermore, the study provides critical information about the relative importance of the peptidases for growth in milk, the order of events in the proteolytic pathway, and the regulation of its individual components.

Transport of beta-casein-derived peptides by the oligopeptide transport system is a crucial step in the proteolytic pathway of Lactococcus lactis

In the proteolytic pathway of Lactococcus lactis, milk proteins (caseins) are hydrolyzed extracellularly to oligopeptides by the proteinase (PrtP). The fate of these peptides, i.e. extracellular hydrolysis followed by amino acid uptake or transport followed by intracellular hydrolysis, has been addressed. Mutants have been constructed that lack a functional di-tripeptide transport system (DtpT) and/or oligopeptide transport system (Opp) but do express the P1-type proteinase (specific for hydrolysis of beta- and to a lesser extent kappa-casein). The wild type strain and the DtpT- mutant accumulate all beta-casein-derived amino acids in the presence of beta-casein as protein substrate and glucose as a source of metabolic energy. The amino acids are not accumulated significantly inside the cells by the Opp- and DtpT- Opp- mutants. When cells are incubated with a mixture of amino acids mimicking the composition of beta-casein, the amino acids are taken up to the same extent in all four strains. Analysis of the extracellular peptide fraction, formed by the action of PrtP on beta-casein, indicates that distinct peptides disappear only when the cells express an active Opp system. These and other experiments indicate that (i) oligopeptide transport is essential for the accumulation of all beta-casein-derived amino acids, (ii) the activity of the Opp system is sufficiently high to support high growth rates on beta-casein provided leucine and histidine are present as free amino acids, and (iii) extracellular peptidase activity is not present in L. lactis.

Kinetics and specificity of peptide uptake by the oligopeptide transport system of Lactococcus lactis

To obtain amino acids for growth, Lactococcus lactis uses a proteolytic system to degrade exogenous proteins such as caseins. The extracellular cell wall-attached proteinase PrtP and the oligopeptide transport system Opp mediate the first two steps in the utilization of caseins. beta-Casein is degraded by PrtP to fragments of 5-30 amino acid residues, and only a limited number of peptides are selected from this pool for uptake via Opp. To study the specificity of Opp and the kinetics of peptide uptake in L. lactis in detail, we used the following strategy: (i) the Opp system was overexpressed; (ii) a 4-fold peptidase mutant was used that is unable to degrade KYGK; (iii) iodinated KYGK was used as the reporter peptide; (iv) libraries of peptides, in which one amino acid position is systematically varied, were used as competitive peptides; and (v) peptides were synthesized on the basis of the beta-casein degradation products, their inhibition of KYGK uptake was determined, and the uptake of these peptides was followed by high-performance liquid chromatography (HPLC). These studies indicate that (i) the Opp system can transport a broad range of peptides from 4 up to at least 18 residues with very little preference for particular side chains and (ii) the kinetics of peptide uptake differ for different substrates tested. Whereas class I peptides such as KYGK exhibit normal Michaelis-Menten kinetics, the level of uptake of the majority of peptides (class II) increases sigmoidally with concentration. Different models for explaining the apparent cooperative effects that are observed for peptide uptake are discussed.

Di-tripeptides and oligopeptides are taken up via distinct transport mechanisms in Lactococcus lactis

Lactococcus lactis ML3 possesses two different peptide transport systems of which the substrate size restriction and specificity have been determined. The first system is the earlier-described proton motive force-dependent di-tripeptide carrier (E. J. Smid, A. J. M. Driessen, and W. N. Konings, J. Bacteriol. 171:292-298, 1989). The second system is a metabolic energy-dependent oligopeptide transport system which transports peptides of four to at least six amino acid residues. The involvement of a specific oligopeptide transport system in the utilization of tetra-alanine and penta-alanine was established in a mutant of L. lactis MG1363 that was selected on the basis of resistance to toxic analogs of alanine and alanine-containing di- and tripeptides. This mutant is unable to transport alanine, dialanine, and trialanine but still shows uptake of tetra-alanine and penta-alanine. The oligopeptide transport system has a lower activity than the di-tripeptide transport system. Uptake of oligopeptides occurs in the absence of a proton motive force and is specifically inhibited by vanadate. The oligopeptide transport system is most likely driven by ATP or a related energy-rich, phosphorylated intermediate.

Specificity of peptide transport systems in Lactococcus lactis: evidence for a third system which transports hydrophobic di- and tripeptides

A proton motive force-driven di-tripeptide carrier protein (DtpT) and an ATP-dependent oligopeptide transport system (Opp) have been described for Lactococcus lactis MG1363. Using genetically well-defined mutants in which dtpT and/or opp were inactivated, we have now established the presence of a third peptide transport system (DtpP) in L. lactis. The specificity of DtpP partially overlaps that of DtpT. DtpP transports preferentially di- and tripeptides that are composed of hydrophobic (branched-chain amino acid) residues, whereas DtpT has a higher specificity for more-hydrophilic and charged peptides. The toxic dipeptide L-phenylalanyl-beta-chloro-L-alanine has been used to select for a di-tripeptide transport-negative mutant with the delta dtpT strain as a genetic background. This mutant is unable to transport di- and tripeptides but still shows uptake of amino acids and oligopeptides. The DtpP system is induced in the presence of di- and tripeptides containing branched-chain amino acids. The use of ionophores and metabolic inhibitors suggests that, similar to Opp, DtpP-mediated peptide transport is driven by ATP or a related energy-rich phosphorylated intermediate.

Reconstruction of the proteolytic pathway for use of beta-casein by Lactococcus lactis

Amino acid auxotrophous bacteria such as Lactococcus lactis use proteins as a source of amino acids. For this process, they possess a complex proteolytic system to degrade the protein(s) and to transport the degradation products into the cell. We have been able to dissect the various steps of the pathway by deleting one or more genes encoding key enzymes/components of the system and using mass spectrometry to analyse the complex peptide mixtures. This approach revealed in detail how L. lactis liberates the required amino acids from beta-casein, the major component of the lactococcal diet. Mutants containing the extracellular proteinase PrtP, but lacking the oligopeptide transport system Opp and the autolysin AcmA, were used to determine the proteinase specificity in vivo. To identify the substrates of Opp present in the casein hydrolysate, the PrtP-generated peptide pool was offered to mutants lacking the proteinase, but containing Opp, and the disappearance of peptides from the medium as well as the intracellular accumulation of amino acids and peptides was monitored in peptidase-proficient and fivefold peptidase-deficient genetic backgrounds. The results are unambiguous and firmly establish that (i) the carboxyl-terminal end of beta-casein is degraded preferentially despite the broad specificity of the proteinase; (ii) peptides smaller than five residues are not formed in vivo; (iii) use of oligopeptides of 5-10 residues becomes only possible after uptake via Opp; (iv) only a few (10-14) of the peptides generated by PrtP are actually used, even though the system facilitates the transport of oligopeptides up to at least 10 residues. The technology described here allows us to monitor the fate of individual peptides in complex mixtures and is applicable to other proteolytic systems.

Bactericidal mode of action of plantaricin C

Plantaricin C is a bacteriocin produced by Lactobacillus plantarum LL441 that kills sensitive cells by acting on the cytoplasmic membrane. In contrast to its lack of impact on immune cells, plantaricin C dissipates the proton motive force and inhibits amino acid transport in sensitive cells. In proteoliposomes, plantaricin C dissipates the transmembrane electrical potential, and in liposomes, it elicits efflux of entrapped carboxy-fluorescein. It is concluded that plantaricin C is a pore-forming bacteriocin that functions in a voltage-independent manner and does not require a specific protein receptor in the target membrane.

The three-dimensional structure of halorhodopsin to 5 A by electron crystallography: A new unbending procedure for two-dimensional crystals by using a global reference structure

Electron microscopy does not, in principle, require highly ordered crystals to determine a high-resolution structure. Nevertheless, crystals of any type help to constrain the molecules into a more limited range of orientations and positions, from which it is easier to carry out structure determination. We describe an improved procedure for determination of crystalline disorder, which we have applied to poorly ordered two-dimensional crystals of the chloride pump halorhodopsin from Halobacterium salinarum. The new image analysis procedure involves the use of a reference projection calculated from a global three-dimensional map to carry out the initial cross-correlation analysis. Coupled with a greater number of images taken with field emission gun microscopes, this has allowed us to calculate a three-dimensional structure for halorhodopsin, in which the seven transmembrane helices and certain molecular features, such as the beta-ionone ring of retinal, are now resolved.

Specificity mutants of the binding protein of the oligopeptide transport system of Lactococcus lactis

The kinetic properties of wild-type and mutant oligopeptide binding proteins of Lactococcus lactis were determined. To observe the properties of the mutant proteins in vivo, the oppA gene was deleted from the chromosome of L. lactis to produce a strain that was totally defective in oligopeptide transport. Amplified expression of the oppA gene resulted in an 8- to 12-fold increase in OppA protein relative to the wild-type level. The amplified expression was paralleled by increased bradykinin binding activity, but had relatively little effect on the overall transport of bradykinin via Opp. Several site-directed mutants were constructed on the basis of a comparison of the primary sequences of OppA from Salmonella enterica serovar Typhimurium and L. lactis, taking into account the known structure of the serovar Typhimurium protein. Putative peptide binding-site residues were mutated. All the mutant OppA proteins exhibited a decreased binding affinity for the high-affinity peptide bradykinin. Except for OppA(D471R), the mutant OppA proteins displayed highly defective bradykinin uptake, whereas the transport of the low-affinity substrate KYGK was barely affected. Cells expressing OppA(D471R) had a similar K(m) for transport, whereas the V(max) was increased more than twofold as compared to the wild-type protein. The data are discussed in the light of a kinetic model and imply that the rate of transport is determined to a large extent by the donation of the peptide from the OppA protein to the translocator complex.

Fate of peptides in peptidase mutants of Lactococcus lactis

The utilization of exogenous peptides was studied in mutants of Lactococcus lactis in which combinations of the peptidase genes pepN, pepC, pepO, pepX and pepT were deleted. Multiple mutants lacking PepN, PepC, PepT plus PepX could not grow on peptides such as Leu-Gly-Gly, Gly-Phe-Leu, Leu-Gly-Pro, Ala-Pro-Leu and Gly-Leu-Gly-Leu, respectively, indicating that no other peptidases are present to release the essential amino acid Leu. In these mutants, peptides accumulate intracellularly, demonstrating that peptides are translocated as whole entities prior to degradation. The mutant lacking all five peptidases could still grow on Gly-Leu and Tyr-Gly-Gly-Phe-Leu, which confirmed the presence of a dipeptidase and led to the identification of an unknown PepO-like endopeptidase. These studies have also shown that the general aminopeptidases PepN, PepC and PepT have overlapping but not identical specificities and differ in their overall activity towards individual peptides. In contrast, PepX has an unique specificity, because it is the only enzyme which can efficiently degrade Ala-Pro-Leu. The concerted action of peptidases in the breakdown of particular peptides revealed how these substrates are utilized as sources of nitrogen.

Electron crystallographic analysis of two-dimensional crystals of sensory rhodopsin II: a 6.9 A projection structure

Sensory rhodopsins, phototaxis receptors in Haloarchaea, were purified and reconstituted into halobacterial lipids to form photoactive two-dimensional crystals. Images of vitreous ice-embedded, flattened, tubular crystals of sensory rhodopsin II (SRII) of Natronobacterium pharaonis were recorded using a field emission gun electron cryo-microscope. Fourier components for the SRII structure were determined either from the separated image transforms from single layers that formed each side of flattened tubes, or by a deconvolution procedure when two layers were stacked in register so that they generated a single crystal lattice by superposition. Most micrographs showed significant diffraction to 6.9 A after computer processing, and the results provide the first intermediate- resolution information obtained for an archaeal sensory rhodopsin. The projection structure of SRII indicates that the helix positions match the seven-helix arrangement of the archaeal transport rhodopsins rather than that of the eukaryotic visual pigments. The structural similarity of SRII to the transport rhodopsins supports models in which the transport and signalling mechanisms of archaeal rhodopsins derive from the same retinal-driven changes in protein conformation.

The mitochondrial ADP/ATP carrier exists and functions as a monomer

For more than 40 years, the oligomeric state of members of the mitochondrial carrier family (SLC25) has been the subject of debate. Initially, the consensus was that they were dimeric, based on the application of a large number of different techniques. However, the structures of the mitochondrial ADP/ATP carrier, a member of the family, clearly demonstrated that its structural fold is monomeric, lacking a conserved dimerisation interface. A re-evaluation of previously published data, with the advantage of hindsight, concluded that technical errors were at the basis of the earlier dimer claims. Here, we revisit this topic, as new claims for the existence of dimers of the bovine ADP/ATP carrier have emerged using native mass spectrometry of mitochondrial membrane vesicles. However, the measured mass does not agree with previously published values, and a large number of post-translational modifications are proposed to account for the difference. Contrarily, these modifications are not observed in electron density maps of the bovine carrier. If they were present, they would interfere with the structure and function of the carrier, including inhibitor and substrate binding. Furthermore, the reported mass does not account for three tightly bound cardiolipin molecules, which are consistently observed in other studies and are important stabilising factors for the transport mechanism. The monomeric carrier has all of the required properties for a functional transporter and undergoes large conformational changes that are incompatible with a stable dimerisation interface. Thus, our view that the native mitochondrial ADP/ATP carrier exists and functions as a monomer remains unaltered.

Cloning and functional expression in Escherichia coli of the gene encoding the di- and tripeptide transport protein of Lactobacillus helveticus

The gene encoding the di- and tripeptide transport protein (DtpT) of Lactobacillus helveticus (DtpTLH) was cloned with the aid of the inverse PCR technique and used to complement the dipeptide transport-deficient and proline-auxotrophic Escherichia coli E1772. Functional expression of the peptide transporter was shown by the uptake of prolyl-[14C] alanine in whole cells and membrane vesicles. Peptide transport via DtpT in membrane vesicles is driven by the proton motive force. The system has specificity for di- and tripeptides but not for amino acids or tetrapeptides. The dtpTLH gene consists of 1,491 bp, which translates into a 497-amino-acid polypeptide. DtpTLH shows 34% identity to the di- and tripeptide transport protein of Lactococcus lactis and is also homologous to various peptide transporters of eukaryotic origin, but the similarity between these proteins is confined mainly to the N-terminal halves.

Characterization of drug-induced human mitochondrial ADP/ATP carrier inhibition

An increasing number of commonly prescribed drugs are known to interfere with mitochondrial function, causing cellular toxicity, but the underlying mechanisms are largely unknown. Although often not considered, mitochondrial transport proteins form a significant class of potential mitochondrial off-targets. So far, most drug interactions have been reported for the mitochondrial ADP/ATP carrier (AAC), which exchanges cytosolic ADP for mitochondrial ATP. Here, we show inhibition of cellular respiratory capacity by only a subset of the 18 published AAC inhibitors, which questions whether all compound do indeed inhibit such a central metabolic process. This could be explained by the lack of a simple, direct model system to evaluate and compare drug-induced AAC inhibition. Methods: For its development, we have expressed and purified human AAC1 (hAAC1) and applied two approaches. In the first, thermostability shift assays were carried out to investigate the binding of these compounds to human AAC1. In the second, the effect of these compounds on transport was assessed in proteoliposomes with reconstituted human AAC1, enabling characterization of their inhibition kinetics. Results: Of the proposed inhibitors, chebulinic acid, CD-437 and suramin are the most potent with IC50-values in the low micromolar range, whereas another six are effective at a concentration of 100 μM. Remarkably, half of all previously published AAC inhibitors do not show significant inhibition in our assays, indicating that they are false positives. Finally, we show that inhibitor strength correlates with a negatively charged surface area of the inhibitor, matching the positively charged surface of the substrate binding site. Conclusion: Consequently, we have provided a straightforward model system to investigate AAC inhibition and have gained new insights into the chemical compound features important for inhibition. Better evaluation methods of drug-induced inhibition of mitochondrial transport proteins will contribute to the development of drugs with an enhanced safety profile.