Protein transport along the presequence pathway.

Most mitochondrial proteins are nuclear-encoded and imported by the protein import machinery based on specific targeting signals. The proteins that carry an amino-terminal targeting signal (presequence) are imported via the presequence import pathway that involves the translocases of the outer and inner membranes - TOM and TIM23 complexes. In this article, we discuss how mitochondrial matrix and inner membrane precursor proteins are imported along the presequence pathway in Saccharomyces cerevisiae with a focus on the dynamics of the TIM23 complex, and further update with some of the key findings that advanced the field in the last few years.

The hydrogenosome of Trichomonas vaginalis.

This review is dedicated to the 50th anniversary of the discovery of hydrogenosomes by Miklós Müller and Donald Lindmark, which we will celebrate the following year. It was a long journey from the first observation of enigmatic rows of granules in trichomonads at the end of the 19th century to their first biochemical characterization in 1973. The key experiments by Müller and Lindmark revealed that the isolated granules contain hydrogen-producing hydrogenase, similar to some anaerobic bacteria-a discovery that gave birth to the field of hydrogenosomes. It is also important to acknowledge the parallel work of the team of Apolena Cerkasovová, Jirí Cerkasov, and Jaroslav Kulda, who demonstrated that these granules, similar to mitochondria, produce ATP. However, the evolutionary origin of hydrogenosomes remained enigmatic until the turn of the millennium, when it was finally accepted that hydrogenosomes and mitochondria evolved from a common ancestor. After a historical introduction, the review provides an overview of hydrogenosome biogenesis, hydrogenosomal protein import, and the relationship between the peculiar structure of membrane translocases and its low inner membrane potential due to the lack of respiratory complexes. Next, it summarizes the current state of knowledge on energy metabolism, the oxygen defense system, and iron/sulfur cluster assembly.

Anaerobic peroxisomes in Entamoeba histolytica metabolize myo-inositol.

Entamoeba histolytica is believed to be devoid of peroxisomes, like most anaerobic protists. In this work, we provided the first evidence that peroxisomes are present in E. histolytica, although only seven proteins responsible for peroxisome biogenesis (peroxins) were identified (Pex1, Pex6, Pex5, Pex11, Pex14, Pex16, and Pex19). Targeting matrix proteins to peroxisomes is reduced to the PTS1-dependent pathway mediated via the soluble Pex5 receptor, while the PTS2 receptor Pex7 is absent. Immunofluorescence microscopy showed that peroxisomal markers (Pex5, Pex14, Pex16, Pex19) are present in vesicles distinct from mitosomes, the endoplasmic reticulum, and the endosome/phagosome system, except Pex11, which has dual localization in peroxisomes and mitosomes. Immunoelectron microscopy revealed that Pex14 localized to vesicles of approximately 90-100 nm in diameter. Proteomic analyses of affinity-purified peroxisomes and in silico PTS1 predictions provided datasets of 655 and 56 peroxisomal candidates, respectively; however, only six proteins were shared by both datasets, including myo-inositol dehydrogenase (myo-IDH). Peroxisomal NAD-dependent myo-IDH appeared to be a dimeric enzyme with high affinity to myo-inositol (Km 0.044 mM) and can utilize also scyllo-inositol, D-glucose and D-xylose as substrates. Phylogenetic analyses revealed that orthologs of myo-IDH with PTS1 are present in E. dispar, E. nutalli and E. moshkovskii but not in E. invadens, and form a monophyletic clade of mostly peroxisomal orthologs with free-living Mastigamoeba balamuthi and Pelomyxa schiedti. The presence of peroxisomes in E. histolytica and other archamoebae breaks the paradigm of peroxisome absence in anaerobes and provides a new potential target for the development of antiparasitic drugs.

Determinism and contingencies shaped the evolution of mitochondrial protein import.

Mitochondrial protein import requires outer membrane receptors that evolved independently in different lineages. Here we used quantitative proteomics and in vitro binding assays to investigate the substrate preferences of ATOM46 and ATOM69, the two mitochondrial import receptors of Trypanosoma brucei The results show that ATOM46 prefers presequence-containing, hydrophilic proteins that lack transmembrane domains (TMDs), whereas ATOM69 prefers presequence-lacking, hydrophobic substrates that have TMDs. Thus, the ATOM46/yeast Tom20 and the ATOM69/yeast Tom70 pairs have similar substrate preferences. However, ATOM46 mainly uses electrostatic, and Tom20 hydrophobic, interactions for substrate binding. In vivo replacement of T. brucei ATOM46 by yeast Tom20 did not restore import. However, replacement of ATOM69 by the recently discovered Tom36 receptor of Trichomonas hydrogenosomes, while not allowing for growth, restored import of a large subset of trypanosomal proteins that lack TMDs. Thus, even though ATOM69 and Tom36 share the same domain structure and topology, they have different substrate preferences. The study establishes complementation experiments, combined with quantitative proteomics, as a highly versatile and sensitive method to compare in vivo preferences of protein import receptors. Moreover, it illustrates the role determinism and contingencies played in the evolution of mitochondrial protein import receptors.

Triplet-pore structure of a highly divergent TOM complex of hydrogenosomes in Trichomonas vaginalis.

Mitochondria originated from proteobacterial endosymbionts, and their transition to organelles was tightly linked to establishment of the protein import pathways. The initial import of most proteins is mediated by the translocase of the outer membrane (TOM). Although TOM is common to all forms of mitochondria, an unexpected diversity of subunits between eukaryotic lineages has been predicted. However, experimental knowledge is limited to a few organisms, and so far, it remains unsettled whether the triplet-pore or the twin-pore structure is the generic form of TOM complex. Here, we analysed the TOM complex in hydrogenosomes, a metabolically specialised anaerobic form of mitochondria found in the excavate Trichomonas vaginalis. We demonstrate that the highly divergent ß-barrel T. vaginalis TOM (TvTom)40-2 forms a translocation channel to conduct hydrogenosomal protein import. TvTom40-2 is present in high molecular weight complexes, and their analysis revealed the presence of four tail-anchored (TA) proteins. Two of them, Tom36 and Tom46, with heat shock protein (Hsp)20 and tetratricopeptide repeat (TPR) domains, can bind hydrogenosomal preproteins and most likely function as receptors. A third subunit, Tom22-like protein, has a short cis domain and a conserved Tom22 transmembrane segment but lacks a trans domain. The fourth protein, hydrogenosomal outer membrane protein 19 (Homp19) has no known homology. Furthermore, our data indicate that TvTOM is associated with sorting and assembly machinery (Sam)50 that is involved in ß-barrel assembly. Visualisation of TvTOM by electron microscopy revealed that it forms three pores and has an unconventional skull-like shape. Although TvTOM seems to lack Tom7, our phylogenetic profiling predicted Tom7 in free-living excavates. Collectively, our results suggest that the triplet-pore TOM complex, composed of three conserved subunits, was present in the last common eukaryotic ancestor (LECA), while receptors responsible for substrate binding evolved independently in different eukaryotic lineages.

Targeting of tail-anchored proteins to Trichomonas vaginalis hydrogenosomes.

Tail-anchored (TA) proteins are membrane proteins that are found in all domains of life. They consist of an N-terminal domain that performs various functions and a single transmembrane domain (TMD) near the C-terminus. In eukaryotes, TA proteins are targeted to the membranes of mitochondria, the endoplasmic reticulum (ER), peroxisomes and in plants, chloroplasts. The targeting of these proteins to their specific destinations correlates with the properties of the C-terminal domain, mainly the TMD hydrophobicity and the net charge of the flanking regions. Trichomonas vaginalis is a human parasite that has adapted to oxygen-poor environment. This adaptation is reflected by the presence of highly modified mitochondria (hydrogenosomes) and the absence of peroxisomes. The proteome of hydrogenosomes is considerably reduced; however, our bioinformatic analysis predicted 120 putative hydrogenosomal TA proteins. Seven proteins were selected to prove their localization. The elimination of the net positive charge in the C-tail of the hydrogenosomal TA4 protein resulted in its dual localization to hydrogenosomes and the ER, causing changes in ER morphology. Domain mutation and swap experiments with hydrogenosomal (TA4) and ER (TAPDI) proteins indicated that the general principles for specific targeting are conserved across eukaryotic lineages, including T. vaginalis; however, there are also significant lineage-specific differences.

N-Terminal Presequence-Independent Import of Phosphofructokinase into Hydrogenosomes of Trichomonas vaginalis.

Mitochondrial evolution entailed the origin of protein import machinery that allows nuclear-encoded proteins to be targeted to the organelle, as well as the origin of cleavable N-terminal targeting sequences (NTS) that allow efficient sorting and import of matrix proteins. In hydrogenosomes and mitosomes, reduced forms of mitochondria with reduced proteomes, NTS-independent targeting of matrix proteins is known. Here, we studied the cellular localization of two glycolytic enzymes in the anaerobic pathogen Trichomonas vaginalis: PPi-dependent phosphofructokinase (TvPPi-PFK), which is the main glycolytic PFK activity of the protist, and ATP-dependent PFK (TvATP-PFK), the function of which is less clear. TvPPi-PFK was detected predominantly in the cytosol, as expected, while all four TvATP-PFK paralogues were imported into T. vaginalis hydrogenosomes, although none of them possesses an NTS. The heterologous expression of TvATP-PFK in Saccharomyces cerevisiae revealed an intrinsic capability of the protein to be recognized and imported into yeast mitochondria, whereas yeast ATP-PFK resides in the cytosol. TvATP-PFK consists of only a catalytic domain, similarly to "short" bacterial enzymes, while ScATP-PFK includes an N-terminal extension, a catalytic domain, and a C-terminal regulatory domain. Expression of the catalytic domain of ScATP-PFK and short Escherichia coli ATP-PFK in T. vaginalis resulted in their partial delivery to hydrogenosomes. These results indicate that TvATP-PFK and the homologous ATP-PFKs possess internal structural targeting information that is recognized by the hydrogenosomal import machinery. From an evolutionary perspective, the predisposition of ancient ATP-PFK to be recognized and imported into hydrogenosomes might be a relict from the early phases of organelle evolution.

Phenotypic and genotypic characterization of coagulase negative staphylococci (CoNS) other than Staphylococcus epidermidis isolated from ocular infections.

PURPOSE: The clinical significance of many species belonging to coagulase-negative staphylococci (CoNS) other than Staphylococcus epidermidis has been increasingly recognized. The present study attempted the characterization of non-S. epidermidis CoNS isolates of ocular origin. METHODS: The isolates were characterized phenotypically by analytical profile index (API) and genotypically by fluorescent amplified fragment length polymorphism (FAFLP). In addition, the presence of genes that are likely to enhance virulence such as biofilm related genes (icaA and icaB) and methicillin resistance (mecA) were detected. RESULTS: API identified seven different species with an identification score varying from 44% to 99%. S. haemolyticus and S. xylosus species identified by API showed good API scores when compared with all other species identified. FAFLP generated 12 clusters from all the isolates and appeared to be more discriminatory. API identification results corresponded well with the FAFLP results only in six clusters. Nearly 40% of isolates showed the presence of mecA and icaAB genes. CONCLUSIONS: Identification results of these two methods corresponded only in 48.4% of the isolates suggesting that the battery of tests using API were not sufficient enough to identify all the species of CoNS. Therefore, it is sensible to rely on two or more identification methods particularly when API species identification has a lower score. FAFLP genotyping appears to be a reliable and rapid alternative identification method for CoNS that can be used in conjunction with any other phenotypic method.

Genotypic Characterization of Staphylococcus aureus Obtained from Humans and Bovine Mastitis Samples in India.

AIM AND BACKGROUND: Staphylococcus aureus is a major human pathogen that also causes important infections in cattle and sheep. The present study aimed to test genetic diversity among strains of S. aureus isolated from cattle (n=34) and humans (n=22) by DNA typing. MATERIALS AND METHODS: Fluorescent amplified fragment length polymorphism (FAFLP) is the genotyping tool used in the study. The presence of the mecA and Panton-Valentine leukocidin (PVL) genes among these strain groups was also checked. RESULTS: A dendrogram deduced from FAFLP showed that all the strains clustered into 10 groups (A-J) with a relative genetic divergence of less than 8%. Sixty-seven percent of the isolates from bovine sources clustered together in two clades (A and H), while another major cluster with 13 isolates (59%) (Cluster G) had all strains from a human host. The remaining strains from both the hosts clustered independently into smaller clusters with the exception of two strains of human origin, which clustered along with a bovine cluster. Thirteen strains belonging to cluster G were highly clonal. About 77% of strains obtained from human infections were methicillin-resistant S. aureus (MRSA), whereas only 29% of strains from bovine origin were MRSA. Only three strains from human origin showed PVL positive, while no strain from cattle had PVL genes. The complete absence of PVL genes in all the bovine strains in the study appears to be significant. CONCLUSIONS: FAFLP can be successfully applied to assess the genetic relationship of S. aureus isolates from different hosts. The study also provided the valuable epidemiological data on S. aureus from bovine sources in India, which is lacking.