The type 2 secretion system (T2SS) is present in some Gram-negative eubacteria and used to secrete proteins across the outer membrane. Here we report that certain representative heteroloboseans, jakobids, malawimonads and hemimastigotes unexpectedly possess homologues of core T2SS components. We show that at least some of them are present in mitochondria, and their behaviour in biochemical assays is consistent with the presence of a mitochondrial T2SS-derived system (miT2SS). We additionally identified 23 protein families co-occurring with miT2SS in eukaryotes. Seven of these proteins could be directly linked to the core miT2SS by functional data and/or sequence features, whereas others may represent different parts of a broader functional pathway, possibly also involving the peroxisome. Its distribution in eukaryotes and phylogenetic evidence together indicate that the miT2SS-centred pathway is an ancestral eukaryotic trait. Our findings thus have direct implications for the functional properties of the early mitochondrion.

• MeSH
• Models, Biological MeSH
• Eukaryota classification   genetics   metabolism   MeSH
• Phylogeny MeSH
• Gram-Negative Bacteria classification   genetics   metabolism   MeSH
• Conserved Sequence MeSH
• Mitochondrial Proteins classification   genetics   metabolism   MeSH
• Mitochondria genetics   metabolism   MeSH
• Evolution, Molecular * MeSH
• Models, Molecular MeSH
• Naegleria classification   genetics   metabolism   MeSH
• Peroxisomes metabolism   MeSH
• Protozoan Proteins classification   genetics   metabolism   MeSH
• Type II Secretion Systems classification   genetics   metabolism   MeSH
• Amino Acid Sequence MeSH
• Sequence Homology, Amino Acid MeSH
• Publication type
• Journal Article MeSH
• Research Support, Non-U.S. Gov't MeSH
• Names of Substances
• Mitochondrial Proteins MeSH
• Protozoan Proteins MeSH
• Type II Secretion Systems MeSH

The discovery that the protist Monocercomonoides exilis completely lacks mitochondria demonstrates that these organelles are not absolutely essential to eukaryotic cells. However, the degree to which the metabolism and cellular systems of this organism have adapted to the loss of mitochondria is unknown. Here, we report an extensive analysis of the M. exilis genome to address this question. Unexpectedly, we find that M. exilis genome structure and content is similar in complexity to other eukaryotes and less "reduced" than genomes of some other protists from the Metamonada group to which it belongs. Furthermore, the predicted cytoskeletal systems, the organization of endomembrane systems, and biosynthetic pathways also display canonical eukaryotic complexity. The only apparent preadaptation that permitted the loss of mitochondria was the acquisition of the SUF system for Fe-S cluster assembly and the loss of glycine cleavage system. Changes in other systems, including in amino acid metabolism and oxidative stress response, were coincident with the loss of mitochondria but are likely adaptations to the microaerophilic and endobiotic niche rather than the mitochondrial loss per se. Apart from the lack of mitochondria and peroxisomes, we show that M. exilis is a fully elaborated eukaryotic cell that is a promising model system in which eukaryotic cell biology can be investigated in the absence of mitochondria.

• Keywords
• Monocercomonoides, oxymonads, protist genomics, amitochondrial eukaryote, cell biology,
• MeSH
• Genome, Protozoan * MeSH
• Intracellular Membranes * MeSH
• Introns MeSH
• Actin Cytoskeleton MeSH
• Mitochondrial Dynamics MeSH
• Oxymonadida enzymology   genetics   ultrastructure   MeSH
• Proteome MeSH
• Publication type
• Journal Article MeSH
• Research Support, Non-U.S. Gov't MeSH
• Names of Substances
• Proteome MeSH

During mitotic prophase, chromosomes of the pathogenic unicellular eukaryote Giardia intestinalis condense in each of the cell's two nuclei. In this study, Giardia chromosomes were investigated using light microscopy, high-resolution field emission scanning electron microscopy, and in situ hybridization. For the first time, we describe the overall morphology, condensation stages, and mitotic segregation of these chromosomes. Despite the absence of several genes involved in the cohesion and condensation pathways in the Giardia genome, we observed chromatin organization similar to those found in eukaryotes, i.e., 10-nm nucleosomal fibrils, 30-nm fibrils coiled to chromomeres or in parallel arrangements, and closely aligned sister chromatids. DNA molecules of Giardia terminate with telomeric repeats that we visualized on each of the four chromatid endings of metaphase chromosomes. Giardia chromosomes lack primary and secondary constrictions, thus preventing their classification based on the position of the centromere. The anaphase poleward segregation of sister chromatids is atypical in orientation and tends to generate lagging chromatids between daughter nuclei. In the Giardia genome database, we identified two putative members of the kleisin family thought to be responsible for condensin ring establishment. Thus far, Giardia chromosomes (300 nm to 1.5 μm) are the smallest chromosomes that were analyzed at the ultrastructural level. This study complements the existing molecular and sequencing data on Giardia chromosomes with cytological and ultrastructural information.

• MeSH
• Adenosine Triphosphatases analysis   MeSH
• Cell Nucleus ultrastructure   MeSH
• Chromosomes physiology   ultrastructure   MeSH
• DNA-Binding Proteins analysis   MeSH
• Giardia lamblia genetics   ultrastructure   MeSH
• Mitosis MeSH
• Multiprotein Complexes analysis   MeSH
• Publication type
• Journal Article MeSH
• Research Support, Non-U.S. Gov't MeSH
• Names of Substances
• Adenosine Triphosphatases MeSH
• condensin complexes MeSH Browser
• DNA-Binding Proteins MeSH
• Multiprotein Complexes MeSH

The parabasalid protist Trichomonas vaginalis is a widespread parasite that affects humans, frequently causing vaginitis in infected women. Trichomonad mitosis is marked by the persistence of the nuclear membrane and the presence of an asymmetric extranuclear spindle with no obvious direct connection to the chromosomes. No centromeric markers have been described in T. vaginalis, which has prevented a detailed analysis of mitotic events in this organism. In other eukaryotes, nucleosomes of centromeric chromatin contain the histone H3 variant CenH3. The principal aim of this work was to identify a CenH3 homolog in T. vaginalis. We performed a screen of the T. vaginalis genome to retrieve sequences of canonical and variant H3 histones. Three variant histone H3 proteins were identified, and the subcellular localization of their epitope-tagged variants was determined. The localization of the variant TVAG_185390 could not be distinguished from that of the canonical H3 histone. The sequence of the variant TVAG_087830 closely resembled that of histone H3. The tagged protein colocalized with sites of active transcription, indicating that the variant TVAG_087830 represented H3.3 in T. vaginalis. The third H3 variant (TVAG_224460) was localized to 6 or 12 distinct spots at the periphery of the nucleus, corresponding to the number of chromosomes in G(1) phase and G(2) phase, respectively. We propose that this variant represents the centromeric marker CenH3 and thus can be employed as a tool to study mitosis in T. vaginalis. Furthermore, we suggest that the peripheral distribution of CenH3 within the nucleus results from the association of centromeres with the nuclear envelope throughout the cell cycle.

• MeSH
• Transcriptional Activation MeSH
• Cell Nucleus genetics   metabolism   MeSH
• Centromere genetics   metabolism   MeSH
• Chromosomes genetics   metabolism   MeSH
• Microscopy, Fluorescence MeSH
• G1 Phase MeSH
• G2 Phase MeSH
• Genome, Protozoan * MeSH
• Histones genetics   metabolism   MeSH
• Nuclear Envelope metabolism   MeSH
• Mitosis MeSH
• Molecular Sequence Data MeSH
• Nucleosomes metabolism   MeSH
• Protozoan Proteins genetics   metabolism   MeSH
• Amino Acid Sequence MeSH
• Sequence Analysis, Protein MeSH
• Transformation, Genetic MeSH
• Trichomonas vaginalis genetics   metabolism   MeSH
• Publication type
• Journal Article MeSH
• Research Support, Non-U.S. Gov't MeSH
• Names of Substances
• Histones MeSH
• Nucleosomes MeSH
• Protozoan Proteins MeSH