Triple immunofluorescent labeling of FtsZ, dynamin, and EF-Tu reveals a loose association between the inner and outer membrane mitochondrial division machinery in the red alga Cyanidioschyzon merolae

Engineering anaerobic electron flow through heterologous rhodoquinone synthesis in model microbial and photosynthetic platforms

Anaerobic conditions facilitate bioproduction by enabling diverse metabolic pathways; however, they disrupt redox balance due to the accumulation of reduced cofactors, limiting metabolic efficiency. Rhodoquinone (RQ), a low-redox-potential quinone, supports electron transport under anaerobic conditions. Unlike menaquinone, RQ is synthesized from ubiquinone through a single enzymatic reaction catalyzed by rhodoquinone biosynthesis protein A (RquA), making it a simple, adaptable metabolic engineering tool. In this study, RQ was synthesized in the menaquinone-deficient Escherichia coli ΔmenA strain via heterologous gene expression of rquA from Euglena gracilis. The engineered strain tripled succinate production under anaerobic conditions compared with the control strain. Redox analysis showed a decreased NADH/NAD+ ratio, reflecting improved electron flow under oxygen-limited conditions. Introducing rquA into a strain with high succinate production further increased succinate yields, confirming compatibility with existing metabolic modifications. To explore broader applications, rquA from Rhodospirillum rubrum was expressed in Cyanidioschyzon merolae mitochondria using a construct with a C. merolae mitochondrial targeting signal. Quinone analysis confirmed RQ synthesis, and the engineered strains produced more succinate anaerobically relative to the controls. Although redox cofactor ratios in C. merolae remained stable, rotenone sensitivity indicated altered mitochondrial electron transport under anaerobic conditions. These findings demonstrate that RQ synthesis enhances anaerobic metabolism in bacterial and eukaryotic systems, providing a versatile tool for metabolic engineering under oxygen-limited conditions.

Long-term live cell cycle imaging of single Cyanidioschyzon merolae cells

Live cell imaging by fluorescence microscopy is a useful tool for elucidating the localization and function of proteins and organelles in single cells. Especially, time-lapse analysis observing the same field sequentially can be used to observe cells of many organisms and analyze the dynamics of intracellular molecules. By single-cell analysis, it is possible to elucidate the characteristics and fluctuations of individual cells, which cannot be elucidated from the data obtained by averaging the characteristics of an ensemble of cells. The primitive red alga Cyanidioschyzon merolae has a very simple structure and is considered a useful model organism for studying the mechanism of organelle division, since the division is performed synchronously with the cell cycle. However, C. merolae does not have a rigid cell wall, and environmental changes such as low temperature or high pH cause morphological change and disruption easily. Therefore, morphological studies of C. merolae typically use fixed cells. In this study, we constructed a long-term time-lapse observation system to analyze the dynamics of proteins in living C. merolae cells. From the results, we elucidate the cell division process of single living cells, including the function of intracellular components.

Function and regulation of the divisome for mitochondrial fission

Mitochondria form dynamic networks in the cell that are balanced by the flux of iterative fusion and fission events of the organelles. It is now appreciated that mitochondrial fission also represents an end-point event in a signalling axis that allows cells to sense and respond to external cues. The fission process is orchestrated by membrane-associated adaptors, influenced by organellar and cytoskeletal interactions and ultimately executed by the dynamin-like GTPase DRP1. Here we invoke the framework of the 'mitochondrial divisome', which is conceptually and operationally similar to the bacterial cell-division machinery. We review the functional and regulatory aspects of the mitochondrial divisome and, within this framework, parse the core from the accessory machinery. In so doing, we transition from a phenomenological to a mechanistic understanding of the fission process.

Cyanidioschyzon merolae aurora kinase phosphorylates evolutionarily conserved sites on its target to regulate mitochondrial division

The mitochondrion is an organelle that was derived from an endosymbiosis. Although regulation of mitochondrial growth by the host cell is necessary for the maintenance of mitochondria, it is unclear how this regulatory mechanism was acquired. To address this, we studied the primitive unicellular red alga Cyanidioschyzon merolae, which has the simplest eukaryotic genome and a single mitochondrion. Here we show that the C. merolae Aurora kinase ortholog CmAUR regulates mitochondrial division through phosphorylation of mitochondrial division ring components. One of the components, the Drp1 ortholog CmDnm1, has at least four sites phosphorylated by CmAUR. Depletion of the phosphorylation site conserved among eukaryotes induced defects such as mitochondrial distribution on one side of the cell. Taken together with the observation that human Aurora kinase phosphorylates Drp1 in vitro, we suggest that the phosphoregulation is conserved from the simplest eukaryotes to mammals, and was acquired at the primitive stage of endosymbiosis.

Onsite GTP fuelling via DYNAMO1 drives division of mitochondria and peroxisomes

Mitochondria and peroxisomes proliferate by division. During division, a part of their membrane is pinched off by constriction of the ring-shaped mitochondrial division (MD) and peroxisome-dividing (POD) machinery. This constriction is mediated by a dynamin-like GTPase Dnm1 that requires a large amount of GTP as an energy source. Here, via proteomics of the isolated division machinery, we show that the 17-kDa nucleoside diphosphate kinase-like protein, dynamin-based ring motive-force organizer 1 (DYNAMO1), locally generates GTP in MD and POD machineries. DYNAMO1 is widely conserved among eukaryotes and colocalizes with Dnm1 on the division machineries. DYNAMO1 converts ATP to GTP, and disruption of its activity impairs mitochondrial and peroxisomal fissions. DYNAMO1 forms a ring-shaped complex with Dnm1 and increases the magnitude of the constricting force. Our results identify DYNAMO1 as an essential component of MD and POD machineries, suggesting that local GTP generation in Dnm1-based machinery regulates motive force for membrane severance.

Mitochondria and peroxisomes require a dynamin-like GTPase to remodel membranes during division. Here the authors identify DYNAMO1, a nucleoside diphosphate kinase-like protein that generates a local source of GTP to promote constriction of the division machinery and produce daughter organelles.

Accelerated triacylglycerol production without growth inhibition by overexpression of a glycerol-3-phosphate acyltransferase in the unicellular red alga Cyanidioschyzon merolae

Microalgae accumulate triacylglycerols (TAGs), a promising feedstock for biodiesel production, under unfavorable environmental or stress conditions for their growth. Our previous analyses revealed that only transcripts of CmGPAT1 and CmGPAT2, both encoding glycerol-3-phosphate acyltransferase, were increased among fatty acid and TAG synthesis genes under TAG accumulation conditions in the red alga Cyanidioschyzon merolae. In this study, to investigate the role of these proteins in TAG accumulation in C. merolae, we constructed FLAG-fused CmGPAT1 and CmGPAT2 overexpression strains. We found that CmGPAT1 overexpression resulted in marked accumulation of TAG even under normal growth conditions, with the maximum TAG productivity increased 56.1-fold compared with the control strain, without a negative impact on algal growth. The relative fatty acid composition of 18:2 in the TAGs and the sn-1/sn-3 positions were significantly increased compared with the control strain, suggesting that CmGPAT1 had a substrate preference for 18:2. Immunoblot analysis after cell fractionation and immunostaining analysis demonstrated that CmGPAT1 localizes in the endoplasmic reticulum (ER). These results indicate that the reaction catalyzed by the ER-localized CmGPAT1 is a rate-limiting step for TAG synthesis in C. merolae, and would be a potential target for improvement of TAG productivity in microalgae.

Defining the dynamin-based ring organizing center on the peroxisome-dividing machinery isolated from Cyanidioschyzon merolae

Organelle division is executed through contraction of a ring-shaped supramolecular dividing machinery. A core component of the machinery is the dynamin-based ring conserved during the division of mitochondrion, plastid and peroxisome. Here, using isolated peroxisome-dividing (POD) machinery from a unicellular red algae, Cyanidioschyzon merolae, we identified a dynamin-based ring organizing center (DOC) that acts as an initiation point for formation of the dynamin-based ring. C. merolae contains a single peroxisome, the division of which can be highly synchronized by light-dark stimulation; thus, intact POD machinery can be isolated in bulk. Dynamin-based ring homeostasis is maintained by the turnover of the GTP-bound form of the dynamin-related protein Dnm1 between the cytosol and division machinery via the DOC. A single DOC is formed on the POD machinery with a diameter of 500-700 nm, and the dynamin-based ring is unidirectionally elongated from the DOC in a manner that is dependent on GTP concentration. During the later step of membrane fission, the second DOC is formed and constructs the double dynamin-based ring to make the machinery thicker. These findings provide new insights to define fundamental mechanisms underlying the dynamin-based membrane fission in eukaryotic cells.

Chloramphenicol acetyltransferase-a new selectable marker in stable nuclear transformation of the red alga Cyanidioschyzon merolae

In this study, we have shown the applicability of chloramphenicol acetyltransferase as a new and convenient selectable marker for stable nuclear transformation as well as potential chloroplast transformation of Cyanidioschyzon merolae-a new model organism, which offers unique opportunities for studding the mitochondrial and plastid physiology as well as various evolutionary, structural, and functional features of the photosynthetic apparatus.

Subcellular distribution of central carbohydrate metabolism pathways in the red alga Cyanidioschyzon merolae

Comprehensive subcellular localization analysis revealed that the subcellular distribution of carbohydrate metabolic pathways in the red alga Cyanidioschyzon is essentially identical with that in Arabidopsis , except the lack of transaldolase. In plants, the glycolysis and oxidative pentose phosphate pathways (oxPPP) are located in both cytosol and plastids. However, in algae, particularly red algae, the subcellular localization of enzymes involved in carbon metabolism is unclear. Here, we identified and examined the localization of enzymes related to glycolysis, oxPPP, and tricarboxylic acid (TCA) and Calvin-Benson cycles in the red alga Cyanidioschyzon merolae. A gene encoding transaldolase of the oxPPP was not found in the C. merolae genome, and no transaldolase activity was detected in cellular extracts. The subcellular localization of 65 carbon metabolic enzymes tagged with green fluorescent protein or hemagglutinin was examined in C. merolae cells. As expected, TCA and Calvin-Benson cycle enzymes were localized to mitochondria and plastids, respectively. The analyses also revealed that the cytosol contains the entire glycolytic pathway and partial oxPPP, whereas the plastid contains a partial glycolytic pathway and complete oxPPP, with the exception of transaldolase. Together, these results suggest that the subcellular distribution of carbohydrate metabolic pathways in C. merolae is essentially identical with that reported in the photosynthetic tissue of Arabidopsis thaliana; however, it appears that substrates typically utilized by transaldolase are consumed by glycolytic enzymes in the plastidic oxPPP of C. merolae.

Localization and phylogenetic analysis of enzymes related to organellar genome replication in the unicellular rhodophyte Cyanidioschyzon merolae

Plants and algae possess plastids and mitochondria harboring their own genomes, which are replicated by the apparatus consisting of DNA polymerase, DNA primase, DNA helicase, DNA topoisomerase, single-stranded DNA maintenance protein, DNA ligase, and primer removal enzyme. In the higher plant Arabidopsis thaliana, organellar replication-related enzymes (OREs) are similar in plastids and mitochondria because many of them are dually targeted to plastids and mitochondria. In the red algae, there is a report about a DNA replicase, plant/protist organellar DNA polymerase, which is localized to both plastids and mitochondria. However, other OREs remain unclear in algae. Here, we identified OREs possibly localized to organelles in the unicellular rhodophyte Cyanidioschyzon merolae. We then examined intracellular localization of green fluorescent protein-fusion proteins of these enzymes in C. merolae, whose cell has a single plastid and a single mitochondrion and is suitable for localization analysis, demonstrating that the plastid and the mitochondrion contain markedly different components of replication machinery. Phylogenetic analyses revealed that the organelle replication apparatus was composed of enzymes of various different origins, such as proteobacterial, cyanobacterial, and eukaryotic, in both red algae and green plants. Especially in the red alga, many enzymes of cyanobacterial origin remained. Finally, on the basis of the results of localization and phylogenetic analyses, we propose a model on the succession of OREs in eukaryotes.

Gene targeting in the red alga Cyanidioschyzon merolae: single- and multi-copy insertion using authentic and chimeric selection markers

The unicellular red alga Cyanidioschyzon merolae is an emerging model organism for studying organelle division and inheritance: the cell is composed of an extremely simple set of organelles (one nucleus, one mitochondrion and one chloroplast), and their genomes are completely sequenced. Although a fruitful set of cytological and biochemical methods have now been developed, gene targeting techniques remain to be fully established in this organism. Thus far, only a single selection marker, URA Cm-Gs , has been available that complements the uracil-auxotrophic mutant M4. URA Cm-Gs , a chimeric URA5.3 gene of C. merolae and the related alga Galdieria sulphuraria, was originally designed to avoid gene conversion of the mutated URA5.3 allele in the parental strain M4. Although an early example of targeted gene disruption by homologous recombination was reported using this marker, the genome structure of the resultant transformants had never been fully characterized. In the current study, we showed that the use of the chimeric URA Cm-Gs selection marker caused multicopy insertion at high frequencies, accompanied by undesired recombination events at the targeted loci. The copy number of the inserted fragments was variable among the transformants, resulting in high yet uneven levels of transgene expression. In striking contrast, when the authentic URA5.3 gene (URA Cm-Cm ) was used as a selection marker, efficient single-copy insertion was observed at the targeted locus. Thus, we have successfully established a highly reliable and reproducible method for gene targeting in C. merolae. Our method will be applicable to a number of genetic manipulations in this organism, including targeted gene disruption, replacement and tagging.

Mitochondrial localization of ferrochelatase in a red alga Cyanidioschyzon merolae

Ferrochelatase (FECH) is an essential enzyme for the final step of heme biosynthesis. In green plants, its activity has been reported in both plastids and mitochondria. However, the precise subcellular localization of FECH remains uncertain. In this study, we analyzed the localization of FECH in the unicellular red alga, Cyanidioschyzon merolae. Immunoblot and enzyme activity analyses of subcellular fractions localized little FECH in the plastid. In addition, immunofluorescence microscopy identified that both intrinsic and hemagglutinin (HA)-tagged FECH are localized in the mitochondrion. We therefore conclude that FECH is localized in the mitochondrion in C. merolae.

Golgi inheritance in the primitive red alga, Cyanidioschyzon merolae

The Golgi body has important roles in modifying, sorting, and transport of proteins and lipids. Eukaryotic cells have evolved in various ways to inherit the Golgi body from mother to daughter cells, which allows the cells to function properly immediately after mitosis. Here we used Cyanidioschyzon merolae, one of the most suitable systems for studies of organelle dynamics, to investigate the inheritance of the Golgi. Two proteins, Sed5 and Got1, were used as Golgi markers. Using immunofluorescence microscopy, we demonstrated that C. merolae contains one to two Golgi bodies per cell. The Golgi body was localized to the perinuclear region during the G1 and S phases and next to the spindle poles in a microtubule-dependent manner during M phase. It was inherited together with spindle poles upon cytokinesis. These observations suggested that Golgi inheritance is dependent on microtubules in C. merolae.

Mitotic inheritance of endoplasmic reticulum in the primitive red alga Cyanidioschyzon merolae

Endoplasmic reticulum (ER) is a major site for secretory protein folding and lipid synthesis. Since ER cannot be synthesized de novo, it must be inherited during the cell cycle. Studying ER inheritance can however be difficult because the ER of typical plant and animal cells is morphologically complex. Therefore, our study used Cyanidioschyzon merolae, a species that has a simple ER structure, to investigate the inheritance of this organelle. Using immunofluorescence microscopy, we demonstrated that C. merolae contains a nuclear ER (nuclear envelope) and a small amount of peripheral ER extending from the nuclear ER. During mitosis, the nuclear ER became dumbbell-shaped and underwent division. Peripheral ER formed ring-like structures during the G1 and S phases, and extended toward the mitochondria and cell division planes during the M phase. These observations indicated that C. merolae undergoes closed mitosis, whereby the nuclear ER does not diffuse, and the peripheral ER contains cell cycle-specific structures.

The coiled-coil protein VIG1 is essential for tethering vacuoles to mitochondria during vacuole inheritance of Cyanidioschyzon merolae

Vacuoles/lysosomes function in endocytosis and in storage and digestion of metabolites. These organelles are inherited by the daughter cells in eukaryotes. However, the mechanisms of this inheritance are poorly understood because the cells contain multiple vacuoles that behave randomly. The primitive red alga Cyanidioschyzon merolae has a minimum set of organelles. Here, we show that C. merolae contains about four vacuoles that are distributed equally between the daughter cells by binding to dividing mitochondria. Binding is mediated by VIG1, a 30-kD coiled-coil protein identified by microarray analyses and immunological assays. VIG1 appears on the surface of free vacuoles in the cytosol and then tethers the vacuoles to the mitochondria. The vacuoles are released from the mitochondrion in the daughter cells following VIG1 digestion. Suppression of VIG1 by antisense RNA disrupted the migration of vacuoles. Thus, VIG1 is essential for tethering vacuoles to mitochondria during vacuole inheritance in C. merolae.

Identification of novel proteins in isolated polyphosphate vacuoles in the primitive red alga Cyanidioschyzon merolae

Plant vacuoles are organelles bound by a single membrane, and involved in various functions such as intracellular digestion, metabolite storage, and secretion. To understand their evolution and fundamental mechanisms, characterization of vacuoles in primitive plants would be invaluable. Algal cells often contain polyphosphate-rich compartments, which are thought to be the counterparts of seed plant vacuoles. Here, we developed a method for isolating these vacuoles from Cyanidioschyzon merolae, and identified their proteins by MALDI TOF-MS. The vacuoles were of unexpectedly high density, and were highly enriched at the boundary between 62 and 80% w/v iodixanol by density-gradient ultracentrifugation. The vacuole-containing fraction was subjected to SDS-PAGE, and a total of 46 proteins were identified, including six lytic enzymes, 13 transporters, six proteins for membrane fusion or vesicle trafficking, five non-lytic enzymes, 13 proteins of unknown function, and three miscellaneous proteins. Fourteen proteins were homologous to known vacuolar or lysosomal proteins from seed plants, yeasts or mammals, suggesting functional and evolutionary relationships between C. merolae vacuoles and these compartments. The vacuolar localization of four novel proteins, namely CMP249C (metallopeptidase), CMJ260C (prenylated Rab receptor), CMS401C (ABC transporter) and CMT369C (o-methyltransferase), was confirmed by labeling with specific antibodies or transient expression of hemagglutinin-tagged proteins. The results presented here provide insights into the proteome of C. merolae vacuoles and shed light on their functions, as well as indicating new features.

The phosphatase laforin crosses evolutionary boundaries and links carbohydrate metabolism to neuronal disease

Lafora disease (LD) is a progressive myoclonic epilepsy resulting in severe neurodegeneration followed by death. A hallmark of LD is the accumulation of insoluble polyglucosans called Lafora bodies (LBs). LD is caused by mutations in the gene encoding the phosphatase laforin, which reportedly exists solely in vertebrates. We utilized a bioinformatics screen to identify laforin orthologues in five protists. These protists evolved from a progenitor red alga and synthesize an insoluble carbohydrate whose composition closely resembles LBs. Furthermore, we show that the kingdom Plantae, which lacks laforin, possesses a protein with laforin-like properties called starch excess 4 (SEX4). Mutations in the Arabidopsis thaliana SEX4 gene results in a starch excess phenotype reminiscent of LD. We demonstrate that Homo sapiens laforin complements the sex4 phenotype and propose that laforin and SEX4 are functional equivalents. Finally, we show that laforins and SEX4 dephosphorylate a complex carbohydrate and form the only family of phosphatases with this activity. These results provide a molecular explanation for the etiology of LD.

A mitochondrial protein affects cell morphology, mitochondrial segregation and virulence in Leishmania

The single mitochondrion of kinetoplastids divides in synchrony with the nucleus and plays a crucial role in cell division. However, despite its importance and potential as a drug target, the mechanism of mitochondrial division and segregation and the molecules involved are only partly understood. In our quest to identify novel mitochondrial proteins in Leishmania, we constructed a hidden Markov model from the targeting motifs of known mitochondrial proteins as a tool to search the Leishmania major genome. We show here that one of the 17 proteins of unknown function that we identified, designated mitochondrial protein X (MIX), is an oligomeric protein probably located in the inner membrane and expressed throughout the Leishmania life cycle. The MIX gene appears to be essential. Moreover, even deletion of one allele from L. major led to abnormalities in cell morphology, mitochondrial segregation and, importantly, to loss of virulence. MIX is unique to kinetoplastids but its heterologous expression in Saccharomyces cerevisiae produced defects in mitochondrial morphology. Our data show that a number of mitochondrial proteins are unique to kinetoplastids and some, like MIX, play a central role in mitochondrial segregation and cell division, as well as virulence.

Origin of mitochondria by intracellular enslavement of a photosynthetic purple bacterium

Mitochondria originated by permanent enslavement of purple non-sulphur bacteria. These endosymbionts became organelles through the origin of complex protein-import machinery and insertion into their inner membranes of protein carriers for extracting energy for the host. A chicken-and-egg problem exists: selective advantages for evolving import machinery were absent until inner membrane carriers were present, but this very machinery is now required for carrier insertion. I argue here that this problem was probably circumvented by conversion of the symbiont protein-export machinery into protein-import machinery, in three phases. I suggest that the first carrier entered the periplasmic space via pre-existing beta-barrel proteins in the bacterial outer membrane that later became Tom40, and inserted into the inner membrane probably helped by a pre-existing inner membrane protein, thereby immediately providing the protoeukaryote host with photosynthesate. This would have created a powerful selective advantage for evolving more efficient carrier import by inserting Tom70 receptors. Massive gene transfer to the nucleus inevitably occurred by mutation pressure. Finally, pressure from harmful, non-selected gene transfer to the nucleus probably caused evolution of the presequence mechanism, and photosynthesis was lost.

Plant mitochondrial dynamics

Higher plant mitochondria are dynamic, pleomorphic organelles. The higher plant chondriome (all mitochondria in a cell collectively) is typically composed of numerous, physically discrete, mitochondria. However, frequent inter-mitochondrial fusion, enabling the mixing and recombination of mtDNA, ensures that the higher plant chondriome functions, at least genetically, as a discontinuous whole. Nothing is known about the genes controlling mitochondrial fusion in plants; there are no plant homologues of most of the genes known to be involved in fusion in other organisms. In contrast, the mitochondrial fission apparatus is generally conserved. Higher plant mitochondria use dynamin-like and Fis-type proteins for division; like yeast and animals, higher plants have lost the mitochondrial-specific form of the prokaryote-derived protein, FtsZ. In addition to being providers of energy for life, mitochondria provide a trigger for death. The role of mitochondrial dynamics in the initiation and promulgation of cell death is conserved in higher plants although there are specific differences in the genes and mechanisms involved relative to other higher eukaryotes.

Cell biology of mitochondrial dynamics

Mitochondria are the product of an ancient endosymbiotic event between an alpha-proteobacterium and an archael host. An early barrier to overcome in this relationship was the control of the bacterium's proliferation within the host. Undoubtedly, the bacterium (or protomitochondrion) would have used its own cell division apparatus to divide at first and, today a remnant of this system remains in some "ancient" and diverse eukaryotes such as algae and amoebae, the most conserved and widespread of all bacterial division proteins, FtsZ. In many of the eukaryotes that still use FtsZ to constrict the mitochondria from the inside, the mitochondria still resemble bacteria in shape and size. Eukaryotes, however, have a mitochondrial morphology that is often highly fluid, and in their tubular networks of mitochondria, division is clearly complemented by mitochondrial fusion. FtsZ is no longer used by these complex eukaryotes, and may have been replaced by other proteins better suited to sustaining complex mitochondrial networks. Although proteins that divide mitochondria from the inside are just beginning to be characterized in higher eukaryotes, many division proteins are known to act on the outside of the organelle. The most widespread of these are the dynamin-like proteins, which appear to have been recruited very early in the evolution of mitochondria. The essential nature of mitochondria dictates that their loss is intolerable to human cells, and that mutations disrupting mitochondrial division are more likely to be fatal than result in disease. To date, only one disease (Charcot-Marie-Tooth disease 2A) has been mapped to a gene that is required for mitochondrial division, whereas two other diseases can be attributed to mutations in mitochondrial fusion genes. Apart from playing a role in regulating the morphology, which might be important for efficient ATP production, research has indicated that the mitochondrial division and fusion proteins can also be important during apoptosis; mitochondrial fragmentation is an early triggering (and under many stimuli, essential) step in the pathway to cell suicide.

Cell cycle-regulated, microtubule-independent organelle division in Cyanidioschyzon merolae

Mitochondrial and chloroplast division controls the number and morphology of organelles, but how cells regulate organelle division remains to be clarified. Here, we show that each step of mitochondrial and chloroplast division is closely associated with the cell cycle in Cyanidioschyzon merolae. Electron microscopy revealed direct associations between the spindle pole bodies and mitochondria, suggesting that mitochondrial distribution is physically coupled with mitosis. Interconnected organelles were fractionated under microtubule-stabilizing condition. Immunoblotting analysis revealed that the protein levels required for organelle division increased before microtubule changes upon cell division, indicating that regulation of protein expression for organelle division is distinct from that of cytokinesis. At the mitochondrial division site, dynamin stuck to one of the divided mitochondria and was spatially associated with the tip of a microtubule stretching from the other one. Inhibition of microtubule organization, proteasome activity or DNA synthesis, respectively, induced arrested cells with divided but shrunk mitochondria, with divided and segregated mitochondria, or with incomplete mitochondrial division restrained at the final severance, and repetitive chloroplast division. The results indicated that mitochondrial morphology and segregation but not division depend on microtubules and implied that the division processes of the two organelles are regulated at distinct checkpoints.

VP1 of foot-and-mouth disease virus induces apoptosis via the Akt signaling pathway

Foot-and-mouth disease virus (FMDV) binds to cellular integrins through an RGD motif in its capsid protein, VP1. It is unclear, however, what kind of cellular event(s) are triggered after the binding of VP1 to the cells. In this study, we show that aqueous soluble recombinant DNA-derived VP1 (rVP1) of FMDV induced apoptosis of BHK-21 cells after binding to integrins. In addition, treatment of BHK-21 cells with rVP1 resulted in deactivation of Akt and enhancement of several proapoptotic responses such as dephosphorylation of glycogen synthase kinase-3beta and cleavage of procaspase-3, -7, and -9. Additional studies revealed that the rVP1 treatment caused apoptosis of cancer cells, including MCF-7 (a breast carcinoma cell line with a functional deletion of the caspase-3 gene) and PC-3 (a sphingosine 1-phosphate receptor subtype 3-deficient androgen-independent prostate cancer cell line). These results suggest that rVP1 of FMDV may be used selectively as a potent apoptotic agent for human cancer by modulating the Akt signaling pathway and that its effect is not primarily dependent on either activation of procaspase-3 or deactivation of sphingosine 1-phosphate receptor subtype 3.