Peroxisome biogenesis

Peroxisomes are eukaryotic organelles that are the subcellular location of important metabolic reactions. In humans, defects in the organelle's function are often lethal. Yet, relative to other organelles, little is known about how cells maintain and propagate peroxisomes or how they direct specific sets of newly synthesized proteins to these organelles (peroxisome biogenesis/assembly). In recent years, substantial progress has been made in elucidating aspects of peroxisome biogenesis and in identifying PEX genes whose products, peroxins, are essential for one or more of these processes. The most progress has been made in understanding the mechanism by which peroxisome matrix proteins are imported into the organelles. Signal sequences responsible for targeting proteins to the organelle have been defined. Potential signal receptor proteins, a receptor docking protein and other components of the import machinery have been identified, along with insights into how they operate. These studies indicate that multiple peroxisomal protein-import mechanisms exist and that these mechanisms are novel, not simply variations of those described for other organelles.     

Melanosome biogenesis: shedding light on the origin of an obscure organelle

Melanosomes are specialized intracellular organelles in which melanin pigments are synthesized and stored. The ontogenesis of these morphologically unique organelles, as well as their relationship to "conventional" organelles of the secretory and endocytic pathways, has for decades been a matter of study - and controversy. Recent work by the groups of Michael Marks and Gra?a Raposo has uncovered the molecular mechanism that results in the formation of the lumenal striations characteristic of melanosome precursor organelles.     

Small sperm,uniparental inheritance and selfish cytoplasmic elements: a comparison of two models

It has previously been suggested that small sperm size may be an adaptation to achieve uniparental inheritance of organelles, and hence to prevent the spread of selfish cytoplasmic elements. Such an explanation for anisogamy implies a mechanism whereby the male gamete eliminates its own cytoplasm prior to fusion with the egg. A model has been presented demonstrating the invasion and persistence of a modifier that acts gametically to kill its own organelles. Here we show, however, that this model is far from robust; indeed, if any cost is associated with the modifier it cannot persist. We also show that despite an empirically demonstrated association between anisogamy and multicellularity, this result also applies if the analysis is applied in the multicellular case. This class of model contrasts with the majority of analyses in which the modifier kills off the incoming gamete’s organelles. We show that these models are highly robust, even if uniparental inheritance is imperfect.     

Mucolipidosis Type IV Protein TRPML1‐Dependent Lysosome Formation

Lysosomes are dynamic organelles that undergo cycles of fusion and fission with themselves and with other organelles. Following fusion with late endosomes to form hybrid organelles, lysosomes are reformed as discrete organelles. This lysosome reformation or formation is a poorly understood process that has not been systematically analyzed and that lacks known regulators. In this study, we quantitatively define the multiple steps of lysosome formation and identify the first regulator of this process.      

Ultrastructural characteristics of freeze-dried smooth muscle

The rapid freezing and vacuum dehydration of tissue has been employed to study the intracellular distribution of diffusible substances at the electron microscopic level. Hower, the ultrastructural detail of freeze-dried tissue is difficult to retain. Consequently, the ultrastructural preservation of freeze-dried smooth muscle has not been sufficient to permit satisfactorily definition of intracellular organelles. Therefore, determinations of the intracellular distribution of soluble ions have not been achieved in freeze-dried smooth muscle. In this study a freeze drying method is presented which provides more satisfactory definition of intracellular organelles than has been available in the past. Using this merens have been preserved. When these muscles are compared to convention preparer of surface vesicles that are observed, and the mitochondria are extremely electron opaque in the freeze-dried tissue. The smooth muscles which have been examined do not have the inherent contrast of other types of tissue nor do they contain the different types of mitochondria that have been observed in nonmuscle tissue.     

Catalase-negative peroxisomes in human embryonic liver

Hepatic peroxisomes in human embryos with a menstrual age of 6 and 7 weeks have been examined via catalase cytochemistry. In the younger sample, the organelles show no catalase activity, their matrix being pale and coarsely reticular. In the 7-week specimen, the peroxisome population consists of catalase-positive and catalase-negative organelles. The latter have a morphology identical to that of the 6-week sample and represent 66% of the population. The positive organelles show a pronounced staining hetereogeneity. Together with the simultaneous presence of negative organelles, this might reflect the onset of catalase import into the peroxisomes during this period. Catalase heterogeneity excludes a continuous exchange of matrix contents; moreover, interconnections between peroxisomes have not been observed, and no cluster formation occurs. The data therefore also suggest that catalase is imported into individual, preexisting organelles in embryonic liver. The three peroxisomal -oxidation enzymes become detectable by immunocytochemistry only later during development. Morphological indications for a rapidly dividing population, such as elongated and/or tailed organelles, have not been observed. Morphometry has revealed that, in these early stages, the organelles are significantly smaller than the peroxisomes of fetal and adult human liver.     

Vacuoles and fungal biology

Fungal vacuoles have long been recognised as versatile organelles, involved in many aspects of protein turnover, cellular homeostasis, membrane trafficking, signalling and nutrition. Recent research has also revealed an expanding repertoire of physiological functions for fungal vacuoles that are vital for fungal growth, differentiation, symbiosis and pathogenesis. Vacuole-mediated long-distance nutrient transporting systems have been shown to facilitate mycelial foraging and long-distance communication in saprophytes and mycorrhizal fungi. Some hyphae of plant and human fungal pathogens can grow under severely nutrient-limited conditions by expanding the vacuolar space rather than synthesising new cytoplasm and organelles. Autophagy has been recognised as a crucial process in plant pathogens for the initiation of appressorium formation. These studies demonstrate the importance of fungal vacuoles as organelles that are essential for many of the attributes that define the activities and roles of fungi in their natural environments.     

Co-operative dynamics in organelles

Some organelles produce elementary life phenomena which are characterized by the spontaneous formation and/or maintenance of ordered macroscopic dynamics like e.g. the shortening of sarcomeres in striated muscle and the transmission of electrical impulses in an axon. It has been widely accepted that such organelles are organized molecular systems where molecular elements work independently under constraint of a more or less rigid and regular structure of the system. On the other hand, such organelles should be regarded as self-organizing systems if the ordered macroscopic dynamics are self-organized. As the macroscopic dynamics gradually emerge, the microscopic dynamics of its elements become linked to each other through a feedback loop. It is crucial for the feedback loop to operate that the macroscopic dynamics are "free" in their behavior. In the present paper, it is pointed out that the traditional view of independent molecular elements has been obtained from experiments in which, by means of external constraint, the macroscopic dynamics is "clamped". Under such conditions, the self-organizing system may behave as an organized one. Based on synergetics we propose criterions for proving self-organizing systems, and, by applying the criterions, we conclude that skeletal muscle actomysin is a co-operative element in the sense of self-organization.     

Golgi biogenesis in simple eukaryotes

The accurate duplication of cellular organelles is important to ensure propagation through successive generations. The semi-conserved replication of DNA and DNA-containing organelles has been well studied, but the mechanisms used to duplicate most other organelles remain elusive. These include the centrosomes, which act as microtubule organizing centres during interphase and orient the mitotic spindle poles during mitosis. Centrosomes can also act as basal bodies, nucleating the growth of cilia or flagella. Even less understood are the mechanisms used to duplicate membrane-bound organelles that do not contain DNA. These include organelles involved in the secretory pathway such as the endoplasmic reticulum and the Golgi apparatus. This review will summarize the current knowledge of Golgi biogenesis in simple eukaryotic organisms, in particular, two protozoan parasites, Toxoplasma gondii and Trypanosoma brucei.     

Association of kinesin with characterized membrane-bounded organelles.

The family of molecular motors known as kinesin has been implicated in the translocation of membrane-bounded organelles along microtubules, but relatively little is known about the interaction of kinesin with organelles. In order to understand these interactions, we have examined the association of kinesin with a variety of organelles. Kinesin was detected in purified organelle fractions, including synaptic vesicles, mitochondria, and coated vesicles, using quantitative immunoblots and immunoelectron microscopy. In contrast, isolated Golgi membranes and nuclear fractions did not contain detectable levels of kinesin. These results demonstrate that the organelle binding capacity of kinesin is selective and specific. The ability to purify membrane-bounded organelles with associated kinesin indicates that at least a portion of the cellular kinesin has a relatively stable association with membrane-bounded organelles in the cell. In addition, immunoelectron microscopy of mitochondria revealed a patch-like pattern in the kinesin distribution, suggesting that the organization of the motor on the organelle membrane may play a role in regulating organelle motility.     

Polyhedral organelles compartmenting bacterial metabolic processes

Bacterial polyhedral organelles are extremely large macromolecular complexes consisting of metabolic enzymes encased within a multiprotein shell that is somewhat reminiscent of a viral capsid. Recent investigations suggest that polyhedral organelles are widely used by bacteria for optimizing metabolic processes. The distribution and diversity of these unique structures has been underestimated because many are not formed during growth on standard laboratory media and because electron microscopy is required for their observation. However, recent physiological studies and genomic analyses tentatively indicate seven functionally distinct organelles distributed among over 40 genera of bacteria. Functional studies conducted thus far are consistent with the idea that polyhedral organelles act as microcompartments that enhance metabolic processes by selectively concentrating specific metabolites. Relatively little is known about how this is achieved at the molecular level. Possible mechanisms include regulation of enzyme activity or efficiency, substrate channeling, a selectively permeable protein shell, and/or differential solubility of metabolites within the organelle. Given their complexity and distinctive structure, it would not be surprising if aspects of their biochemical mechanism are unique. Therefore, the unusual structure of polyhedral organelles raises intriguing questions about their assembly, turnover, and molecular evolution, very little of which is understood.     

ER calcium and the functions of intracellular organelles

Cytosolic calcium has long been known as a second messenger of major significance. Recently it has become apparent that calcium stored in cellular organelles can also be an important regulator of cellular functions. The endoplasmic reticulum (ER) is usually the largest store of releasable calcium in the cell. The diverse signalling functions of calcium populating the endoplasmic reticulum and its interactions with other organelles are illustrated in Figure ?? and described in this paper.     

Myosin-dependent transport in neurons

Axonal transport in neurons has been shown to be microtubule dependent, driven by the molecular motor proteins kinesin and dynein. However, organelles undergoing fast transport can often pause or rapidly change directions without apparent dissociation from their transport tracks. Cytoskeletal polymers such as neurofilaments and microtubules have also been shown to make infrequent but rapid movements in axons indicating that their transport is likely to involve molecular motors. In addition, neurons have multiple compartments that are devoid of microtubules where transport of organelles is still seen to occur. These areas are rich in other cytoskeletal polymers such as actin filaments. Transported organelles have been shown to associate with multiple motor proteins including myosins. This suggests that nonmicrotubule-based transport may be myosin driven. In this review we will focus our attention on myosin motors known to be present in neurons and evaluate the evidence that they contribute to transport or other functions in the different compartments of the neuron.     

Mobile reticular organization of plastids and mitochondria in plant cells

Results of confocal, fluorescent and video microscopy of plant cell organelles and of stromule network uniting them are reviewed. The vast information on the structure of stromules, their mobility, proposed functions and development has been analyzed, in addition to factors stimulating and suppressing this development. Structural similarity between the network of stromules in living cells, observed by confocal fluorescence microscopy, and the endoplasmic reticulum, seen on micrographs of preparations fixed for electron microscopy is discussed. As a result of this discussion, a conclusion is made with regard to the identity of these endomembranous networks. The intercellular symplastic organization is shown for both networks in plant tissues. The existence of a common transport and trophic compartment is proposed that includes organelles, intercellular endoplasmic reticulum and its derivatives, phloem and xylem. The trophic system development might have been induced in the course of endosymbiogenesis with some bacterial precursors of organelles.     

Hermansky–Pudlak Syndrome and Pale Ear: Melanosome‐Making for the Millennium

Hermansky–Pudlak syndrome (HPS) is a rare autosomal recessive disorder characterized principally by oculocutaneous albinism, a bleeding tendency, and a ceroid‐lipofuscin lysosomal storage disease. These clinical manifestations of HPS are associated with defects of multiple cytoplasmic organelles – melanosomes, platelet granules, and lysosomes – suggesting that the HPS gene product is involved in some shared feature of the biogenesis or functions of these diverse organelles. The HPS gene has been cloned, and a number of pathologic mutations of the gene have been identified. Functional studies indicate that the HPS protein is part of a high‐molecular weight complex involved in the biogenesis of early melanosomes. Additional disorders with similarities to HPS have been identified in man, mouse, flies, and yeast, and it is rapidly becoming clear that understanding these disorders will shed new light on the mechanisms by which cells traffic newly synthesized proteins through the cytoplasm to assemble functional organelles.     

A light and electron microscopic study of the quadriflagellate green algaSpermatozopsis exsultans

The green flagellateSpermatozopsis exsultans Korshikov has been studied in culture by light and electron microscopy. The organism is naked, bears four flagella and is conspicuously spirally twisted. The ultrastructure and location of cell organelles (except the flagellar apparatus) has been investigated in detail using an absolute configuration analysis. With the exception of a doubling of the flagella and of the secondary cytoskeletal microtubule system,S. exsultans has the exact same complement of organelles occupying the same relative positions as has been described forS. similis. The two species are therefore correctly placed in the same genus. The usefulness of absolute orientations of cell organelles for green algal taxonomy and phylogeny is stressed.Dedicated to Prof.M. Mix on the occasion of her 60th birthday.     

Organelles in Blastocystis that blur the distinction between mitochondria and hydrogenosomes

Blastocystis is a unicellular stramenopile of controversial pathogenicity in humans. Although it is a strict anaerobe, Blastocystis has mitochondrion-like organelles with cristae, a transmembrane potential and DNA. An apparent lack of several typical mitochondrial pathways has led some to suggest that these organelles might be hydrogenosomes, anaerobic organelles related to mitochondria. We generated 12,767 expressed sequence tags (ESTs) from Blastocystis and identified 115 clusters that encode putative mitochondrial and hydrogenosomal proteins. Among these is the canonical hydrogenosomal protein iron-only [FeFe] hydrogenase that we show localizes to the organelles. The organelles also have mitochondrial characteristics, including pathways for amino acid metabolism, iron-sulfur cluster biogenesis, and an incomplete tricarboxylic acid cycle as well as a mitochondrial genome. Although complexes I and II of the electron transport chain (ETC) are present, we found no evidence for complexes III and IV or F1Fo ATPases. The Blastocystis organelles have metabolic properties of aerobic and anaerobic mitochondria and of hydrogenosomes. They are convergently similar to organelles recently described in the unrelated ciliate Nyctotherus ovalis. These findings blur the boundaries between mitochondria, hydrogenosomes, and mitosomes, as currently defined, underscoring the disparate selective forces that shape these organelles in eukaryotes.     

In vitro assays demonstrate that pollen tube organelles use kinesin-related motor proteins to move along microtubules

The movement of pollen tube organelles relies on cytoskeletal elements. Although the movement of organelles along actin filaments in the pollen tube has been studied widely and is becoming progressively clear, it remains unclear what role microtubules play. Many uncertainties about the role of microtubules in the active transport of pollen tube organelles and/or in the control of this process remain to be resolved. In an effort to determine if organelles are capable of moving along microtubules in the absence of actin, we extracted organelles from tobacco pollen tubes and analyzed their ability to move along in vitro-polymerized microtubules under different experimental conditions. Regardless of their size, the organelles moved at different rates along microtubules in the presence of ATP. Cytochalasin D did not inhibit organelle movement, indicating that actin filaments are not required for organelle transport in our assay. The movement of organelles was cytosol independent, which suggests that soluble factors are not necessary for the organelle movement to occur and that microtubule-based motor proteins are present on the organelle surface. By washing organelles with KI, it was possible to release proteins capable of gliding carboxylated beads along microtubules. Several membrane fractions, which were separated by Suc density gradient centrifugation, showed microtubule-based movement. Proteins were extracted by KI treatment from the most active organelle fraction and then analyzed with an ATP-sensitive microtubule binding assay. Proteins isolated by the selective binding to microtubules were tested for the ability to glide microtubules in the in vitro motility assay, for the presence of microtubule-stimulated ATPase activity, and for cross-reactivity with anti-kinesin antibodies. We identified and characterized a 105-kD organelle-associated motor protein that is functionally, biochemically, and immunologically related to kinesin. This work provides clear evidence that the movement of pollen tube organelles is not just actin based; rather, they show a microtubule-based motion as well. This unexpected finding suggests new insights into the use of pollen tube microtubules, which could be used for short-range transport, as actin filaments are in animal cells.     

The <Emphasis Type="Italic">Arabidopsis</Emphasis> calmodulin-like proteins AtCML30 and AtCML3 are targeted to mitochondria and peroxisomes,respectively

Calmodulin (CaM) is a ubiquitous sensor/transducer of calcium signals in eukaryotic organisms. While CaM mediated calcium regulation of cytosolic processes is well established, there is growing evidence for the inclusion of organelles such as chloroplasts, mitochondria and peroxisomes into the calcium/calmodulin regulation network. A number of CaM-binding proteins have been identified in these organelles and processes such as protein import into chloroplasts and mitochondria have been shown to be governed by CaM regulation. What have been missing to date are the mediators of this regulation since no CaM or calmodulin-like protein (CML) has been identified in any of these organelles. Here we show that two Arabidopsis CMLs, AtCML3 and AtCML30, are localized in peroxisomes and mitochondria, respectively. AtCML3 is targeted via an unusual C-terminal PTS1-like tripeptide while AtCML30 utilizes an N-terminal, non-cleavable transit peptide. Both proteins possess the typical structure of CaMs, with two pairs of EF-hand motifs separated by a short linker domain. They furthermore display common characteristics, such as calcium-dependent alteration of gel mobility and calcium-dependent exposure of a hydrophobic surface. This indicates that they can function in a similar manner as canonical CaMs. The presence of close homologues to AtCML3 and AtCML30 in other plants further indicates that organellar targeting of these CMLs is not a specific feature of Arabidopsis. The identification of peroxisomal and mitochondrial CMLs is an important step in the understanding how these organelles are integrated into the cellular calcium/calmodulin signaling pathways.