What Are The Membranous Organelles

The traditional view of life on World divides the living world into 2 major groups, prokaryotes and eukaryotes. These two groups were originally suggested to differ in very basic respects. While eukaryotes had complex cell structures including a cytoskeleton and intracellular membrane-bounded organelles, prokaryotes were believed to lack them. In fact, numerous textbooks and current sources still note this distinction and concur information technology to be truthful. For example, in Campbell's Biology [Campbell, 1993, p. 515] it is stated without equivocation: 'Prokaryotic cells lack membrane-enclosed organelles.' In 'Functional Anatomy of Prokaryotic and Eukaryotic Cells' [Tortora et al., 2009, chapt. four] it is similarly claimed that 'Prokaryotes lack membrane-enclosed organelles, specialized structures that carry on various activities'. In the current Wikipedia, under 'Prokaryote' the following statement tin be found: 'The prokaryotes are a group of organisms whose cells lack a jail cell nucleus (karyon) or any other membrane-bounded organelles'. In the same online compendium nether 'Organelle', one can read: 'whilst prokaryotes do non possess organelles per se, some do contain protein-based microcompartments'. Proteinaceous microcompartments will be the subject field of a forthcoming Journal of Molecular Microbiology and Biotechnology written symposium, but this one will bear witness that these generalizations, suggesting a lack of subcellular compartmentalization in prokaryotes, are blatantly in mistake [Murat et al., 2010a].

Intracellular Membranes of Escherichia coli and Other Leaner

In this written symposium on membrane-divisional organelles in bacteria, we consider many of the well-characterized intracellular and extracellular vesicular structures of known function. We begin with vesicular structures found within the bacterial workhorse, Escherichia coli. This organism tin exist induced to produce extensive intracellular membranes (ICMs) and vesicles, particularly when certain integral membrane proteins are produced in large quantities [Arechaga et al., 2000, 2003] (see fig. 1 and symposium commodity entitled 'Membrane invaginations in bacteria and mitochondria: mutual features and evolutionary scenarios' by Arechaga). Protein overproduction using recombinant Dna technologies often results in the formation of inclusion bodies, consisting primarily of denatured or partially denatured protein in the cell cytoplasm [Carrio and Villaverde, 2002]. The task of renaturing inclusion body polypeptides to their native structures represents a major challenge in biotechnology, merely substantial success has been achieved [Baneres et al., 2011; Schlapschy and Skerra, 2011].

Fig. 1

Electron micrographs of thin sections of E. coli cells overproducing the b-subunit of the F-type ATPase. Tiptop: 3 h afterwards initiation of overproduction at 37°; lesser: 3 h after initiation of overproduction at 25°. Reproduced from Arechaga et al. [2000], with permission.

Some overproduced membrane proteins appear in various forms within the cell cytoplasm. These include cytoplasmic micelles (CMs) and intracellular membrane vesicles (ICVs) in add-on to ICMs [Aboulwafa and Saier, 2011; Arechaga et al., 2000; Bogdanov and Dowhan, 2012] (encounter symposium article entitled 'Subcellular localization of integral membrane proteins in Escherichia coli' past Bogdanov et al.). Mesosomes, intracellular extensions of the plasma membrane, have been identified in both Gram-positive and Gram-negative bacteria, although they have been characterized well-nigh extensively in the former organism where they may play roles in 'extracellular' digestion [Cherepova et al., 1986; Greenawalt and Whiteside, 1975; Li et al., 2008; Santhana Raj et al., 2007].

Mesosomes in Gram-positive leaner appear to event from invagination of the plasma membrane, which is also thought to be the origin of the ICMs and ICVs in Eastward. coli [Arechaga et al., 2000; Biriuzova et al., 1980; Hirata, 1979]. It is possible that the ICM in E. coli [Bogdanov and Dowhan, 2012] is of similar function, structure and origin as previously studied mesosomes in other leaner. Moreover, chromatophorous ICMs of photosynthetic bacteria and magnetosomes of magnetotactic bacteria are too believed to accept their origin in plasma membrane sites of invagination. It may be that all of these ICMs have related modes of biogenesis.

Chromatophores (ICMs) in Photosynthetic Bacteria

For over 60 years it has been recognized that many photosynthetic leaner possess intracellular pigmented membrane structures (the ICM; fig. 2) that are capable of catalyzing light-driven reactions including proton motive force (pmf)-driven ATP synthesis [Schachman et al., 1952] or photophosphorylation [Pardee et al., 1952]. This intracellular photosynthetic apparatus, quantitatively different in lipid and protein limerick from the cytoplasmic membranes of these organisms, assumes diverse morphological types, some continuous with and others discontinuous with the plasma membranes, depending on the organism under study. The biogenesis of these photosynthetic membranes continues to be an exciting area of research with the potential of revealing novel mechanisms of membrane differentiation. In the symposium article by Drews entitled 'The intracytoplasmic membranes of majestic bacteria - associates of energy-transducing complexes', the energy-transducing complexes that are responsible for calorie-free-driven electron flow and photophosphorylation are analyzed and reviewed. This commodity focuses on purple α-proteobacterial species of the genus Rhodobacter, in which the ICMs contain the calorie-free-harvesting complexes as well as the bacteriochlorophyll-containing reaction centers where conversion of light energy into a pmf is initiated.

Fig. 2

Transmission electron micrograph of the ICM in negatively stained thin sections of a fresh R. sphaeroides cell. The asterisk indicates a storage granule. Reproduced with permission from Adams et al. [2011].

In the symposium article by Woronowicz et al. entitled 'Structural and functional proteomics of intracytoplasmic membrane assembly in Rhodobacter sphaeroides', a temporal and spatial proteomic approach is taken to the report of photosynthetic ICM structure, function and associates. As besides noted in the commodity past Drews, the ICMs appear to event from invagination of the plasma membrane, and these invagination sites likewise as ICM vesicles have been isolated and characterized. Many of the proteins that incorporate these membranes and the photosynthetic complexes they incorporate have been identified, and four pigmented fractions accept been separated, the reaction heart-lite harvesting one (RC-LH1) core complex, the LH2 peripheral antenna and ii fractions with distinct associations of LH2 with core complexes. The ratios of these dissimilar constituent complexes proved to alter as ICM evolution gain. Other proteins, many of which were identified, as well cofractionate with these complexes, providing functional and biogenic insight. Changes have been followed under dissimilar growth weather condition showing, for instance, that vesiculation of plasma membrane growth initiation sites to form vesicular ICMs is quickly arrested upon introduction of oxic weather condition. The experimental approaches used to ascertain their properties are briefly presented in the symposium article by Woronowicz et al.

Magnetosomes in Magnetotactic Bacteria

Magnetotactic bacteria and the bondage of magnetosomes that permit these organisms to align in the Earth'south magnetic field is the topic of discussion in the symposium articles by Lower and Bazylinski entitled 'The bacterial magnetosome: a unique prokaryotic organelle' and by Murat entitled 'Magnetosomes: how practise they stay in shape?' These membrane-bounded organelles are found in a diversity of bacteria. These bacterial cytoplasmic organelles contain Fe₃O₄ (magnetite) or in anaerobic leaner, Atomic number 26₃S₄ (greigite), frequently as small cubo-octahedral crystals. Bondage of magnets grow by deposition of new membrane-enclosed magnets at the ends of the chains. The mechanisms of biogenesis and coupling of magnetic field detection to a response are still under intensive study, but much information is already bachelor (see the symposium articles past Lower and Bazylinski and past Murat).

Magnetotaxis is well documented for animals that employ the Earth'southward magnetic field for navigation purposes. These animals are capable of sensing the Globe's magnetic field through the use of magnets linked to nerves [Frankel and Bazylinski, 2006, 2009; Jogler and Schuler, 2009; Lefevre et al., 2011, 2012].This is truthful for birds (eastward.k. homing pigeons), bees (i.e. for foraging), fish (i.due east. for migration) and body of water turtles (i.e. for migration). Humans can also respond to magnetic fields. Some take an immutable sense of direction, and homo tissue culture cells respond to imposed magnetic fields. Imposition of stiff magnetic fields (100× that of the Earth) to the brains of epilepsy patients causes a ten-fold increase in the frequency of seizures. Furthermore, continual exposure to power lines has been reported to increase the incidence of cancer in people. Crystals of magnetite (Atomic number 26₃O₄) have been identified in human brains and the brain tissues of many animals using magnetic resonance imaging [Wiltschko and Wiltschko, 2012].

In bacterial magnetosomes, magnetite or greigite (and likewise other sulfides such as pyrite) crystals are surrounded by a membrane of lipids like to those of the plasma membrane but containing unique proteins. The magnetic crystals align in chains yielding big magnetic moments. Each chain has up to 100 magnetosomes per bacterium (fig. 3). They orient in the Globe'south magnetic field and thereby allow the bacteria to move sideways up and downward in the water cavalcade in response to geomagnetism. In the northern hemisphere, they orient northward and swim towards the due south pole of a magnet. The reverse is truthful for those in the southern hemisphere. The two types of bacteria are not fundamentally dissimilar and interconvert at loftier rates relative to mutation rates [Lefevre et al., 2009; Wang et al., 2008].

Fig. three

Low-resolution magnetosome chains (a) and high-resolution view of membrane-bounded magnetite crystals (b) in Magnetospirillum glyphiswaldense every bit revealed by transmission electron microscopy. Reproduced from Bazylinski and Schuler [2009], with permission.

Magnetotactic leaner come up from several diverse bacterial kingdoms, so magnetotaxis may exist very sometime. In fact, ancient magnetofossils have been characterized (see the symposium commodity past Lower and Bazylinski). The membranes of magnetosomes apparently arise by invagination of the plasma membrane, simply they incorporate a unique prepare of proteins that biomineralize and course chains [Komeili et al., 2006; Murat et al., 2010b; Staniland et al., 2007; Tanaka et al., 2006].

Equally noted to a higher place, many magnetosome crystals are of similar sizes and shapes. The question is: why? If also small, they do non take the mass to overcome the energy of thermal vibration to maintain stable motion. However, magnetosome crystals are single-domain particles. If too big, individual domains orient randomly when the multidomain structures are formed (see the symposium article past Murat). These cancel each other out, yielding weak total magnetic moments. Obligate microaerophiles primarily use Fe_iiiO₄ (magnetite) crystals. Nevertheless, some bacteria prefer to be in the oxic-anoxic transition zone, where Fe²⁺ is nowadays in a soluble grade. Fe³⁺ is largely insoluble as oxides and other ferric salts. Some of these bacteria use magnetotaxis to stay in the region of loftier Fe²⁺ concentration but depression O₂ tension (i.e. if they go down, at that place is more than Fe²⁺ and less O₂, if they go up, there is more than O₂ and less Iron²⁺). Other bacteria employ their magnetosomes to seek nutrient-rich sediments [Jogler and Schuler, 2009] (see also the symposium article by Lower and Bazylinski).

Anammoxosomes and Nuclear Envelopes in Planctomycetes

The Planctomycetes represent an unusual and relatively newly discovered group of organelle-containing leaner with unique properties that are just now becoming appreciated and understood. These organisms have a complex internal cell structure. Their outermost membrane has been considered to exist the cytoplasmic membrane although it may showroom some of the backdrop of more than typical Gram-negative bacterial outer membranes. They likewise take a membrane-bounded nucleoid, coordinating in some respects to the nuclei of eukaryotic cells (see symposium article by Fuerst and Sagulenko entitled 'Nested bacterial boxes: nuclear and other intracellular compartments in planctomycetes'). Planctomycetes besides have free energy-producing, mitochondrion-like organelles referred to as anammoxosomes (anaerobic ammonium oxidation organelles) with unusually rigid lipids called 'ladderanes' considering of their inflexible ladder-like structures (encounter symposium article by van Teeseling et al. entitled 'The anammoxosome organelle is crucial for the energy metabolism of anaerobic ammonium oxidizing bacteria'). These lipids probably contribute to energy conservation past creating a more than H⁺ leak-proof membrane. Such a feature may be important as anammoxosomes probably generate a pmf across this membrane via a quinone-dependent process analogous to and peradventure mechanistically similar to the process catalyzed by the membrane cytochrome bc1 complex, present in many bacteria, and equivalent to the electron flow complex Iii of mitochondria. The pmf thus generated tin then be used to make ATP via an F-blazon ATPase that has been found in association with the anammoxosome membrane. Since the anammoxosome is a carve up membrane-enclosed compartment with a distinct lipid and protein composition serving a unique function, it qualifies as an intracellular organelle by whatsoever definition.

Anammoxosomes are only establish in Planctomycetes, only not in all of these organisms. These organelles perchance evolved specifically to compartmentalize the enzymes catalyzing NH₄ ⁺ oxidation and to permit energy production from the primary reaction they catalyze: anaerobic NH₄ ⁺ oxidation. Compartmentalization may also be required considering an intermediate in NH₄ ⁺ oxidation is hydrazine (H_twoN-NH₂), a highly reactive and toxic substance that could destroy nucleic acids if these molecules came in direct contact with them. These compelling arguments, set forth in the article by van Teeseling et al. may well provide the ground for their evolution. This logic appears to exist equally applicable to the evolution of other prokaryotic organelles also as eukaryotic organelles that probably evolved by entirely different mechanisms via very different pathways.

Special Delivery: Outer Membrane Vesicle Trafficking in Prokaryotes

Although the observation that Gram-negative bacteria bleb off outer membrane vesicles (OMVs), releasing them into the external medium, was fabricated over twoscore years ago, their biological roles have get a focus of study only within the past few years. Recent progress in this expanse has revealed that bacterial OMVs are utilized for several processes including: (ane) delivery of toxins to eukaryotic cells, (2) protein and DNA transfer betwixt bacterial cells, (3) trafficking of cell-cell signals, (4) delivery of proteases and antibiotics and (5) removal of harmful incorrectly folded proteins. Some of these roles announced to exist generalized among Gram-negative bacteria while others are restricted to specific bacterial species [Mashburn-Warren and Whiteley, 2006]. The symposium article by Whiteley and associates entitled 'Bacterial outer membrane vesicles in trafficking, advice and host-pathogen interactions' and the contribution by Manning and Kuehn entitled 'Functional advantages conferred past extracellular prokaryotic membrane vesicles' discuss several of these functions. Additionally, in the symposium article entitled 'The role of membrane vesicles in secretion of Lysobacter sp. bacteriolytic enzymes', Vasilyeva et al., present a well-characterized example of the use of these OMVs for secretion of bacteriolytic enzymes, important in microbial interactions in many environments.

Many leaner utilize extracellular signals to communicate and coordinate social activities, a process referred to every bit quorum sensing. Some quorum signals have hydrophobic character, and how these signals are trafficked between leaner inside a population is of great interest. The opportunistic human pathogen, Pseudomonas aeruginosa, packages the signaling molecule, 2-heptyl-3-hydroxy-4-quinolone (Pseudomonas quinolone betoken; PQS), into membrane vesicles that serve to traffic this molecule inside a population. Removal of these vesicles from the bacterial population halts cell-jail cell advice and inhibits PQS-controlled group beliefs.

PQS actively mediates its own packaging and the packaging of other antimicrobial quinolones produced by P. aeruginosa into vesicles. Thus, prokaryotes possess signal trafficking systems with features common to those used by higher organisms. Novel mechanisms for the delivery of signals critical for coordinating group behavior accept been proposed [Mashburn and Whiteley, 2005; Schertzer and Whiteley, 2012].

The extracellular matrix helps define the architecture and infrastructure of bacterial biofilms and also contributes to their resilient nature. How structural characteristics help to bridge the gap between the chemical and physical aspects of the matrix is currently nether critical investigation. Schooling and Beveridge [2006] showed that OMVs are a mutual particulate feature of the matrix of Pseudomonas aeruginosa biofilms. Biofilms grown using different model systems and growth conditions contain OMVs when thin sectioned for transmission electron microscopy, and mechanically disrupted biofilms revealed OMVs in association with intercellular materials. Characterization of planktonic and biofilm-derived OMVs revealed quantitative and qualitative differences betwixt the two and indicated functional roles, such every bit proteolytic action and binding of antibiotics. The essential ubiquity of OMVs was supported by observations of biofilms from a variety of natural environments outside the laboratory and established OMVs every bit common biofilm constituents. They appear to be important and relatively unacknowledged particulate components of the matrix of Gram-negative or mixed bacterial biofilms [Schooling and Beveridge, 2006; Zhong, 2011].

OMVs, released past pathogenic bacteria, can transmit virulence factors to host cells [Kuehn and Kesty, 2005]. These structures are not merely a outcome of membrane instability and are formed by a more directed procedure. Kuehn and Kesty [2005] and McBroom et al. [2006] showed that only a few low-vesiculation mutants and no zippo mutants were recovered following screening for such mutants, suggesting that vesiculation may be a cardinal characteristic of Gram-negative bacterial growth. Gene disruptions were identified that caused differences in vesicle production ranging from a 5-fold decrease to a 200-fold increase relative to wild-type levels. These disruptions included loci governing outer membrane components and peptidoglycan synthesis and constituents of the σ^E cell envelope stress response system. Detergent sensitivity, leakiness and growth characteristics of the novel vesiculation mutant strains did not correlate with vesiculation levels, demonstrating that vesicle production is not predictive of envelope instability [McBroom et al., 2006].

Conditions that impair poly peptide folding in the Gram-negative bacterial envelope crusade stress. The destabilizing furnishings of various types of stress in this compartment are recognized and countered by a number of signal transduction mechanisms [Baumgarten et al., 2012]. Data presented by McBroom and Kuehn [2007] revealed that a bacterial stress response includes release of OMVs. Native vesicles are equanimous of outer membrane and periplasmic materials, and they are released from the bacterial surface without loss of membrane integrity.

The quantity of vesicle release correlates directly with the level of protein accumulation in the prison cell envelope. Accumulation of material occurs under stress, and is exacerbated upon impairment of the normal housekeeping and stress-responsive mechanisms of the prison cell. Mutations that cause increased vesiculation enhance bacterial survival upon challenge with stressing agents or accumulation of toxic misfolded proteins. Preferential packaging of a misfolded protein into vesicles for removal indicates that the vesiculation process can selectively eliminate unwanted textile. Production of bacterial OMVs is thus an independent, general, envelope stress response [Manning and Kuehn, 2011; McBroom and Kuehn, 2007].

Acidocalcisomes and the Evolution of Intracellular Compartmentalization

Acidocalcisomes are calcium/polyphosphate-rich acidic membrane-enclosed organelles that are found in organisms belonging to the iii domains of life (fig. iv) [Docampo and Moreno, 2011; Ramos et al., 2010]. Their membranes may contain a diverseness of transport systems including aquaporins, ion-pumping ATPases, cation exchangers and H⁺-pumping pyrophosphatases [Rohloff et al., 2011; Seufferheld et al., 2011]. Their functions include storage of cations and polyphosphates, osmo-, pH- and Ca²⁺-homeostasis, and energy metabolism [Docampo et al., 2005]. They superficially resemble eukaryotic lysosomes in their sizes, acidic properties and contents [Moreno and Docampo, 2009].

Fig. 4

An acidocalcisome in an intact cell of Agrobacterium tumefaciens. Reproduced with permission from Docampo and Moreno [2011].

The ii symposium manufactures past Caetano-Anollés and Seufferheld are entitled 'The coevolutionary roots of biochemistry and cellular organization challenge the RNA globe prototype' and 'Phylogenomics supports a cellularly structured urancestor'. In the starting time of these 2 articles, these authors examine the origins and evolution of circuitous cellular structures past using phylogenomic approaches amidst others. These studies suggest to the authors that the final common universal ancestor of all extant living organisms on World, the urancestor, already had complex intracellular structures. They argue for the gradual coevolution of nucleic acids and proteins and discard the notion of an ancient RNA earth. In their second article, the authors hash out intracellular and extracellular compartments including acidocalcisomes and mitochondria. They consider the channeling of redox energy to satisfy the metabolic needs of Globe's earliest inhabitants. Thus, information technology is argued that the urancestor was relatively complex. The authors present molecular and microfossil show to support their claims. They extrapolate back iii.4 billion years, suggesting that primordial microbial communities were already in existence at that time. They thus advise that cellular compartmentalization and energy interconversion mechanisms were early inventions.

Final Remarks

The compendium of manufactures presented in this Journal of Molecular Microbiology and Biotechnology written symposium reveals the almost ubiquity of intracellular and extracellular membrane-bounded structures that serve unique functions in prokaryotes. Contempo research in Eastward. coli and other leaner suggests that ICMs may occur in a large range of bacteria that had previously been thought to lack such structures. Similarly, recognition that OMVs in Gram-negative leaner serve a plethora of interesting functions provides novel impetus to written report these structures in much greater detail. The recent decision that ICMs in E. coli, photosynthetic bacteria and magnetotactic bacteria may all derive from the plasma membrane past invagination leads to the heady possibility that chromatophore and magnetosome biogenesis may share mechanistic features with ICM formation in E. coli. This unifying consideration leads to the proposal that studies in the prokaryotic workhorse, E. coli, may prove to be applicative to organellar phenomena in other prokaryotes as well as eukaryotes.

The recent discovery of prokaryotic organelles similar to those in eukaryotes (i.east. nuclear envelopes, anammoxosomes and acidocalcisomes) has led some investigators to propose that the urancestor of the iii domains of life possessed some types of organelles. Whether true or non, the presence of these structures has far-reaching implications for our understanding of prokaryotic complexity. It also suggests new approaches to the study of organellar biology. Intracellular membrane differentiation in bacteria is probable to reveal novel unifying principles applicable to all forms of life on Earth.

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