Cell biology of prokaryotic organelles

Exploration and analytical techniques for membrane curvature-sensing proteins in bacteria

The mechanism by which cells regulate protein localization is an important topic in the field of bacterial biology. In certain instances, the morphology of the biological membrane has been demonstrated to function as a spatial cue for the subcellular localization of proteins. These proteins are capable of sensing membrane curvature and are involved in a number of physiological functions such as cytokinesis and the formation of membrane-bound organelles. This review presents recent advances in the in vitro evaluation of curvature-sensing properties using artificially controlled membranes and purified proteins, as well as microscopic live cell assays. However, these evaluation methodologies often require sophisticated experiments, and the number of identified curvature sensors remains limited. Thus, we present a comprehensive exploration of recently reported curvature-sensing proteins. Subsequently, we summarize the known curvature-sensing proteins in bacteria, in conjunction with the analytical methodologies employed in this field. Finally, future prospects and further requirements in the study of curvature-sensing proteins are discussed.

Membraneless Compartmentalization of Cell-Free Transcription-Translation by Polymer-Assisted Liquid-Liquid Phase Separation

Living cells use liquid-liquid phase separation (LLPS) to compartmentalize metabolic functions into mesoscopic-sized droplets. Deciphering the mechanisms at play in LLPS is therefore critical to understanding the structuration and functions of cells at the subcellular level. Although observed and achieved to a significant degree of control in vivo, the reconstitution of LLPS integrating advanced biological functions, such as gene expression, has been so far limited in vitro. LLPS of cell-free transcription-translation (TXTL) reactions require multi-step experimental approaches that lack biomimetic and have relatively poor efficacy, thus limiting their usage in cell-free engineered systems such as synthetic cells. Here the polymer-assisted LLPS of TXTL reactions are reported as the single-pot one-step compartmentalization of a model complex metabolic system obtain without using solvents or surfactants. LLPS occurs by adding the biocompatible polymers poly(ethylene glycol), poly(vinyl alcohol), and dextran to a TXTL reaction, that remains highly active. These polymers serve as partitioning agents that localize TXTL in mesoscopic-sized droplets rich in dextran. Cytoplasmic and membrane-interacting proteins are synthesized preferentially inside these droplets, and localize either uniformly or preferentially at the interface, depending on their nature. The LLPS-TXTL system presented in this work is a step toward the design of synthetic membraneless active organelles.

The landscape of lncRNAs in cell granules: Insights into their significance in cancer

Cellular compartmentalization, achieved through membrane-based compartments, is a fundamental aspect of cell biology that contributes to the evolutionary success of cells. While organelles have traditionally been the focus of research, membrane-less organelles (MLOs) are emerging as critical players, exhibiting distinct morphological features and unique molecular compositions. Recent research highlights the pivotal role of long noncoding RNAs (lncRNAs) in MLOs and their involvement in various cellular processes across different organisms. In the context of cancer, dysregulation of MLO formation, influenced by altered lncRNA expression, impacts chromatin organization, oncogenic transcription, signaling pathways, and telomere lengthening. This review synthesizes the current understanding of lncRNA composition within MLOs, delineating their functions and exploring how their dysregulation contributes to human cancers. Environmental challenges in tumorigenesis, such as nutrient deprivation and hypoxia, induce stress granules, promoting cancer cell survival and progression. Advancements in biochemical techniques, particularly single RNA imaging methods, offer valuable tools for studying RNA functions within live cells. However, detecting low-abundance lncRNAs remains challenging due to their limited expression levels. The correlation between lncRNA expression and pathological conditions, particularly cancer, should be explored, emphasizing the importance of single-cell studies for precise biomarker identification and the development of personalized therapeutic strategies. This article is categorized under: RNA Export and Localization > RNA Localization RNA in Disease and Development > RNA in Disease RNA Interactions with Proteins and Other Molecules > RNA-Protein Complexes.

Building Synthetic Cells─From the Technology Infrastructure to Cellular Entities

The de novo construction of a living organism is a compelling vision. Despite the astonishing technologies developed to modify living cells, building a functioning cell "from scratch" has yet to be accomplished. The pursuit of this goal alone has─and will─yield scientific insights affecting fields as diverse as cell biology, biotechnology, medicine, and astrobiology. Multiple approaches have aimed to create biochemical systems manifesting common characteristics of life, such as compartmentalization, metabolism, and replication and the derived features, evolution, responsiveness to stimuli, and directed movement. Significant achievements in synthesizing each of these criteria have been made, individually and in limited combinations. Here, we review these efforts, distinguish different approaches, and highlight bottlenecks in the current research. We look ahead at what work remains to be accomplished and propose a "roadmap" with key milestones to achieve the vision of building cells from molecular parts.

From Protein Film Electrochemistry to Nanoconfined Enzyme Cascades and the Electrochemical Leaf

Protein film electrochemistry (PFE) has given unrivalled insight into the properties of redox proteins and many electron-transferring enzymes, allowing investigations of otherwise ill-defined or intractable topics such as unstable Fe-S centers and the catalytic bias of enzymes. Many enzymes have been established to be reversible electrocatalysts when attached to an electrode, and further investigations have revealed how unusual dependences of catalytic rates on electrode potential have stark similarities with electronics. A special case, the reversible electrochemistry of a photosynthetic enzyme, ferredoxin-NADP⁺ reductase (FNR), loaded at very high concentrations in the 3D nanopores of a conducting metal oxide layer, is leading to a new technology that brings PFE to myriad enzymes of other classes, the activities of which become controlled by the primary electron exchange. This extension is possible because FNR-based recycling of NADP(H) can be coupled to a dehydrogenase, and thence to other enzymes linked in tandem by the tight channelling of cofactors and intermediates within the nanopores of the material. The earlier interpretations of catalytic wave-shapes and various analogies with electronics are thus extended to initiate a field perhaps aptly named "cascade-tronics", in which the flow of reactions along an enzyme cascade is monitored and controlled through an electrochemical analyzer. Unlike in photosynthesis where FNR transduces electron transfer and hydride transfer through the unidirectional recycling of NADPH, the "electrochemical leaf" (e-Leaf) can be used to drive reactions in both oxidizing and reducing directions. The e-Leaf offers a natural way to study how enzymes are affected by nanoconfinement and crowding, mimicking the physical conditions under which enzyme cascades operate in living cells. The reactions of the trapped enzymes, often at very high local concentration, are thus studied electrochemically, exploiting the potential domain to control rates and direction and the current-rate analogy to derive kinetic data. Localized NADP(H) recycling is very efficient, resulting in very high cofactor turnover numbers and new opportunities for controlling and exploiting biocatalysis.

The relevance of chromatin architecture to genome rearrangements in Drosophila

DNA within chromosomes in the nucleus is non-randomly organized into chromosome territories, compartments and topologically associated domains (TADs). Chromosomal rearrangements have the potential to alter chromatin organization and modify gene expression leading to selection against these structural variants. Drosophila pseudoobscura has a wealth of naturally occurring gene arrangements that were generated by overlapping inversion mutations caused by two chromosomal breaks that rejoin the central region in reverse order. Unlike humans, Drosophila inversion heterozygotes do not have negative effects associated with crossing over during meiosis because males use achiasmate mechanisms for proper segregation, and aberrant recombinant meiotic products generated in females are lost in polar bodies. As a result, Drosophila populations are found to harbour extensive inversion polymorphisms. It is not clear, however, whether chromatin architecture constrains which inversions breakpoints persist in populations. We mapped the breakpoints of seven inversions in D. pseudoobscura to the TAD map to determine if persisting inversion breakpoints are more likely to occur at boundaries between TADs. Our results show that breakpoints occur at TAD boundaries more than expected by chance. Some breakpoints may alter gene expression within TADs supporting the hypothesis that position effects contribute to inversion establishment. This article is part of the theme issue 'Genomic architecture of supergenes: causes and evolutionary consequences'.

Dynamic Reconfiguration of Subcompartment Architectures in Artificial Cells

Artificial cells are minimal structures constructed from biomolecular building blocks designed to mimic cellular processes, behaviors, and architectures. One near-ubiquitous feature of cellular life is the spatial organization of internal content. We know from biology that organization of content (including in membrane-bound organelles) is linked to cellular functions and that this feature is dynamic: the presence, location, and degree of compartmentalization changes over time. Vesicle-based artificial cells, however, are not currently able to mimic this fundamental cellular property. Here, we describe an artificial cell design strategy that addresses this technological bottleneck. We create a series of artificial cell architectures which possess multicompartment assemblies localized either on the inner or on the outer surface of the artificial cell membrane. Exploiting liquid-liquid phase separation, we can also engineer spatially segregated regions of condensed subcompartments attached to the cell surface, aligning with coexisting membrane domains. These structures can sense changes in environmental conditions and respond by reversibly transitioning from condensed multicompartment layers on the membrane surface to a dispersed state in the cell lumen, mimicking the dynamic compartmentalization found in biological cells. Likewise, we engineer exosome-like subcompartments that can be released to the environment. We can achieve this by using two types of triggers: chemical (addition of salts) and mechanical (by pulling membrane tethers using optical traps). These approaches allow us to control the compartmentalization state of artificial cells on population and single-cell levels.

Therapeutic Applications of Magnetotactic Bacteria and Magnetosomes: A Review Emphasizing on the Cancer Treatment

Magnetotactic bacteria (MTB) are aquatic microorganisms have the ability to biomineralize magnetosomes, which are membrane-enclosed magnetic nanoparticles. Magnetosomes are organized in a chain inside the MTB, allowing them to align with and traverse along the earth’s magnetic field. Magnetosomes have several potential applications for targeted cancer therapy when isolated from the MTB, including magnetic hyperthermia, localized medication delivery, and tumour monitoring. Magnetosomes features and properties for various applications outperform manufactured magnetic nanoparticles in several ways. Similarly, the entire MTB can be regarded as prospective agents for cancer treatment, thanks to their flagella’s ability to self-propel and the magnetosome chain’s ability to guide them. MTBs are conceptualized as nanobiots that can be guided and manipulated by external magnetic fields and are driven to hypoxic areas, such as tumor sites, while retaining the therapeutic and imaging characteristics of isolated magnetosomes. Furthermore, unlike most bacteria now being studied in clinical trials for cancer treatment, MTB are not pathogenic but might be modified to deliver and express certain cytotoxic chemicals. This review will assess the current and prospects of this burgeoning research field and the major obstacles that must be overcome before MTB can be successfully used in clinical treatments.

The cell as a bag of RNA

Genomic sequencing has provided insight into the genetic characterization of many organisms, and we are now seeing sequencing technologies turned towards phenotypic characterization of cells, tissues, and whole organisms. In particular, single-cell transcriptomic techniques are revolutionizing certain aspects of cell biology and enabling fundamental discoveries about cellular diversity, cell state, and cell type identity. I argue here that much of this progress depends on abstracting one's view of the cell to regard it as a 'bag of RNA'.

Biomolecular condensates in neurodegeneration and cancer

The intracellular environment is partitioned into functionally distinct compartments containing specific sets of molecules and reactions. Biomolecular condensates, also referred to as membrane-less organelles, are diverse and abundant cellular compartments that lack membranous enclosures. Molecules assemble into condensates by phase separation; multivalent weak interactions drive molecules to separate from their surroundings and concentrate in discrete locations. Biomolecular condensates exist in all eukaryotes and in some prokaryotes, and participate in various essential house-keeping, stress-response and cell type-specific processes. An increasing number of recent studies link abnormal condensate formation, composition and material properties to a number of disease states. In this review, we discuss current knowledge and models describing the regulation of condensates and how they become dysregulated in neurodegeneration and cancer. Further research on the regulation of biomolecular phase separation will help us to better understand their role in cell physiology and disease.

Prokaryotic and Mitochondrial Lipids: A Survey of Evolutionary Origins

Mitochondria and bacteria share a myriad of properties since it is believed that the powerhouses of the eukaryotic cell have evolved from a prokaryotic origin. Ribosomal RNA sequences, DNA architecture and metabolism are strikingly similar in these two entities. Proteins and nucleic acids have been a hallmark for comparison between mitochondria and prokaryotes. In this chapter, similarities (and differences) between mitochondrial and prokaryotic membranes are addressed with a focus on structure-function relationship of different lipid classes. In order to be suitable for the theme of the book, a special emphasis is reserved to the effects of bioactive sphingolipids, mainly ceramide, on mitochondrial membranes and their roles in initiating programmed cell death.

Comparative Genomic Analysis Reveals Novel Microcompartment-Associated Metabolic Pathways in the Human Gut Microbiome

Bacterial microcompartments are self-assembling subcellular structures surrounded by a semipermeable protein shell and found only in bacteria, but not archaea or eukaryotes. The general functions of the bacterial microcompartments are to concentrate enzymes, metabolites, and cofactors for multistep pathways; maintain the cofactor ratio; protect the cell from toxic metabolic intermediates; and protect the encapsulated pathway from unwanted side reactions. The bacterial microcompartments were suggested to play a significant role in organisms of the human gut microbiome, especially for various pathogens. Here, we used a comparative genomics approach to analyze the bacterial microcompartments in 646 individual genomes of organisms commonly found in the human gut microbiome. The bacterial microcompartments were found in 150 (23.2%) analyzed genomes. These microcompartments include previously known ones for the utilization of ethanolamine, 1,2-propanediol, choline, and fucose/rhamnose. Moreover, we reconstructed two novel pathways associated with the bacterial microcompartments. These pathways are catabolic pathways for the utilization of 1-amino-2-propanol/1-amino-2-propanone and xanthine. Remarkably, the xanthine utilization pathway does not demonstrate similarity to previously known microcompartment-associated pathways. Thus, we describe a novel type of bacterial microcompartment.

Construction and validation of the Tn5-PLtetO-1-msfGFP transposon as a tool to probe protein expression and localization

In this study we report the design, construction and validation of a novel transposon aimed to systematically screen for protein localization and expression patterns in prokaryotes using fluorescence microscopy. Upon random insertion in an open reading frame in the proper frame and orientation, the transposon creates an N-terminal fluorescent protein fusion to the msfGFP reporter. Moreover, in order to examine the localization of fusion proteins whose native expression might be too low or absent, the transposon was fitted with a P_LtetO-1 promoter that makes the expression of the generated fluorescent protein fusions controllable by anhydrotetracycline. Importantly, upon flipping out the P_LtetO-1 promoter and neighboring antibiotic resistance marker, an in-frame "sandwich" msfGFP fusion is created in which the N- and C-terminal portions of the targeted protein are again controlled by its native promoter.

How bacteria arrange their organelles

The structures responsible for photosynthesis in bacteria use the nucleoid and two unique proteins as a scaffold to position themselves.

Protein gradients on the nucleoid position the carbon-fixing organelles of cyanobacteria

Carboxysomes are protein-based bacterial organelles encapsulating key enzymes of the Calvin-Benson-Bassham cycle. Previous work has implicated a ParA-like protein (hereafter McdA) as important for spatially organizing carboxysomes along the longitudinal axis of the model cyanobacterium Synechococcus elongatus PCC 7942. Yet, how self-organization of McdA emerges and contributes to carboxysome positioning is unknown. Here, we identify a small protein, termed McdB that localizes to carboxysomes and drives emergent oscillatory patterning of McdA on the nucleoid. Our results demonstrate that McdB directly stimulates McdA ATPase activity and its release from DNA, driving carboxysome-dependent depletion of McdA locally on the nucleoid and promoting directed motion of carboxysomes towards increased concentrations of McdA. We propose that McdA and McdB are a previously unknown class of self-organizing proteins that utilize a Brownian-ratchet mechanism to position carboxysomes in cyanobacteria, rather than a cytoskeletal system. These results have broader implications for understanding spatial organization of protein mega-complexes and organelles in bacteria.

Cyanobacteria are tiny organisms that can harness the energy of the sun to power their cells. Many of the tools required for this complex photosynthetic process are packaged into small compartments inside the cell, the carboxysomes. In Synechococcus elongatus, a cyanobacterium that is shaped like a rod, the carboxysomes are positioned at regular intervals along the length of the cell. This ensures that, when the bacterium splits itself in half to reproduce, both daughter cells have the same number of carboxysomes.

Researchers know that, in S. elongatus, a protein called McdA can oscillate from one end of the cell to the other. This protein is responsible for the carboxysomes being in the right place, and some scientists believe that it helps to create an internal skeleton that anchors and drags the compartments into position.

Here, MacCready et al. propose another mechanism and, by combining various approaches, identify a new partner for McdA. This protein, called McdB, is present on the carboxysomes. McdB also binds to McdA, which itself attaches to the nucleoid – the region in the cell that contains the DNA. McdB forces McdA to release itself from DNA, causing the protein to reposition itself along the nucleoid. Because McdB attaches to McdA, the carboxysomes then follow suit, constantly seeking the highest concentrations of McdA bound to nearby DNA. Instead of relying on a cellular skeleton, these two proteins can organize themselves on their own using the nucleoid as a scaffold; in turn, they distribute carboxysomes evenly along the length of a cell.

Plants also obtain their energy from the sun via photosynthesis, but they do not carry carboxysomes. Scientists have tried to introduce these compartments inside plant cells, hoping that it could generate crops with higher yields. Knowing how carboxysomes are organized so they can be passed down from one generation to the next could be important for these experiments.

Applications of Magnetotactic Bacteria, Magnetosomes and Magnetosome Crystals in Biotechnology and Nanotechnology: Mini-Review

Magnetotactic bacteria (MTB) biomineralize magnetosomes, which are defined as intracellular nanocrystals of the magnetic minerals magnetite (Fe₃O₄) or greigite (Fe₃S₄) enveloped by a phospholipid bilayer membrane. The synthesis of magnetosomes is controlled by a specific set of genes that encode proteins, some of which are exclusively found in the magnetosome membrane in the cell. Over the past several decades, interest in nanoscale technology (nanotechnology) and biotechnology has increased significantly due to the development and establishment of new commercial, medical and scientific processes and applications that utilize nanomaterials, some of which are biologically derived. One excellent example of a biological nanomaterial that is showing great promise for use in a large number of commercial and medical applications are bacterial magnetite magnetosomes. Unlike chemically-synthesized magnetite nanoparticles, magnetosome magnetite crystals are stable single-magnetic domains and are thus permanently magnetic at ambient temperature, are of high chemical purity, and display a narrow size range and consistent crystal morphology. These physical/chemical features are important in their use in biotechnological and other applications. Applications utilizing magnetite-producing MTB, magnetite magnetosomes and/or magnetosome magnetite crystals include and/or involve bioremediation, cell separation, DNA/antigen recovery or detection, drug delivery, enzyme immobilization, magnetic hyperthermia and contrast enhancement of magnetic resonance imaging. Metric analysis using Scopus and Web of Science databases from 2003 to 2018 showed that applied research involving magnetite from MTB in some form has been focused mainly in biomedical applications, particularly in magnetic hyperthermia and drug delivery.

Organelle Formation in Bacteria and Archaea

Uncovering the mechanisms that underlie the biogenesis and maintenance of eukaryotic organelles is a vibrant and essential area of biological research. In comparison, little attention has been paid to the process of compartmentalization in bacteria and archaea. This lack of attention is in part due to the common misconception that organelles are a unique evolutionary invention of the "complex" eukaryotic cell and are absent from the "primitive" bacterial and archaeal cells. Comparisons across the tree of life are further complicated by the nebulous criteria used to designate subcellular structures as organelles. Here, with the aid of a unified definition of a membrane-bounded organelle, we present some of the recent findings in the study of lipid-bounded organelles in bacteria and archaea.

Tailoring lumazine synthase assemblies for bionanotechnology

The cage-forming protein lumazine synthase is readily modified, evolved and assembled with other components.

Preparation of Bacterial Magnetosomes for Proteome Analysis

Magnetotactic bacteria form unique prokaryotic organelles, termed magnetosomes, which consist of membrane-enclosed magnetite nanoparticles. Analysis of magnetosome biogenesis has been greatly facilitated by proteomic methods. These, however, require pure, highly enriched magnetosomes. Here, we describe the purification of magnetosomes from Magnetospirillum gryphiswaldense using high pressure cell disruption, and sequential purification by magnetic enrichment and sucrose density ultracentrifugation. The resulting enriched magnetosomes can be subsequently subjected to proteomic analyses or biotechnological applications.

Shedding light on cell compartmentation in the candidate phylum Poribacteria by high resolution visualisation and transcriptional profiling

Assigning functions to uncultivated environmental microorganisms continues to be a challenging endeavour. Here, we present a new microscopy protocol for fluorescence in situ hybridisation-correlative light and electron microscopy (FISH-CLEM) that enabled, to our knowledge for the first time, the identification of single cells within their complex microenvironment at electron microscopy resolution. Members of the candidate phylum Poribacteria, common and uncultivated symbionts of marine sponges, were used towards this goal. Cellular 3D reconstructions revealed bipolar, spherical granules of low electron density, which likely represent carbon reserves. Poribacterial activity profiles were retrieved from prokaryotic enriched sponge metatranscriptomes using simulation-based optimised mapping. We observed high transcriptional activity for proteins related to bacterial microcompartments (BMC) and we resolved their subcellular localisation by combining FISH-CLEM with immunohistochemistry (IHC) on ultra-thin sponge tissue sections. In terms of functional relevance, we propose that the BMC-A region may be involved in 1,2-propanediol degradation. The FISH-IHC-CLEM approach was proven an effective toolkit to combine -omics approaches with functional studies and it should be widely applicable in environmental microbiology.

Segregation of prokaryotic magnetosomes organelles is driven by treadmilling of a dynamic actin-like MamK filament

BACKGROUND

The navigation of magnetotactic bacteria relies on specific intracellular organelles, the magnetosomes, which are membrane-enclosed crystals of magnetite aligned into a linear chain. The magnetosome chain acts as a cellular compass, aligning the cells in the geomagnetic field in order to search for suitable environmental conditions in chemically stratified water columns and sediments. During cytokinesis, magnetosome chains have to be properly positioned, cleaved and separated in order to be evenly passed into daughter cells. In Magnetospirillum gryphiswaldense, the assembly of the magnetosome chain is controlled by the actin-like MamK, which polymerizes into cytoskeletal filaments that are connected to magnetosomes through the acidic MamJ protein. MamK filaments were speculated to recruit the magnetosome chain to cellular division sites, thus ensuring equal organelle inheritance. However, the underlying mechanism of magnetic organelle segregation has remained largely unknown.

RESULTS

Here, we performed in vivo time-lapse fluorescence imaging to directly track the intracellular movement and dynamics of magnetosome chains as well as photokinetic and ultrastructural analyses of the actin-like cytoskeletal MamK filament. We show that magnetosome chains undergo rapid intracellular repositioning from the new poles towards midcell into the newborn daughter cells, and the driving force for magnetosomes movement is likely provided by the pole-to-midcell treadmilling growth of MamK filaments. We further discovered that splitting and equipartitioning of magnetosome chains occurs with unexpectedly high accuracy, which depends directly on the dynamics of MamK filaments.

CONCLUSION

We propose a novel mechanism for prokaryotic organelle segregation that, similar to the type-II bacterial partitioning system of plasmids, relies on the action of cytomotive actin-like filaments together with specific connectors, which transport the magnetosome cargo in a fashion reminiscent of eukaryotic actin-organelle transport and segregation mechanisms.

Genetic and Ultrastructural Analysis Reveals the Key Players and Initial Steps of Bacterial Magnetosome Membrane Biogenesis

Magnetosomes of magnetotactic bacteria contain well-ordered nanocrystals for magnetic navigation and have recently emerged as the most sophisticated model system to study the formation of membrane bounded organelles in prokaryotes. Magnetosome biosynthesis is thought to begin with the formation of a dedicated compartment, the magnetosome membrane (MM), in which the biosynthesis of a magnetic mineral is strictly controlled. While the biomineralization of magnetosomes and their subsequent assembly into linear chains recently have become increasingly well studied, the molecular mechanisms and early stages involved in MM formation remained poorly understood. In the Alphaproteobacterium Magnetospirillum gryphiswaldense, approximately 30 genes were found to control magnetosome biosynthesis. By cryo-electron tomography of several key mutant strains we identified the gene complement controlling MM formation in this model organism. Whereas the putative magnetosomal iron transporter MamB was most crucial for the process and caused the most severe MM phenotype upon elimination, MamM, MamQ and MamL were also required for the formation of wild-type-like MMs. A subset of seven genes (mamLQBIEMO) combined within a synthetic operon was sufficient to restore the formation of intracellular membranes in the absence of other genes from the key mamAB operon. Tracking of de novo magnetosome membrane formation by genetic induction revealed that magnetosomes originate from unspecific cytoplasmic membrane locations before alignment into coherent chains. Our results indicate that no single factor alone is essential for MM formation, which instead is orchestrated by the cumulative action of several magnetosome proteins.

One of the most intriguing examples for membrane-bounded prokaryotic organelles are magnetosomes which consist of well-ordered chains of perfectly shaped magnetic nanocrystals that in many aquatic bacteria serve as geomagnetic field sensors to direct their swimming towards microoxic zones at the bottom of natural waters. In the model bacterium Magnetospirillum gryphiswaldense and related magnetotactic microorganisms, magnetosomes are formed by a complex pathway that is orchestrated by more than 30 genes. However, the initial and most crucial step of magnetosome biosynthesis, formation and differentiation of a dedicated intracellular membrane compartment for controlled biomineralization of magnetite crystals, remained only poorly understood. By ultrastructural analysis of several mutants and genetic induction of de novo magnetosome synthesis, we identified the key determinants and early steps of magnetosome membrane biogenesis. Our results suggest that formation of intracellular membranes in bacteria is mediated by a cumulative action of several factors, but apparently is differently controlled than intracellular membrane remodeling in eukaryotic cells.

Dynamic Remodeling of the Magnetosome Membrane Is Triggered by the Initiation of Biomineralization

Magnetotactic bacteria produce chains of membrane-bound organelles that direct the biomineralization of magnetic nanoparticles. These magnetosome compartments are a model for studying the biogenesis and subcellular organization of bacterial organelles. Previous studies have suggested that discrete gene products build and assemble magnetosomes in a stepwise fashion. Here, using an inducible system, we show that the stages of magnetosome formation are highly dynamic and interconnected. During de novo formation, magnetosomes first organize into discontinuous chain fragments that are subsequently connected by the bacterial actin-like protein MamK. We also find that magnetosome membranes are not uniform in size and can grow in a biomineralization-dependent manner. In the absence of biomineralization, magnetosome membranes stall at a diameter of ~50 nm. Those that have initiated biomineralization then expand to significantly larger sizes and accommodate mature magnetic particles. We speculate that such a biomineralization-dependent checkpoint for membrane growth establishes the appropriate conditions within the magnetosome to ensure successful nucleation and growth of magnetic particles.

Magnetotactic bacteria make magnetic nanoparticles inside membrane-bound organelles called magnetosomes; however, it is unclear how the magnetosome membrane controls the biomineralization that occurs within this bacterial organelle. We placed magnetosome formation under inducible control in Magnetospirillum magneticum AMB-1 and used electron cryo-tomography to capture magnetosomes in their near-native state as they form de novo. An inducible system provided the key evidence that magnetosome membranes grow continuously unless they have not properly initiated biomineralization. Our finding that the size of a bacterial organelle impacts its biochemical function is a fundamental advance that impacts our perception of organelle formation and can inform future attempts aimed at creating designer magnetic particles.

The origin and evolution of the acidocalcisome and its interactions with other organelles

Acidocalcisomes are acidic calcium stores that have been found from bacteria to human cells. They are rich in phosphorus compounds in the form of orthophosphate (P_i), pyrophosphate (PP_i), and polyphosphate (polyP) and their acidity is maintained by proton pumps such as the vacuolar proton pyrophosphatase (V-H⁺-PPase, or VP1), the vacuolar proton ATPase (V-H⁺-ATPase), or both. Recent studies in trypanosomatids and in other species have revealed their role in phosphate metabolism, and cation and water homeostasis, as suggested by the presence of novel pumps, transporters, and channels. An important role in autophagy has also been described. The study of the biogenesis of acidocalcisomes as well as of the interactions of these lysosome-related organelles with other organelles have uncovered important roles in calcium signaling and osmoregulation. Significantly, despite conservation of acidocalcisomes across all of cellular life, there is evidence for intimate integration of these organelles with eukaryotic cellular functions, and which are directly relevant to parasites.

Bacterial microcompartments: widespread prokaryotic organelles for isolation and optimization of metabolic pathways

Prokaryotes use subcellular compartments for a variety of purposes. An intriguing example is a family of complex subcellular organelles known as bacterial microcompartments (MCPs). MCPs are widely distributed among bacteria and impact processes ranging from global carbon fixation to enteric pathogenesis. Overall, MCPs consist of metabolic enzymes encased within a protein shell, and their function is to optimize biochemical pathways by confining toxic or volatile metabolic intermediates. MCPs are fundamentally different from other organelles in having a complex protein shell rather than a lipid-based membrane as an outer barrier. This unusual feature raises basic questions about organelle assembly, protein targeting and metabolite transport. In this review, we discuss the three best-studied MCPs highlighting atomic-level models for shell assembly, targeting sequences that direct enzyme encapsulation, multivalent proteins that organize the lumen enzymes, the principles of metabolite movement across the shell, internal cofactor recycling, a potential system of allosteric regulation of metabolite transport and the mechanism and rationale behind the functional diversification of the proteins that form the shell. We also touch on some potential biotechnology applications of an unusual compartment designed by nature to optimize metabolic processes within a cellular context.

Encapsulation as a Strategy for the Design of Biological Compartmentalization

Compartmentalization is one of the defining features of life. Through intracellular spatial control, cells are able to organize and regulate their metabolism. One of the most broadly used organizational principles in nature is encapsulation. Cellular processes can be encapsulated within either membrane-bound organelles or proteinaceous compartments that create distinct microenvironments optimized for a given task. Further challenges addressed through intracellular compartmentalization are toxic or volatile pathway intermediates, slow turnover rates and competing side reactions. This review highlights a selection of naturally occurring membrane- and protein-based encapsulation systems in microbes and their recent applications and emerging opportunities in synthetic biology. We focus on examples that use engineered cellular organization to control metabolic pathway flux for the production of useful compounds and materials.

Opposite and Coordinated Rotation of Amphitrichous Flagella Governs Oriented Swimming and Reversals in a Magnetotactic Spirillum

Current knowledge regarding the mechanism that governs flagellar motor rotation in response to environmental stimuli stems mainly from the study of monotrichous and peritrichous bacteria. Little is known about how two polar flagella, one at each cell pole of the so-called amphitrichous bacterium, are coordinated to steer the swimming. Here we fluorescently labeled the flagella of Magnetospirillum magneticum AMB-1 cells and took advantage of the magnetically controllable swimming of this bacterium to investigate flagellar rotation in moving cells. We identified three motility behaviors (runs, tumbles, and reversals) and two characteristic fluorescence patterns likely corresponding to flagella rotating in opposite directions. Each AMB-1 locomotion mode was systematically associated with particular flagellar patterns at the poles which led us to conclude that, while cell runs are allowed by the asymmetrical rotation of flagellar motors, their symmetrical rotation triggers cell tumbling. Our observations point toward a precise coordination of the two flagellar motors which can be temporarily unsynchronized during tumbling.

IMPORTANCE

Motility is essential for bacteria to search for optimal niches and survive. Many bacteria use one or several flagella to explore their environment. The mechanism by which bipolarly flagellated cells coordinate flagellar rotation is poorly understood. We took advantage of the genetic amenability and magnetically controlled swimming of the spirillum-shaped magnetotactic bacterium Magnetospirillum magneticum AMB-1 to correlate cell motion with flagellar rotation. We found that asymmetric rotation of the flagella (counterclockwise at the lagging pole and clockwise at the leading pole) enables cell runs whereas symmetric rotation triggers cell tumbling. Taking into consideration similar observations in spirochetes, bacteria possessing bipolar ribbons of periplasmic flagella, we propose a conserved motility paradigm for spirillum-shaped bipolarly flagellated bacteria.

An intracellular nanotrap redirects proteins and organelles in live bacteria

Owing to their small size and enhanced stability, nanobodies derived from camelids have previously been used for the construction of intracellular “nanotraps,” which enable redirection and manipulation of green fluorescent protein (GFP)-tagged targets within living plant and animal cells. By taking advantage of intracellular compartmentalization in the magnetic bacterium Magnetospirillum gryphiswaldense, we demonstrate that proteins and even entire organelles can be retargeted also within prokaryotic cells by versatile nanotrap technology. Expression of multivalent GFP-binding nanobodies on magnetosomes ectopically recruited the chemotaxis protein CheW₁-GFP from polar chemoreceptor clusters to the midcell, resulting in a gradual knockdown of aerotaxis. Conversely, entire magnetosome chains could be redirected from the midcell and tethered to one of the cell poles. Similar approaches could potentially be used for building synthetic cellular structures and targeted protein knockdowns in other bacteria.

Importance   Intrabodies are commonly used in eukaryotic systems for intracellular analysis and manipulation of proteins within distinct subcellular compartments. In particular, so-called nanobodies have great potential for synthetic biology approaches because they can be expressed easily in heterologous hosts and actively interact with intracellular targets, for instance, by the construction of intracellular “nanotraps” in living animal and plant cells. Although prokaryotic cells also exhibit a considerable degree of intracellular organization, there are few tools available equivalent to the well-established methods used in eukaryotes. Here, we demonstrate the ectopic retargeting and depletion of polar membrane proteins and entire organelles to distinct compartments in a magnetotactic bacterium, resulting in a gradual knockdown of magneto-aerotaxis. This intracellular nanotrap approach has the potential to be applied in other bacteria for building synthetic cellular structures, manipulating protein function, and creating gradual targeted knockdowns. Our findings provide a proof of principle for the universal use of fluorescently tagged proteins as targets for nanotraps to fulfill these tasks.

Intrabodies are commonly used in eukaryotic systems for intracellular analysis and manipulation of proteins within distinct subcellular compartments. In particular, so-called nanobodies have great potential for synthetic biology approaches because they can be expressed easily in heterologous hosts and actively interact with intracellular targets, for instance, by the construction of intracellular “nanotraps” in living animal and plant cells. Although prokaryotic cells also exhibit a considerable degree of intracellular organization, there are few tools available equivalent to the well-established methods used in eukaryotes. Here, we demonstrate the ectopic retargeting and depletion of polar membrane proteins and entire organelles to distinct compartments in a magnetotactic bacterium, resulting in a gradual knockdown of magneto-aerotaxis. This intracellular nanotrap approach has the potential to be applied in other bacteria for building synthetic cellular structures, manipulating protein function, and creating gradual targeted knockdowns. Our findings provide a proof of principle for the universal use of fluorescently tagged proteins as targets for nanotraps to fulfill these tasks.

Interplay between two bacterial actin homologs, MamK and MamK-Like, is required for the alignment of magnetosome organelles in Magnetospirillum magneticum AMB-1

Many bacterial species contain multiple actin-like proteins tasked with the execution of crucial cell biological functions. MamK, an actin-like protein found in magnetotactic bacteria, is important in organizing magnetosome organelles into chains that are used for navigation along geomagnetic fields. MamK and numerous other magnetosome formation factors are encoded by a genetic island termed the magnetosome island. Unlike most magnetotactic bacteria, Magnetospirillum magneticum AMB-1 (AMB-1) contains a second island of magnetosome-related genes that was named the magnetosome islet. A homologous copy of mamK, mamK-like, resides within this islet and encodes a protein capable of filament formation in vitro. Previous work had shown that mamK-like is expressed in vivo, but its function, if any, had remained unknown. Though MamK-like is highly similar to MamK, it contains a mutation that in MamK and other actins blocks ATPase activity in vitro and filament dynamics in vivo. Here, using genetic analysis, we demonstrate that mamK-like has an in vivo role in assisting organelle alignment. In addition, MamK-like forms filaments in vivo in a manner that is dependent on the presence of MamK and the two proteins interact in a yeast two-hybrid assay. Surprisingly, despite the ATPase active-site mutation, MamK-like is capable of ATP hydrolysis in vitro and promotes MamK filament turnover in vivo. Taken together, these experiments suggest that direct interactions between MamK and MamK-like contribute to magnetosome alignment in AMB-1.

Compartmentalization and organelle formation in bacteria

A number of bacterial species rely on compartmentalization to gain specific functionalities that will provide them with a selective advantage. Here, we will highlight several of these modes of bacterial compartmentalization with an eye toward describing the mechanisms of their formation and their evolutionary origins. Spore formation in Bacillus subtilis, outer membrane biogenesis in Gram-negative bacteria and protein diffusion barriers of Caulobacter crescentus will be used to demonstrate the physical, chemical, and compositional remodeling events that lead to compartmentalization. In addition, magnetosomes and carboxysomes will serve as models to examine the interplay between cytoskeletal systems and the subcellular positioning of organelles.

The FtsZ-like protein FtsZm of Magnetospirillum gryphiswaldense likely interacts with its generic homolog and is required for biomineralization under nitrate deprivation

Midcell selection, septum formation, and cytokinesis in most bacteria are orchestrated by the eukaryotic tubulin homolog FtsZ. The alphaproteobacterium Magnetospirillum gryphiswaldense (MSR-1) septates asymmetrically, and cytokinesis is linked to splitting and segregation of an intracellular chain of membrane-enveloped magnetite crystals (magnetosomes). In addition to a generic, full-length ftsZ gene, MSR-1 contains a truncated ftsZ homolog (ftsZm) which is located adjacent to genes controlling biomineralization and magnetosome chain formation. We analyzed the role of FtsZm in cell division and biomineralization together with the full-length MSR-1 FtsZ protein. Our results indicate that loss of FtsZm has a strong effect on microoxic magnetite biomineralization which, however, could be rescued by the presence of nitrate in the medium. Fluorescence microscopy revealed that FtsZm-mCherry does not colocalize with the magnetosome-related proteins MamC and MamK but is confined to asymmetric spots at midcell and at the cell pole, coinciding with the FtsZ protein position. In Escherichia coli, both FtsZ homologs form distinct structures but colocalize when coexpressed, suggesting an FtsZ-dependent recruitment of FtsZm. In vitro analyses indicate that FtsZm is able to interact with the FtsZ protein. Together, our data suggest that FtsZm shares key features with its full-length homolog but is involved in redox control for magnetite crystallization.

A bacterial tubulovesicular network

We report the presence of a membranous tubulovesicular network in the planctomycete bacterium Gemmata obscuriglobus. This endomembrane system interacts with membrane coat proteins and is capable of protein internalization and degradation. Taken together, this suggests that the planctomycetal bacterium could illuminate the emergence of complex endomembrane systems.

Co-ordinate synthesis and protein localization in a bacterial organelle by the action of a penicillin-binding-protein

Organelles with specialized form and function occur in diverse bacteria. Within the Alphaproteobacteria, several species extrude thin cellular appendages known as stalks, which function in nutrient uptake, buoyancy and reproduction. Consistent with their specialization, stalks maintain a unique molecular composition compared with the cell body, but how this is achieved remains to be fully elucidated. Here we dissect the mechanism of localization of StpX, a stalk-specific protein in Caulobacter crescentus. Using a forward genetics approach, we identify a penicillin-binding-protein, PbpC, which is required for the localization of StpX in the stalk. We show that PbpC acts at the stalked cell pole to anchor StpX to rigid components of the outer membrane of the elongating stalk, concurrent with stalk synthesis. Stalk-localized StpX in turn functions in cellular responses to copper and zinc, suggesting that the stalk may contribute to metal homeostasis in Caulobacter. Together, these results identify a novel role for a penicillin-binding-protein in compartmentalizing a bacterial organelle it itself helps create, raising the possibility that cell wall-synthetic enzymes may broadly serve not only to synthesize the diverse shapes of bacteria, but also to functionalize them at the molecular level.

Stably bridging a great divide: localization of the SpoIIQ landmark protein in Bacillus subtilis

Many bacterial proteins involved in fundamental processes such as cell shape maintenance, cell cycle regulation, differentiation, division and motility localize dynamically to specific subcellular regions. However, the mechanisms underlying dynamic protein localization are incompletely understood. Using the SpoIIQ protein in Bacillus subtilis as a case study, two reports present important novel insights into how a protein finds its right place at the right time and remains stably bound. During sporulation, SpoIIQ localizes in clusters in the forespore membrane at the interface that separates the forespore and mother cell and functions as a landmark protein for SpoIIIAH in the mother cell membrane. The extracellular domains of SpoIIQ and SpoIIIAH interact directly effectively bridging the gap between the two membranes. Here, SpoIIQ localization is shown to depend on two pathways, one involves SpoIIIAH, the second involves two peptidoglycan-degrading enzymes SpoIIP and SpoIID; and, SpoIIQ is only delocalized in the absence of all three proteins. Importantly, in the absence of SpoIIIAH, SpoIIQ apparently localizes normally. However, FRAP experiments demonstrated that SpoIIQ is not stably maintained in the clusters in this mutant. Thus, a second targeting pathway can mask significant changes in the localization of a protein.

Three-dimensional reconstruction of bacteria with a complex endomembrane system

The apparently complex membrane organization of Gemmata obscuriglobus, and probably all PVC superphylum members, comprises interconnected invaginations and is topologically identical to the “classical” Gram-negative bacterial membrane system.

The division of cellular space into functionally distinct membrane-defined compartments has been one of the major transitions in the history of life. Such compartmentalization has been claimed to occur in members of the Planctomycetes, Verrucomicrobiae, and Chlamydiae bacterial superphylum. Here we have investigated the three-dimensional organization of the complex endomembrane system in the planctomycete bacteria Gemmata obscuriglobus. We reveal that the G. obscuriglobus cells are neither compartmentalized nor nucleated as none of the spaces created by the membrane invaginations are closed; instead, they are all interconnected. Thus, the membrane organization of G. obscuriglobus, and most likely all PVC members, is not different from, but an extension of, the “classical” Gram-negative bacterial membrane system. Our results have implications for our definition and understanding of bacterial cell organization, the genesis of complex structure, and the origin of the eukaryotic endomembrane system.

The compartmentalization of cellular space has been an important evolutionary innovation, allowing for the functional specialization of cellular space. This compartmentalization is extensively developed in eukaryotes and although not as complex and developed, compartments with specialized function are known to occur in bacteria and can be surprisingly sophisticated. Nevertheless, members of the Planctomycetes, Verrucomicrobiae, and Chlamydiae (PVC) bacterial superphylum are exceptional in displaying diverse and extensive intracellular membranous organization. We investigated the three-dimensional organization of the complex endomembrane system in the planctomycete bacterium Gemmata obscuriglobus. We reveal that the G. obscuriglobus cells are neither compartmentalized nor nucleated, contrary to previous claims, as none of the spaces created by the membrane invaginations is topologically closed; instead, they are all interconnected. The organization of cellular space is similar to that of a classical Gram-negative bacterium modified by the presence of large invaginations of the inner membrane inside the cytoplasm. Thus, the membrane organization of G. obscuriglobus, and most likely all PVC members, is not fundamentally different from, but is rather an extension of, the “classical” Gram-negative bacterial membrane system.

Functional advantages conferred by extracellular prokaryotic membrane vesicles

The absence of subcellular organelles is a characteristic typically used to distinguish prokaryotic from eukaryotic cells. But recent discoveries do not support this dogma. Over the past 50 years, researchers have begun to appreciate and characterize Gram-negative bacterial outer membrane-derived vesicles and Gram-positive and archaeal membrane vesicles. These extracellular, membrane-bound organelles can perform a variety of functions, including binding and delivery of DNA, transport of virulence factors, protection of the cell from outer membrane targeting antimicrobials and ridding the cell of toxic envelope proteins. Here, we review the contributions of these extracellular organelles to prokaryotic physiology and compare these with the contributions of the bacterial interior membrane-bound organelles responsible for harvesting light energy and for generating magnetic crystals of heavy metals. Understanding the roles of these multifunctional extracellular vesicle organelles as microbial tools will help us to better realize the diverse interactions that occur in our polymicrobial world.

Tiny cells meet big questions: a closer look at bacterial cell biology

While studying actin assembly as a graduate student with Matt Welch at the University of California at Berkeley, my interest was piqued by reports of surprising observations in bacteria: the identification of numerous cytoskeletal proteins, actin homologues fulfilling spindle-like functions, and even the presence of membrane-bound organelles. Curiosity about these phenomena drew me to Lucy Shapiro's lab at Stanford University for my postdoctoral research. In the Shapiro lab, and now in my lab at Johns Hopkins, I have focused on investigating the mechanisms of bacterial cytokinesis. Spending time as both a eukaryotic cell biologist and a bacterial cell biologist has convinced me that bacterial cells present the same questions as eukaryotic cells: How are chromosomes organized and accurately segregated? How is force generated for cytokinesis? How is polarity established? How are signals transduced within and between cells? These problems are conceptually similar between eukaryotes and bacteria, although their solutions can differ significantly in specifics. In this Perspective, I provide a broad view of cell biological phenomena in bacteria, the technical challenges facing those of us who peer into bacterial cells, and areas of common ground as research in eukaryotic and bacterial cell biology moves forward.

Evolution of intracellular compartmentalization

Cells compartmentalize their biochemical functions in a variety of ways, notably by creating physical barriers that separate a compartment via membranes or proteins. Eukaryotes have a wide diversity of membrane-based compartments, many that are lineage- or tissue-specific. In recent years, it has become increasingly evident that membrane-based compartmentalization of the cytosolic space is observed in multiple prokaryotic lineages, giving rise to several types of distinct prokaryotic organelles. Endosymbionts, previously believed to be a hallmark of eukaryotes, have been described in several bacteria. Protein-based compartments, frequent in bacteria, are also found in eukaryotes. In the present review, we focus on selected intracellular compartments from each of these three categories, membrane-based, endosymbiotic and protein-based, in both prokaryotes and eukaryotes. We review their diversity and the current theories and controversies regarding the evolutionary origins. Furthermore, we discuss the evolutionary processes acting on the genetic basis of intracellular compartments and how those differ across the domains of life. We conclude that the distinction between eukaryotes and prokaryotes no longer lies in the existence of a compartmentalized cell plan, but rather in its complexity.

General protein diffusion barriers create compartments within bacterial cells

In eukaryotes, the differentiation of cellular extensions such as cilia or neuronal axons depends on the partitioning of proteins to distinct plasma membrane domains by specialized diffusion barriers. However, examples of this compartmentalization strategy are still missing for prokaryotes, although complex cellular architectures are also widespread among this group of organisms. This study reveals the existence of a protein-mediated membrane diffusion barrier in the stalked bacterium Caulobacter crescentus. We show that the Caulobacter cell envelope is compartmentalized by macromolecular complexes that prevent the exchange of both membrane and soluble proteins between the polar stalk extension and the cell body. The barrier structures span the cross-sectional area of the stalk and comprise at least four proteins that assemble in a cell-cycle-dependent manner. Their presence is critical for cellular fitness because they minimize the effective cell volume, allowing faster adaptation to environmental changes that require de novo synthesis of envelope proteins.

Untangling the evolution of Rab G proteins: implications of a comprehensive genomic analysis

BACKGROUND

Membrane-bound organelles are a defining feature of eukaryotic cells, and play a central role in most of their fundamental processes. The Rab G proteins are the single largest family of proteins that participate in the traffic between organelles, with 66 Rabs encoded in the human genome. Rabs direct the organelle-specific recruitment of vesicle tethering factors, motor proteins, and regulators of membrane traffic. Each organelle or vesicle class is typically associated with one or more Rab, with the Rabs present in a particular cell reflecting that cell's complement of organelles and trafficking routes.

RESULTS

Through iterative use of hidden Markov models and tree building, we classified Rabs across the eukaryotic kingdom to provide the most comprehensive view of Rab evolution obtained to date. A strikingly large repertoire of at least 20 Rabs appears to have been present in the last eukaryotic common ancestor (LECA), consistent with the 'complexity early' view of eukaryotic evolution. We were able to place these Rabs into six supergroups, giving a deep view into eukaryotic prehistory.

CONCLUSIONS

Tracing the fate of the LECA Rabs revealed extensive losses with many extant eukaryotes having fewer Rabs, and none having the full complement. We found that other Rabs have expanded and diversified, including a large expansion at the dawn of metazoans, which could be followed to provide an account of the evolutionary history of all human Rabs. Some Rab changes could be correlated with differences in cellular organization, and the relative lack of variation in other families of membrane-traffic proteins suggests that it is the changes in Rabs that primarily underlies the variation in organelles between species and cell types.

Physical interaction between coat morphogenetic proteins SpoVID and CotE is necessary for spore encasement in Bacillus subtilis

Endospore formation by Bacillus subtilis is a complex and dynamic process. One of the major challenges of sporulation is the assembly of a protective, multilayered, proteinaceous spore coat, composed of at least 70 different proteins. Spore coat formation can be divided into two distinct stages. The first is the recruitment of proteins to the spore surface, dependent on the morphogenetic protein SpoIVA. The second step, known as encasement, involves the migration of the coat proteins around the circumference of the spore in successive waves, a process dependent on the morphogenetic protein SpoVID and the transcriptional regulation of individual coat genes. We provide genetic and biochemical evidence supporting the hypothesis that SpoVID promotes encasement of the spore by establishing direct protein-protein interactions with other coat morphogenetic proteins. It was previously demonstrated that SpoVID directly interacts with SpoIVA and the inner coat morphogenetic protein, SafA. Here, we show by yeast two-hybrid and pulldown assays that SpoVID also interacts directly with the outer coat morphogenetic protein, CotE. Furthermore, by mutational analysis, we identified a specific residue in the N-terminal domain of SpoVID that is essential for the interaction with CotE but dispensable for the interaction with SafA. We propose an updated model of coat assembly and spore encasement that incorporates several physical interactions between the principal coat morphogenetic proteins.

Biogenesis and subcellular organization of the magnetosome organelles of magnetotactic bacteria

Bacterial cells, like their eukaryotic counterparts, are capable of constructing lipid-based organelles that carry out essential biochemical functions. The magnetosomes of magnetotactic bacteria are one such compartment that is quickly becoming a model for exploring the process of organelle biogenesis in bacteria. Magnetosomes consist of a lipid-bilayer compartment that houses a magnetic crystal. By arranging magnetosomes into chains within the cell, magnetotactic bacteria create an internal compass that is used for navigation along magnetic fields. Over the past decade, a number of studies have elucidated the possible factors involved in the formation of the magnetosome membrane and biomineralization of magnetic minerals. Here, we highlight some of these recent advances with a particular focus on the cell biology of magnetosome formation.

Analysis of the CtrA pathway in Magnetospirillum reveals an ancestral role in motility in alphaproteobacteria

Developmental events across the prokaryotic life cycle are highly regulated at the transcriptional and posttranslational levels. Key elements of a few regulatory networks are conserved among phylogenetic groups of bacteria, although the features controlled by these conserved systems are as diverse as the organisms encoding them. In this work, we probed the role of the CtrA regulatory network, conserved throughout the Alphaproteobacteria, in the magnetotactic bacterium Magnetospirillum magneticum strain AMB-1, which possesses unique intracellular organization and compartmentalization. While we have shown that CtrA in AMB-1 is not essential for viability, it is required for motility, and its putative phosphorylation state dictates the ability of CtrA to activate the flagellar biosynthesis gene cascade. Gene expression analysis of strains expressing active and inactive CtrA alleles points to the composition of the extended CtrA regulon, including both direct and indirect targets. These results, combined with a bioinformatic study of the AMB-1 genome, enabled the prediction of an AMB-1-specific CtrA binding site. Further, phylogenetic studies comparing CtrA sequences from alphaproteobacteria in which the role of CtrA has been experimentally examined reveal an ancestral role of CtrA in the regulation of motility and suggest that its essential functions in other alphaproteobacteria were acquired subsequently.