The Peptidoglycan-Binding Protein SjcF1 Influences Septal Junction Function and Channel Formation in the Filamentous Cyanobacterium Anabaena

SepT, a novel protein specific to multicellular cyanobacteria, influences peptidoglycan growth and septal nanopore formation in Anabaena sp. PCC 7120

IMPORTANCE

Multicellular organization is a requirement for the development of complex organisms, and filamentous cyanobacteria such as Anabaena represent a paradigmatic case of bacterial multicellularity. The Anabaena filament can include hundreds of communicated cells that exchange nutrients and regulators and, depending on environmental conditions, can include different cell types specialized in distinct biological functions. Hence, the specific features of the Anabaena filament and how they are propagated during cell division represent outstanding biological issues. Here, we studied SepT, a novel coiled-coil-rich protein of Anabaena that is located in the intercellular septa and influences the formation of the septal specialized structures that allow communication between neighboring cells along the filament, a fundamental trait for the performance of Anabaena as a multicellular organism.

SepN is a septal junction component required for gated cell-cell communication in the filamentous cyanobacterium Nostoc

Multicellular organisms require controlled intercellular communication for their survival. Strains of the filamentous cyanobacterium Nostoc regulate cell-cell communication between sister cells via a conformational change in septal junctions. These multi-protein cell junctions consist of a septum spanning tube with a membrane-embedded plug at both ends, and a cap covering the plug on the cytoplasmic side. The identities of septal junction components are unknown, with exception of the protein FraD. Here, we identify and characterize a FraD-interacting protein, SepN, as the second component of septal junctions in Nostoc. We use cryo-electron tomography of cryo-focused ion beam-thinned cyanobacterial filaments to show that septal junctions in a sepN mutant lack a plug module and display an aberrant cap. The sepN mutant exhibits highly reduced cell-cell communication rates, as shown by fluorescence recovery after photobleaching experiments. Furthermore, the mutant is unable to gate molecule exchange through septal junctions and displays reduced filament survival after stress. Our data demonstrate the importance of controlling molecular diffusion between cells to ensure the survival of a multicellular organism.

Kinopus chlorellivorus gen. nov., sp. nov. (Vampyrellida, Rhizaria), a New Algivorous Protist Predator Isolated from Large-Scale Outdoor Cultures of Chlorella sorokiniana

The vampyrellids (Vampyrellida, Rhizaria) are a major group of predatory amoebae that have attracted significant attention because of their diversity of feeding strategies. The crucial roles they play in important processes such as suppressing soil disease and controlling aquatic algae, and as microbial contaminants in outdoor large-scale algal cultures, have also received increasing attention.

The Making of a Heterocyst in Cyanobacteria

Heterocyst differentiation that occurs in some filamentous cyanobacteria, such as Anabaena sp. PCC 7120, provides a unique model for prokaryotic developmental biology. Heterocyst cells are formed in response to combined-nitrogen deprivation and possess a microoxic environment suitable for nitrogen fixation following extensive morphological and physiological reorganization. A filament of Anabaena is a true multicellular organism, as nitrogen and carbon sources are exchanged among different cells and cell types through septal junctions to ensure filament growth. Because heterocysts are terminally differentiated cells and unable to divide, their activity is an altruistic behavior dedicated to providing fixed nitrogen for neighboring vegetative cells. Heterocyst development is also a process of one-dimensional pattern formation, as heterocysts are semiregularly intercalated among vegetative cells. Morphogens form gradients along the filament and interact with each other in a fashion that fits well into the Turing model, a mathematical framework to explain biological pattern formation. Expected final online publication date for the Annual Review of Microbiology, Volume 76 is September 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

A TonB-Like Protein, SjdR, Is Involved in the Structural Definition of the Intercellular Septa in the Heterocyst-Forming Cyanobacterium Anabaena

Cyanobacteria are photosynthetic organisms with a Gram-negative envelope structure. Certain filamentous species such as Anabaena sp. strain PCC 7120 can fix dinitrogen upon depletion of combined nitrogen. Because the nitrogen-fixing enzyme, nitrogenase, is oxygen sensitive, photosynthesis and nitrogen fixation are spatially separated in Anabaena. Nitrogen fixation takes place in specialized cells called heterocysts, which differentiate from vegetative cells. During heterocyst differentiation, a microoxic environment is created by dismantling photosystem II and restructuring the cell wall. Moreover, solute exchange between the different cell types is regulated to limit oxygen influx into the heterocyst. The septal zone containing nanopores for solute exchange is constricted between heterocysts and vegetative cells, and cyanophycin plugs are located at the heterocyst poles. We identified a protein previously annotated as TonB1 that is largely conserved among cyanobacteria. A mutant of the encoding gene formed heterocysts but was impaired in diazotrophic growth. Mutant heterocysts appeared elongated and exhibited abnormal morphological features, including a reduced cyanophycin plug, an enhanced septum size, and a restricted nanopore zone in the septum. In spite of this, the intercellular transfer velocity of the fluorescent marker calcein was increased in the mutant compared to the wild type. Thus, the protein is required for proper formation of septal structures, expanding our emerging understanding of Anabaena peptidoglycan plasticity and intercellular solute exchange, and is therefore renamed SjdR (septal junction disk regulator). Notably, calcium supplementation compensated for the impaired diazotrophic growth and alterations in septal peptidoglycan in the sjdR mutant, emphasizing the importance of calcium for cell wall structure. IMPORTANCE Multicellularity in bacteria confers an improved adaptive capacity to environmental conditions and stresses. This includes an enhanced capability of resource utilization through a distribution of biochemical processes between constituent cells. This specialization results in a mutual dependency of different cell types, as is the case for nitrogen-fixing heterocysts and photosynthetically active vegetative cells in Anabaena. In this cyanobacterium, intercellular solute exchange is facilitated through nanopores in the peptidoglycan between adjacent cells. To ensure functionality of the specialized cells, septal size as well as the position, size, and frequency of nanopores in the septum need to be tightly established. The novel septal junction disk regulator SjdR characterized here is conserved in the cyanobacterial phylum. It influences septal size and septal nanopore distribution. Consequently, its absence severely affects the intercellular communication and the strains' growth capacity under nitrogen depletion. Thus, SjdR is involved in septal structure remodeling in cyanobacteria.

Heterocyst Septa Contain Large Nanopores That Are Influenced by the Fra Proteins in the Filamentous Cyanobacterium Anabaena sp. Strain PCC 7120

Multicellular heterocyst-forming cyanobacteria, such as Anabaena, grow as chains of cells forming filaments that, under diazotrophic conditions, contain two cell types: vegetative cells that perform oxygenic photosynthesis and N₂-fixing heterocysts. Along the filament, the intercellular septa contain a thick peptidoglycan layer that forms septal disks. Proteinaceous septal junctions connect the cells in the filament traversing the septal disks through nanopores. The fraCDE operon encodes proteins needed to make long filaments in Anabaena. FraC and FraD, located at the intercellular septa, are involved in the formation of septal junctions. Using a superfolder-green fluorescent protein (GFP) fusion, we found in this study that FraE is mainly localized to the poles of the heterocysts, consistent with the requirement of FraE for constriction of the heterocyst poles to form the "heterocyst neck." A fraE insertional mutant was impaired by 22% to 38% in transfer of fluorescent calcein from vegetative cells to heterocysts. Septal disks were inspected in murein sacculi from heterocyst-enriched preparations. Unexpectedly, the diameter of the nanopores in heterocyst septa was about 1.5- to 2-fold larger than in vegetative cell septa. The number of these nanopores was 76% and 6% of the wild-type number in fraE and fraC fraD mutants, respectively. Our results show that FraE is mainly involved in heterocyst maturation, whereas FraC and FraD are needed for the formation of the large nanopores of heterocyst septa, as they are for vegetative cell nanopores. Additionally, arrays of small pores conceivably involved in polysaccharide export were observed close to the septal disks in the heterocyst murein sacculus preparations. IMPORTANCE Intercellular communication, an essential attribute of multicellularity, is required for diazotrophic growth in heterocyst-forming cyanobacteria such as Anabaena, in which the cells are connected by proteinaceous septal junctions that are structural analogs of metazoan connexons. The septal junctions allow molecular intercellular diffusion traversing the septal peptidoglycan through nanopores. In Anabaena the fraCDE operon encodes septal proteins involved in intercellular communication. FraC and FraD are components of the septal junctions along the filament, whereas here we show that FraE is mainly present at the heterocyst poles. We found that the intercellular septa in murein sacculi from heterocysts contain nanopores that are larger than those in vegetative cells, establishing a previously unknown difference between heterocyst and vegetative cell septa in Anabaena.

Cell-cell communication through septal junctions in filamentous cyanobacteria

Septal junctions are cell-cell connections that mediate intercellular communication in filamentous cyanobacteria. The septal peptidoglycan is perforated by dozens of 20 nm-wide nanopores, through which these proteinaceous structures traverse, physically connecting adjacent cells. On each cytoplasmic side, every septal junction contains a flexible cap structure that closes the connection in a reversible manner upon stress. This gating mechanism reminds of the gap junctions from metazoans and represents a primordial control system for cell-cell communication. In this review, we summarize the knowledge about formation of the nanopore array as the framework for incorporation of cell-cell connecting septal junctions. Furthermore, the architecture of septal junctions, proteins involved in septal junction constitution and regulation of intercellular communication will be addressed.

Coexistence of Communicating and Noncommunicating Cells in the Filamentous Cyanobacterium Anabaena

Multicellularity is found in bacteria as well as in eukaryotes, and the filamentous heterocyst-forming (N₂-fixing) cyanobacteria represent a simple and ancient paradigm of multicellular organisms. Multicellularity generally involves cell-cell adhesion and communication.

In filamentous heterocyst-forming (N₂-fixing) cyanobacteria, septal junctions join adjacent cells, mediating intercellular communication, and are thought to traverse the septal peptidoglycan through nanopores. Fluorescence recovery after photobleaching (FRAP) analysis with the fluorescent marker calcein showed that cultures of Anabaena sp. strain PCC 7120 grown in the presence of combined nitrogen contained a substantial fraction of noncommunicating cells (58% and 80% of the tested vegetative cells in nitrate- and ammonium-grown cultures, respectively), whereas cultures induced for nitrogen fixation contained far fewer noncommunicating cells (16%). A single filament could have communicating and noncommunicating cells. These observations indicate that all (or most of) the septal junctions in a cell can be coordinately regulated and are coherent with the need for intercellular communication, especially under diazotrophic conditions. Consistently, intercellular exchange was observed to increase in response to N deprivation and to decrease rapidly in response to the presence of ammonium in the medium or to nitrate assimilation. Proteins involved in the formation of septal junctions have been identified in Anabaena and include SepJ, FraC, and FraD. Here, we reevaluated rates of intercellular transfer of calcein and the number of nanopores in mutants lacking these proteins and found a strong positive correlation between the two parameters only in cultures induced for nitrogen fixation. Thus, whereas the presence of a substantial number of noncommunicating cells appears to impair the correlation, data obtained in diazotrophic cultures support the idea that the nanopores are the structures that hold the septal junctions.

IMPORTANCE Multicellularity is found in bacteria as well as in eukaryotes, and the filamentous heterocyst-forming (N₂-fixing) cyanobacteria represent a simple and ancient paradigm of multicellular organisms. Multicellularity generally involves cell-cell adhesion and communication. The cells in the cyanobacterial filaments are joined by proteinaceous septal junctions that mediate molecular diffusion. The septal junctions traverse the septal peptidoglycan, which bears holes termed nanopores. Our results show that the septal junctions can be coordinately regulated in a cell and emphasize the relationship between septal junctions and nanopores to build intercellular communication structures, which are essential for the multicellular behavior of heterocyst-forming cyanobacteria.

Structural Determinants and Their Role in Cyanobacterial Morphogenesis

Cells have to erect and sustain an organized and dynamically adaptable structure for an efficient mode of operation that allows drastic morphological changes during cell growth and cell division. These manifold tasks are complied by the so-called cytoskeleton and its associated proteins. In bacteria, FtsZ and MreB, the bacterial homologs to tubulin and actin, respectively, as well as coiled-coil-rich proteins of intermediate filament (IF)-like function to fulfil these tasks. Despite generally being characterized as Gram-negative, cyanobacteria have a remarkably thick peptidoglycan layer and possess Gram-positive-specific cell division proteins such as SepF and DivIVA-like proteins, besides Gram-negative and cyanobacterial-specific cell division proteins like MinE, SepI, ZipN (Ftn2) and ZipS (Ftn6). The diversity of cellular morphologies and cell growth strategies in cyanobacteria could therefore be the result of additional unidentified structural determinants such as cytoskeletal proteins. In this article, we review the current advances in the understanding of the cyanobacterial cell shape, cell division and cell growth.

Two novel heteropolymer-forming proteins maintain the multicellular shape of the cyanobacterium Anabaena sp. PCC 7120

Polymerizing and filament-forming proteins are instrumental for numerous cellular processes such as cell division and growth. Their function in stabilization and localization of protein complexes and replicons is achieved by a filamentous structure. Known filamentous proteins assemble into homopolymers consisting of single subunits - for example, MreB and FtsZ in bacteria - or heteropolymers that are composed of two subunits, for example, keratin and α/β tubulin in eukaryotes. Here, we describe two novel coiled-coil-rich proteins (CCRPs) in the filament-forming cyanobacterium Anabaena sp. PCC 7120 (hereafter Anabaena) that assemble into a heteropolymer and function in the maintenance of the Anabaena multicellular shape (termed trichome). The two CCRPs - Alr4504 and Alr4505 (named ZicK and ZacK) - are strictly interdependent for the assembly of protein filaments in vivo and polymerize nucleotide independently in vitro, similar to known intermediate filament (IF) proteins. A ΔzicKΔzacK double mutant is characterized by a zigzagged cell arrangement and hence a loss of the typical linear Anabaena trichome shape. ZicK and ZacK interact with themselves, with each other, with the elongasome protein MreB, the septal junction protein SepJ and the divisome associate septal protein SepI. Our results suggest that ZicK and ZacK function in cooperation with SepJ and MreB to stabilize the Anabaena trichome and are likely essential for the manifestation of the multicellular shape in Anabaena. Our study reveals the presence of filament-forming IF-like proteins whose function is achieved through the formation of heteropolymers in cyanobacteria.

The role of the cytoskeletal proteins MreB and FtsZ in multicellular cyanobacteria

Multiseriate and true‐branching cyanobacteria are at the peak of prokaryotic morphological complexity. However, little is known about the mechanisms governing multiplanar cell division and morphogenesis. Here, we study the function of the prokaryotic cytoskeletal proteins, MreB and FtsZ in Fischerella muscicola PCC 7414 and Chlorogloeopsis fritschii PCC 6912. Vancomycin and HADA labeling revealed a mixed apical, septal, and lateral trichome growth mode in F. muscicola, whereas C. fritschii exhibits septal growth. In all morphotypes from both species, MreB forms either linear filaments or filamentous strings and can interact with FtsZ. Furthermore, multiplanar cell division in F. muscicola likely depends on FtsZ dosage. Our results lay the groundwork for future studies on cytoskeletal proteins in morphologically complex cyanobacteria.

Structure and Function of a Bacterial Gap Junction Analog

Multicellular lifestyle requires cell-cell connections. In multicellular cyanobacteria, septal junctions enable molecular exchange between sister cells and are required for cellular differentiation. The structure of septal junctions is poorly understood, and it is unknown whether they are capable of controlling intercellular communication. Here, we resolved the in situ architecture of septal junctions by electron cryotomography of cryo-focused ion beam-milled cyanobacterial filaments. Septal junctions consisted of a tube traversing the septal peptidoglycan. Each tube end comprised a FraD-containing plug, which was covered by a cytoplasmic cap. Fluorescence recovery after photobleaching showed that intercellular communication was blocked upon stress. Gating was accompanied by a reversible conformational change of the septal junction cap. We provide the mechanistic framework for a cell junction that predates eukaryotic gap junctions by a billion years. The conservation of a gated dynamic mechanism across different domains of life emphasizes the importance of controlling molecular exchange in multicellular organisms.

Cyanobacterial Septal Junctions: Properties and Regulation

Heterocyst-forming cyanobacteria are multicellular organisms that grow as chains of cells (filaments or trichomes) in which the cells exchange regulators and nutrients. In this article, we review the morphological, physiological and genetic data that have led to our current understanding of intercellular communication in these organisms. Intercellular molecular exchange appears to take place by simple diffusion through proteinaceous structures, known as septal junctions, which connect the adjacent cells in the filament and traverse the septal peptidoglycan through perforations known as nanopores. Proteins that are necessary to produce, and that may be components of, the septal junctions-SepJ, FraC and FraD-have been identified in the heterocyst-forming cyanobacterium Anabaena sp. strain PCC 7120 model. Additionally, several proteins that are necessary to produce a normal number of nanopores and functional septal junctions have been identified, including AmiC-type amidases, peptidoglycan-binding proteins and some membrane transporters. Available reports and reevaluation of intercellular molecular transfer data for some mutants of Anabaena suggest that the septal junctions can be regulated, likely by a mechanism of gating.

Specific mutations in the permease domain of septal protein SepJ differentially affect functions related to multicellularity in the filamentous cyanobacterium Anabaena

Filamentous, heterocyst-forming cyanobacteria are multicellular organisms in which growth requires the activity of two interdependent cell types that exchange nutrients and regulators. Vegetative cells provide heterocysts with reduced carbon, and heterocysts provide vegetative cells with fixed nitrogen. Additionally, heterocyst differentiation from vegetative cells is regulated by inhibitors of differentiation produced by prospective heterocysts and heterocysts. Proteinaceous structures known as septal junctions join the cells in the filament. The SepJ protein is involved in formation of septal junctions in the model heterocyst-forming cyanobacterium Anabaena sp. strain PCC 7120. SepJ bears extra-membrane and membrane (permease) domains and is located at the cell poles in the intercellular septa of the filament. Here we created Anabaena mutants that produce SepJ proteins altered in the permease domain. Some of these mutant SepJ proteins did not provide functions needed for Anabaena to form long filaments and (in some cases) differentiate heterocysts, identifying amino acids and amino acid stretches that are important for the structure or function of the protein. Some other mutant SepJ proteins fulfilled filamentation and heterocyst differentiation functions but failed to provide normal communication function assessed via the intercellular transfer of the fluorescent marker calcein. These mutant SepJ proteins bore mutations in amino acids located at the cytoplasmic face of the permease, which could affect access of the fluorescent marker to the septal junctions. Overall, the data are consistent with the idea that SepJ carries out multiple roles in the multicellular function of the Anabaena filament.

Diversity of Growth Patterns Probed in Live Cyanobacterial Cells Using a Fluorescent Analog of a Peptidoglycan Precursor

Cyanobacteria were the first oxygenic photosynthetic organisms during evolution and were ancestors of plastids. Cyanobacterial cells exhibit an extraordinary diversity in their size and shape, and bacterial cell morphology largely depends on the synthesis and the dynamics of the peptidoglycan (PG) layer. Here, we used a fluorescence analog of the PG synthesis precursor D-Ala, 7-Hydroxycoumarin-amino-D-alanine (HADA), to probe the PG synthesis pattern in live cells of cyanobacteria with different morphology. They displayed diverse synthesis patterns, with some strains showing an intensive HADA incorporation at the septal region, whereas others gave an HADA signal distributed around the cells. Growth zones covering several cells at the tips of the filament were present in some filamentous strains such as in Arthrospira. In Anabaena PCC 7120, which is capable of differentiating heterocysts for N₂ fixation, PG synthesis followed the cell division cycle. In addition, an HADA incorporation was strongly activated from 12 to 15 h following the initiation of heterocyst development, indicating a thickening of the PG layer in heterocysts. The PG synthesis pattern is diverse in cyanobacteria and responds to developmental regulation. The use of fluorescent analogs may serve as a useful tool for understanding the mechanisms of cell growth and morphogenesis operating in these organisms.

Surfaceome and Proteosurfaceome in Parietal Monoderm Bacteria: Focus on Protein Cell-Surface Display

The cell envelope of parietal monoderm bacteria (archetypal Gram-positive bacteria) is formed of a cytoplasmic membrane (CM) and a cell wall (CW). While the CM is composed of phospholipids, the CW is composed at least of peptidoglycan (PG) covalently linked to other biopolymers, such as teichoic acids, polysaccharides, and/or polyglutamate. Considering the CW is a porous structure with low selective permeability contrary to the CM, the bacterial cell surface hugs the molecular figure of the CW components as a well of the external side of the CM. While the surfaceome corresponds to the totality of the molecules found at the bacterial cell surface, the proteinaceous complement of the surfaceome is the proteosurfaceome. Once translocated across the CM, secreted proteins can either be released in the extracellular milieu or exposed at the cell surface by associating to the CM or the CW. Following the gene ontology (GO) for cellular components, cell-surface proteins at the CM can either be integral (GO: 0031226), i.e., the integral membrane proteins, or anchored to the membrane (GO: 0046658), i.e., the lipoproteins. At the CW (GO: 0009275), cell-surface proteins can be covalently bound, i.e., the LPXTG-proteins, or bound through weak interactions to the PG or wall polysaccharides, i.e., the cell wall binding proteins. Besides monopolypeptides, some proteins can associate to each other to form supramolecular protein structures of high molecular weight, namely the S-layer, pili, flagella, and cellulosomes. After reviewing the cell envelope components and the different molecular mechanisms involved in protein attachment to the cell envelope, perspectives in investigating the proteosurfaceome in parietal monoderm bacteria are further discussed.

Role of Two Cell Wall Amidases in Septal Junction and Nanopore Formation in the Multicellular Cyanobacterium Anabaena sp. PCC 7120

Filamentous cyanobacteria have developed a strategy to perform incompatible processes in one filament by differentiating specialized cell types, N₂-fixing heterocysts and CO₂-fixing, photosynthetic, vegetative cells. These bacteria can be considered true multicellular organisms with cells exchanging metabolites and signaling molecules via septal junctions, involving the SepJ and FraCD proteins. Previously, it was shown that the cell wall lytic N-acetylmuramyl-L-alanine amidase, AmiC2, is essential for cell-cell communication in Nostoc punctiforme. This enzyme perforates the septal peptidoglycan creating an array of nanopores, which may be the framework for septal junction complexes. In Anabaena sp. PCC 7120, two homologs of AmiC2, encoded by amiC1 and amiC2, were identified and investigated in two different studies. Here, we compare the function of both AmiC proteins by characterizing different Anabaena amiC mutants, which was not possible in N. punctiforme, because there the amiC1 gene could not be inactivated. This study shows the different impact of each protein on nanopore array formation, the process of cell-cell communication, septal protein localization, and heterocyst differentiation. Inactivation of either amidase resulted in significant reduction in nanopore count and in the rate of fluorescent tracer exchange between neighboring cells measured by FRAP analysis. In an amiC1 amiC2 double mutant, filament morphology was affected and heterocyst differentiation was abolished. Furthermore, the inactivation of amiC1 influenced SepJ localization and prevented the filament-fragmentation phenotype that is characteristic of sepJ or fraC fraD mutants. Our findings suggest that both amidases are to some extent redundant in their function, and describe a functional relationship of AmiC1 and septal proteins SepJ and FraCD.

Septal protein SepJ from the heterocyst-forming cyanobacterium Anabaena forms multimers and interacts with peptidoglycan

Heterocyst‐forming cyanobacteria grow as filaments that can be hundreds of cells long. Proteinaceous septal junctions provide cell–cell binding and communication functions in the filament. In Anabaena sp. strain PCC 7120, the SepJ protein is important for the formation of septal junctions. SepJ consists of integral membrane and extramembrane sections – the latter including linker and coiled‐coil domains. SepJ (predicted MW, 81.3 kDa) solubilized from Anabaena membranes was found in complexes of about 296–334 kDa, suggesting that SepJ forms multimeric complexes. We constructed an Anabaena strain producing a double‐tagged SepJ protein (SepJ‐GFP‐His₁₀) and isolated the tagged protein by a two‐step affinity chromatography procedure. Analysis of the purified protein preparation provided no indication of the presence of specific SepJ partners, but suggested that SepJ is processed to remove an N‐terminal fragment. Additionally, pull‐down experiments showed that His₆‐tagged versions of SepJ and of the SepJ coiled‐coil domain interact with Anabaena peptidoglycan (PG). Our results indicate that SepJ forms multimers, that it interacts with PG, and that the coiled‐coil domain is involved in this interaction. These observations support the idea that SepJ is a component of the septal junctions that join the cells in the Anabaena filament.

The plastid outer membrane localized LPTD1 is important for glycerolipid remodelling under phosphate starvation

Glycerolipid synthesis in plants is coordinated between plastids and the endoplasmic reticulum (ER). A central step within the glycerolipid synthesis is the transport of phosphatidic acid from ER to chloroplasts. The chloroplast outer envelope protein TGD4 belongs to the LptD family conserved in bacteria and plants and selectively binds and may transport phosphatidic acid. We describe a second LptD-family protein in A. thaliana (atLPTD1; At2g44640) characterized by a barrel domain with an amino-acid signature typical for cyanobacterial LptDs. It forms a cation selective channel in vitro with a diameter of about 9 Å. atLPTD1 levels are induced under phosphate starvation. Plants expressing an RNAi construct against atLPTD1 show a growth phenotype under normal conditions. Expressing the RNAi against atLPTD1 in the tgd4-1 background renders the plants more sensitive to light stress or phosphate limitation than the individual mutants. Moreover, lipid analysis revealed that digalactosyldiacylglycerol and sulfoquinovosyldiacylglycerol levels remain constant in the RNAi mutants under phosphate starvation, while these two lipids are enhanced in wild-type. Based on our results, we propose a function of atLPTD1 in the transport of lipids from ER to chloroplast under phosphate starvation, which is combinatory with the function of TGD4.

Specific Glucoside Transporters Influence Septal Structure and Function in the Filamentous, Heterocyst-Forming Cyanobacterium Anabaena sp. Strain PCC 7120

When deprived of combined nitrogen, some filamentous cyanobacteria contain two cell types: vegetative cells that fix CO₂ through oxygenic photosynthesis and heterocysts that are specialized in N₂ fixation. In the diazotrophic filament, the vegetative cells provide the heterocysts with reduced carbon (mainly in the form of sucrose) and heterocysts provide the vegetative cells with combined nitrogen. Septal junctions traverse peptidoglycan through structures known as nanopores and appear to mediate intercellular molecular transfer that can be traced with fluorescent markers, including the sucrose analog esculin (a coumarin glucoside) that is incorporated into the cells. Uptake of esculin by the model heterocyst-forming cyanobacterium Anabaena sp. strain PCC 7120 was inhibited by the α-glucosides sucrose and maltose. Analysis of Anabaena mutants identified components of three glucoside transporters that move esculin into the cells: GlsC (Alr4781) and GlsP (All0261) are an ATP-binding subunit and a permease subunit of two different ABC transporters, respectively, and HepP (All1711) is a major facilitator superfamily (MFS) protein that was shown previously to be involved in formation of the heterocyst envelope. Transfer of fluorescent markers (especially calcein) between vegetative cells of Anabaena was impaired by mutation of glucoside transporter genes. GlsP and HepP interact in bacterial two-hybrid assays with the septal junction-related protein SepJ, and GlsC was found to be necessary for the formation of a normal number of septal peptidoglycan nanopores and for normal subcellular localization of SepJ. Therefore, beyond their possible role in nutrient uptake in Anabaena, glucoside transporters influence the structure and function of septal junctions.IMPORTANCE Heterocyst-forming cyanobacteria have the ability to perform oxygenic photosynthesis and to assimilate atmospheric CO₂ and N₂ These organisms grow as filaments that fix these gases specifically in vegetative cells and heterocysts, respectively. For the filaments to grow, these types of cells exchange nutrients, including sucrose, which serves as a source of reducing power and of carbon skeletons for the heterocysts. Movement of sucrose between cells in the filament takes place through septal junctions and has been traced with a fluorescent sucrose analog, esculin, that can be taken up by the cells. Here, we identified α-glucoside transporters of Anabaena that mediate uptake of esculin and, notably, influence septal structure and the function of septal junctions.

Overexpression of SepJ alters septal morphology and heterocyst pattern regulated by diffusible signals in Anabaena

Filamentous, N2 -fixing, heterocyst-forming cyanobacteria grow as chains of cells that are connected by septal junctions. In the model organism Anabaena sp. strain PCC 7120, the septal protein SepJ is required for filament integrity, normal intercellular molecular exchange, heterocyst differentiation, and diazotrophic growth. An Anabaena strain overexpressing SepJ made wider septa between vegetative cells than the wild type, which correlated with a more spread location of SepJ in the septa as observed with a SepJ-GFP fusion, and contained an increased number of nanopores, the septal peptidoglycan perforations that likely accommodate septal junctions. The septa between heterocysts and vegetative cells, which are narrow in wild-type Anabaena, were notably enlarged in the SepJ-overexpressing mutant. Intercellular molecular exchange tested with fluorescent tracers was increased for the SepJ-overexpressing strain specifically in the case of calcein transfer between vegetative cells and heterocysts. These results support an association between calcein transfer, SepJ-related septal junctions, and septal peptidoglycan nanopores. Under nitrogen deprivation, the SepJ-overexpressing strain produced an increased number of contiguous heterocysts but a decreased percentage of total heterocysts. These effects were lost or altered in patS and hetN mutant backgrounds, supporting a role of SepJ in the intercellular transfer of regulatory signals for heterocyst differentiation.

Mutation of the murC and murB Genes Impairs Heterocyst Differentiation in Anabaena sp. Strain PCC 7120

To stabilize cellular integrity in the face of environmental perturbations, most bacteria, including cyanobacteria, synthesize and maintain a strong, flexible, three-dimensional peptidoglycan lattice. Anabaena sp. strain PCC 7120 is a filamentous cyanobacterium capable of differentiating morphologically distinct nitrogen-fixing heterocyst cells in a periodic pattern. While heterocyst development has been shown to require proper peptidoglycan remodeling, the role of peptidoglycan synthesis has remained unclear. Here we report the identification of two peptidoglycan synthesis genes, murC (alr5065) and murB (alr5066), as required for heterocyst development. The murC and murB genes are predicted to encode a UDP-N-acetylmuramate:L-alanine ligase and a UDP-N-acetylenolpyruvoylglucosamine reductase, respectively, and we confirm enzymatic function through complementation of Escherichia coli strains deficient for these enzymes. Cells depleted of either murC or murB expression failed to differentiate heterocysts under normally inducing conditions and displayed decreased filament integrity. To identify the stage(s) of development affected by murC or murB depletion, the spatial distribution of expression of the patterning marker gene, patS, was examined. Whereas murB depletion did not affect the pattern of patS expression, murC depletion led to aberrant expression of patS in all cells of the filament. Finally, expression of gfp controlled by the region of DNA immediately upstream of murC was enriched in differentiating cells and was repressed by the transcription factor NtcA. Collectively, the data in this work provide evidence for a direct link between peptidoglycan synthesis and the maintenance of a biological pattern in a multicellular organism.

IMPORTANCE

Multicellular organisms that differentiate specialized cells must regulate morphological changes such that both cellular integrity and the dissemination of developmental signals are preserved. Here we show that the multicellular bacterium Anabaena, which differentiates a periodic pattern of specialized heterocyst cells, requires peptidoglycan synthesis by the murine ligase genes murC (alr5065) and murB (alr5066) for maintenance of patterned gene expression, filament integrity, and overall development. This work highlights the significant influence that intracellular structure and intercellular connections can have on the execution of a developmental program.

Enabling cell-cell communication via nanopore formation: structure, function and localization of the unique cell wall amidase AmiC2 of Nostoc punctiforme

To orchestrate a complex life style in changing environments, the filamentous cyanobacterium Nostoc punctiforme facilitates communication between neighboring cells through septal junction complexes. This is achieved by nanopores that perforate the peptidoglycan (PGN) layer and traverse the cell septa. The N-acetylmuramoyl-l-alanine amidase AmiC2 (Npun_F1846; EC 3.5.1.28) in N. punctiforme generates arrays of such nanopores in the septal PGN, in contrast to homologous amidases that mediate daughter cell separation after cell division in unicellular bacteria. Nanopore formation is therefore a novel property of AmiC homologs. Immunofluorescence shows that native AmiC2 localizes to the maturing septum. The high-resolution crystal structure (1.12 Å) of its catalytic domain (AmiC2-cat) differs significantly from known structures of cell splitting and PGN recycling amidases. A wide and shallow binding cavity allows easy access of the substrate to the active site, which harbors an essential zinc ion. AmiC2-cat exhibits strong hydrolytic activity in vitro. A single point mutation of a conserved glutamate near the zinc ion results in total loss of activity, whereas zinc removal leads to instability of AmiC2-cat. An inhibitory α-helix, as found in the Escherichia coli AmiC(E. coli) structure, is absent. Taken together, our data provide insight into the cell-biological, biochemical and structural properties of an unusual cell wall lytic enzyme that generates nanopores for cell-cell communication in multicellular cyanobacteria. The novel structural features of the catalytic domain and the unique biological function of AmiC2 hint at mechanisms of action and regulation that are distinct from other amidases.

DATABASE

The AmiC2-cat structure has been deposited in the Protein Data Bank under accession number 5EMI.