DipM is required for peptidoglycan hydrolysis during chloroplast division

The Chloroplast Envelope of Angiosperms Contains a Peptidoglycan Layer

Plastids in plants are assumed to have evolved from cyanobacteria as they have maintained several bacterial features. Recently, peptidoglycans, as bacterial cell wall components, have been shown to exist in the envelopes of moss chloroplasts. Phylogenomic comparisons of bacterial and plant genomes have raised the question of whether such structures are also part of chloroplasts in angiosperms. To address this question, we visualized canonical amino acids of peptidoglycan around chloroplasts of Arabidopsis and Nicotiana via click chemistry and fluorescence microscopy. Additional detection by different peptidoglycan-binding proteins from bacteria and animals supported this observation. Further Arabidopsis experiments with D-cycloserine and AtMurE knock-out lines, both affecting putative peptidoglycan biosynthesis, revealed a central role of this pathway in plastid genesis and division. Taken together, these results indicate that peptidoglycans are integral parts of plastids in the whole plant lineage. Elucidating their biosynthesis and further roles in the function of these organelles is yet to be achieved.

Host-symbiont interactions in Angomonas deanei include the evolution of a host-derived dynamin ring around the endosymbiont division site

The trypanosomatid Angomonas deanei is a model to study endosymbiosis. Each cell contains a single β-proteobacterial endosymbiont that divides at a defined point in the host cell cycle and contributes essential metabolites to the host metabolism. Additionally, one endosymbiont gene, encoding an ornithine cyclodeaminase (OCD), was transferred by endosymbiotic gene transfer (EGT) to the nucleus. However, the molecular mechanisms mediating the intricate host/symbiont interactions are largely unexplored. Here, we used protein mass spectrometry to identify nucleus-encoded proteins that co-purify with the endosymbiont. Expression of fluorescent fusion constructs of these proteins in A. deanei confirmed seven host proteins to be recruited to specific sites within the endosymbiont. These endosymbiont-targeted proteins (ETPs) include two proteins annotated as dynamin-like protein and peptidoglycan hydrolase that form a ring-shaped structure around the endosymbiont division site that remarkably resembles organellar division machineries. The EGT-derived OCD was not among the ETPs, but instead localizes to the glycosome, likely enabling proline production in the glycosome. We hypothesize that recalibration of the metabolic capacity of the glycosomes that are closely associated with the endosymbiont helps to supply the endosymbiont with metabolites it is auxotrophic for and thus supports the integration of host and endosymbiont metabolic networks. Hence, scrutiny of endosymbiosis-induced protein re-localization patterns in A. deanei yielded profound insights into how an endosymbiotic relationship can stabilize and deepen over time far beyond the level of metabolite exchange.

Coordination mechanism of cell and cyanelle division in the glaucophyte alga Cyanophora sudae

In unicellular algae with a single chloroplast, two mechanisms coordinate cell and chloroplast division: the S phase-specific expression of chloroplast division genes and the permission of cell cycle progression from prophase to metaphase by the onset of chloroplast division. This study investigated whether a similar mechanism exists in a unicellular alga with multiple chloroplasts using the glaucophyte alga Cyanophora sudae, which contains four chloroplasts (cyanelles). Cells with eight cyanelles appeared after the S phase arrest with a topoisomerase inhibitor camptothecin, suggesting that the mechanism of S phase-specific expression of cyanelle division genes was conserved in this alga. Inhibition of peptidoglycan synthesis by β-lactam antibiotic ampicillin arrested cells in the S-G2 phase, and inhibition of septum invagination with cephalexin resulted in cells with two nuclei and one cyanelle, despite inhibition of cyanelle division. This indicates that even in the unicellular alga with four chloroplasts, the cell cycle progresses to the M phase following the progression of chloroplast division to a certain division stage. These results suggested that C. sudae has two mechanisms for coordinating cell and cyanelle division, similar to the unicellular algae with a single chloroplast.

A histidine auxotroph mutant is defective for cell separation and allows the identification of crucial factors for cell division in Brucella abortus

The pathogenic bacterium Brucella abortus invades and multiplies inside host cells. To grow inside host cells, B. abortus requires a functional histidine biosynthesis pathway. Here, we show that a B. abortus histidine auxotroph mutant also displays an unexpected chaining phenotype. The intensity of this phenotype varies according to the culture medium and is exacerbated inside host cells. Chains of bacteria consist of contiguous peptidoglycan, and likely result from the defective cleavage of peptidoglycan at septa. Genetic suppression of the chaining phenotype unearthed two essential genes with a role in B. abortus cell division, dipM and cdlP. Loss of function of dipM and cdlP generates swelling at the division site. While DipM is strictly localized at the division site, CdlP is localized at the growth pole and the division site. Altogether, the unexpected chaining phenotype of a hisB mutant allowed the discovery of new crucial actors in cell division in B. abortus.

Genes encoding lipid II flippase MurJ and peptidoglycan hydrolases are required for chloroplast division in the moss Physcomitrella patens

Homologous genes for the peptidoglycan precursor flippase MurJ, and peptidoglycan hydrolases: lytic transglycosylase MltB, and DD-carboxypeptidase VanY are required for chloroplast division in the moss Physcomitrella patens. The moss Physcomitrella patens is used as a model plant to study plastid peptidoglycan biosynthesis. In bacteria, MurJ flippase transports peptidoglycan precursors from the cytoplasm to the periplasm. In this study, we identified a MurJ homolog (PpMurJ) in the P. patens genome. Bacteria employ peptidoglycan degradation and recycling pathways for cell division. We also searched the P. patens genome for genes homologous to bacterial peptidoglycan hydrolases and identified genes homologous for the lytic transglycosylase mltB, N-acetylglucosaminidase nagZ, and LD-carboxypeptidase ldcA in addition to a putative DD-carboxypeptidase vanY reported previously. Moreover, we found a ß-lactamase-like gene (Pplactamase). GFP fusion proteins with either PpMltB or PpVanY were detected in the chloroplasts, whereas fusion proteins with PpNagZ, PpLdcA, or Pplactamase localized in the cytoplasm. Experiments seeking PpMurJ-GFP fusion proteins failed. PpMurJ gene disruption in P. patens resulted in the appearance of macrochloroplasts in protonemal cells. Compared with the numbers of chloroplasts in wild-type plants (38.9 ± 4.9), PpMltB knockout and PpVanY knockout had lower numbers of chloroplasts (14.3 ± 6.7 and 28.1 ± 5.9, respectively). No differences in chloroplast numbers were observed after PpNagZ, PpLdcA, or Pplactamase single-knockout. Chloroplast numbers in PpMltB/PpVanY double-knockout cells were similar to those in PpMltB single-knockout cells. Zymogram analysis of the recombinant PpMltB protein revealed its peptidoglycan hydrolase activity. Our results imply that PpMurJ, PpMltB and PpVanY play a critical role in chloroplast division in the moss P. patens.

Analysis of an improved Cyanophora paradoxa genome assembly

Glaucophyta are members of the Archaeplastida, the founding group of photosynthetic eukaryotes that also includes red algae (Rhodophyta), green algae, and plants (Viridiplantae). Here we present a high-quality assembly, built using long-read sequences, of the ca. 100 Mb nuclear genome of the model glaucophyte Cyanophora paradoxa. We also conducted a quick-freeze deep-etch electron microscopy (QFDEEM) analysis of C. paradoxa cells to investigate glaucophyte morphology in comparison to other organisms. Using the genome data, we generated a resolved 115-taxon eukaryotic tree of life that includes a well-supported, monophyletic Archaeplastida. Analysis of muroplast peptidoglycan (PG) ultrastructure using QFDEEM shows that PG is most dense at the cleavage-furrow. Analysis of the chlamydial contribution to glaucophytes and other Archaeplastida shows that these foreign sequences likely played a key role in anaerobic glycolysis in primordial algae to alleviate ATP starvation under night-time hypoxia. The robust genome assembly of C. paradoxa significantly advances knowledge about this model species and provides a reference for exploring the panoply of traits associated with the anciently diverged glaucophyte lineage.

The monoplastidic bottleneck in algae and plant evolution

Plastids in plants and algae evolved from the endosymbiotic integration of a cyanobacterium by a heterotrophic eukaryote. New plastids can only emerge through fission; thus, the synchronization of bacterial division with the cell cycle of the eukaryotic host was vital to the origin of phototrophic eukaryotes. Most of the sampled algae house a single plastid per cell and basal-branching relatives of polyplastidic lineages are all monoplastidic, as are some non-vascular plants during certain stages of their life cycle. In this Review, we discuss recent advances in our understanding of the molecular components necessary for plastid division, including those of the peptidoglycan wall (of which remnants were recently identified in moss), in a wide range of phototrophic eukaryotes. Our comparison of the phenotype of 131 species harbouring plastids of either primary or secondary origin uncovers that one prerequisite for an algae or plant to house multiple plastids per nucleus appears to be the loss of the bacterial genes minD and minE from the plastid genome. The presence of a single plastid whose division is coupled to host cytokinesis was a prerequisite of plastid emergence. An escape from such a monoplastidic bottleneck succeeded rarely and appears to be coupled to the evolution of additional layers of control over plastid division and a complex morphology. The existence of a quality control checkpoint of plastid transmission remains to be demonstrated and is tied to understanding the monoplastidic bottleneck.

Protein Localization in the Cyanobacterium Anabaena sp. PCC7120 Using Immunofluorescence Labeling

Techniques such as immunoflorescence are widely used to determine subcellular distribution of proteins. Here we report on a method to immunolocalize proteins in Anabaena sp. PCC7120 with fluorophore-conjugated antibodies by fluorescence microscopy. This method improves the permeabilization of cyanobacterial cells and minimizes the background fluorescence for non-specific attachments. In this protocol, rabbit antibodies were raised against the synthetic peptide of CyDiv protein ( Mandakovic et al., 2016 ). The secondary antibody conjugated to the fluorophore Alexa488 was used due to its different emission range in comparison to the autofluorescence of the cyanobacterium.

Chloroplast division checkpoint in eukaryotic algae

Chloroplasts evolved from a cyanobacterial endosymbiont. It is believed that the synchronization of endosymbiotic and host cell division, as is commonly seen in existing algae, was a critical step in establishing the permanent organelle. Algal cells typically contain one or only a small number of chloroplasts that divide once per host cell cycle. This division is based partly on the S-phase-specific expression of nucleus-encoded proteins that constitute the chloroplast-division machinery. In this study, using the red alga Cyanidioschyzon merolae, we show that cell-cycle progression is arrested at the prophase when chloroplast division is blocked before the formation of the chloroplast-division machinery by the overexpression of Filamenting temperature-sensitive (Fts) Z2-1 (Fts72-1), but the cell cycle progresses when chloroplast division is blocked during division-site constriction by the overexpression of either FtsZ2-1 or a dominant-negative form of dynamin-related protein 5B (DRP5B). In the cells arrested in the prophase, the increase in the cyclin B level and the migration of cyclin-dependent kinase B (CDKB) were blocked. These results suggest that chloroplast division restricts host cell-cycle progression so that the cell cycle progresses to the metaphase only when chloroplast division has commenced. Thus, chloroplast division and host cell-cycle progression are synchronized by an interactive restriction that takes place between the nucleus and the chloroplast. In addition, we observed a similar pattern of cell-cycle arrest upon the blockage of chloroplast division in the glaucophyte alga Cyanophora paradoxa, raising the possibility that the chloroplast division checkpoint contributed to the establishment of the permanent organelle.

CyDiv, a Conserved and Novel Filamentous Cyanobacterial Cell Division Protein Involved in Septum Localization

Cell division in bacteria has been studied mostly in Escherichia coli and Bacillus subtilis, model organisms for Gram-negative and Gram-positive bacteria, respectively. However, cell division in filamentous cyanobacteria is poorly understood. Here, we identified a novel protein, named CyDiv (Cyanobacterial Division), encoded by the all2320 gene in Anabaena sp. PCC 7120. We show that CyDiv plays a key role during cell division. CyDiv has been previously described only as an exclusive and conserved hypothetical protein in filamentous cyanobacteria. Using polyclonal antibodies against CyDiv, we showed that it localizes at different positions depending on cell division timing: poles, septum, in both daughter cells, but also in only one of the daughter cells. The partial deletion of CyDiv gene generates partial defects in cell division, including severe membrane instability and anomalous septum localization during late division. The inability to complete knock out CyDiv strains suggests that it is an essential gene. In silico structural protein analyses and our experimental results suggest that CyDiv is an FtsB/DivIC-like protein, and could therefore, be part of an essential late divisome complex in Anabaena sp. PCC 7120.

Thylakoid Development and Galactolipid Synthesis in Cyanobacteria

Cyanobacteria carry out oxygenic photosynthesis and share many features with chloroplasts, including thylakoid membranes, which are mainly composed of membrane lipids and protein complexes that mediate photosynthetic electron transport. Although the functions of the various thylakoid protein complexes have been well characterized, the details underlying the biogenesis of thylakoid membranes remain unclear. Galactolipids are the major constituents of the thylakoid membrane system, and all the genes involved in galactolipid biosynthesis were recently identified. In this chapter, I summarize recent advances in our understanding of the factors involved in thylakoid development, including regulatory proteins and enzymes that mediate lipid biosynthesis.

Prospective function of FtsZ proteins in the secondary plastid of chlorarachniophyte algae

BACKGROUND

Division of double-membraned plastids (primary plastids) is performed by constriction of a ring-like division complex consisting of multiple plastid division proteins. Consistent with the endosymbiotic origin of primary plastids, some of the plastid division proteins are descended from cyanobacterial cell division machinery, and the others are of host origin. In several algal lineages, complex plastids, the "secondary plastids", have been acquired by the endosymbiotic uptake of primary plastid-bearing algae, and are surrounded by three or four membranes. Although homologous genes for primary plastid division proteins have been found in genome sequences of secondary plastid-bearing organisms, little is known about the function of these proteins or the mechanism of secondary plastid division.

RESULTS

To gain insight into the mechanism of secondary plastid division, we characterized two plastid division proteins, FtsZD-1 and FtsZD-2, in chlorarachniophyte algae. FtsZ homologs were encoded by the nuclear genomes and carried an N-terminal plastid targeting signal. Immunoelectron microscopy revealed that both FtsZD-1 and FtsZD-2 formed a ring-like structure at the midpoint of bilobate plastids with a projecting pyrenoid in Bigelowiella natans. The ring was always associated with a shallow plate-like invagination of the two innermost plastid membranes. Furthermore, gene expression analysis confirmed that transcripts of ftsZD genes were periodically increased soon after cell division during the B. natans cell cycle, which is not consistent with the timing of plastid division.

CONCLUSIONS

Our findings suggest that chlorarachniophyte FtsZD proteins are involved in partial constriction of the inner pair of plastid membranes, but not in the whole process of plastid division. It is uncertain how the outer pair of plastid membranes is constricted, and as-yet-unknown mechanism is required for the secondary plastid division in chlorarachniophytes.

An ancestral bacterial division system is widespread in eukaryotic mitochondria

Bacterial division initiates at the site of a contractile Z-ring composed of polymerized FtsZ. The location of the Z-ring in the cell is controlled by a system of three mutually antagonistic proteins, MinC, MinD, and MinE. Plastid division is also known to be dependent on homologs of these proteins, derived from the ancestral cyanobacterial endosymbiont that gave rise to plastids. In contrast, the mitochondria of model systems such as Saccharomyces cerevisiae, mammals, and Arabidopsis thaliana seem to have replaced the ancestral α-proteobacterial Min-based division machinery with host-derived dynamin-related proteins that form outer contractile rings. Here, we show that the mitochondrial division system of these model organisms is the exception, rather than the rule, for eukaryotes. We describe endosymbiont-derived, bacterial-like division systems comprising FtsZ and Min proteins in diverse less-studied eukaryote protistan lineages, including jakobid and heterolobosean excavates, a malawimonad, stramenopiles, amoebozoans, a breviate, and an apusomonad. For two of these taxa, the amoebozoan Dictyostelium purpureum and the jakobid Andalucia incarcerata, we confirm a mitochondrial localization of these proteins by their heterologous expression in Saccharomyces cerevisiae. The discovery of a proteobacterial-like division system in mitochondria of diverse eukaryotic lineages suggests that it was the ancestral feature of all eukaryotic mitochondria and has been supplanted by a host-derived system multiple times in distinct eukaryote lineages.

FtsZ-independent septal recruitment and function of cell wall remodelling enzymes in chlamydial pathogens

The nature and assembly of the chlamydial division septum is poorly defined due to the paucity of a detectable peptidoglycan (PG)-based cell wall, the inhibition of constriction by penicillin and the presence of coding sequences for cell wall precursor and remodelling enzymes in the reduced chlamydial (pan-)genome. Here we show that the chlamydial amidase (AmiA) is active and remodels PG in Escherichia coli. Moreover, forward genetics using an E. coli amidase mutant as entry point reveals that the chlamydial LysM-domain protein NlpD is active in an E. coli reporter strain for PG endopeptidase activity (ΔnlpI). Immunolocalization unveils NlpD as the first septal (cell-wall-binding) protein in Chlamydiae and we show that its septal sequestration depends on prior cell wall synthesis. Since AmiA assembles into peripheral clusters, trimming of a PG-like polymer or precursors occurs throughout the chlamydial envelope, while NlpD targets PG-like peptide crosslinks at the chlamydial septum during constriction.

FtsZ-less prokaryotic cell division as well as FtsZ- and dynamin-less chloroplast and non-photosynthetic plastid division

The chloroplast division machinery is a mixture of a stromal FtsZ-based complex descended from a cyanobacterial ancestor of chloroplasts and a cytosolic dynamin-related protein (DRP) 5B-based complex derived from the eukaryotic host. Molecular genetic studies have shown that each component of the division machinery is normally essential for normal chloroplast division. However, several exceptions have been found. In the absence of the FtsZ ring, non-photosynthetic plastids are able to proliferate, likely by elongation and budding. Depletion of DRP5B impairs, but does not stop chloroplast division. Chloroplasts in glaucophytes, which possesses a peptidoglycan (PG) layer, divide without DRP5B. Certain parasitic eukaryotes possess non-photosynthetic plastids of secondary endosymbiotic origin, but neither FtsZ nor DRP5B is encoded in their genomes. Elucidation of the FtsZ- and/or DRP5B-less chloroplast division mechanism will lead to a better understanding of the function and evolution of the chloroplast division machinery and the finding of the as-yet-unknown mechanism that is likely involved in chloroplast division. Recent studies have shown that FtsZ was lost from a variety of prokaryotes, many of which lost PG by regressive evolution. In addition, even some of the FtsZ-bearing bacteria are able to divide when FtsZ and PG are depleted experimentally. In some cases, alternative mechanisms for cell division, such as budding by an increase of the cell surface-to-volume ratio, are proposed. Although PG is believed to have been lost from chloroplasts other than in glaucophytes, there is some indirect evidence for the existence of PG in chloroplasts. Such information is also useful for understanding how non-photosynthetic plastids are able to divide in FtsZ-depleted cells and the reason for the retention of FtsZ in chloroplast division. Here we summarize information to facilitate analyses of FtsZ- and/or DRP5B-less chloroplast and non-photosynthetic plastid division.