Carboxysomes: 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.
- Centre National de la Recherché Scientifique, France
The emergence of cell biology techniques proved that bacterial cells are compartmentalized. Indeed, many of the activities performed inside bacterial cells are restricted to specific regions, like the cell wall, or to organelles (Murat et al., 2010; Mauriello et al., 2018). Photosynthetic cyanobacteria contain organelles called carboxysomes: specialized compartments that encapsulate the key enzymes for photosynthesis in a protein shell. In the cyanobacterium Synechococcus elongatus, carboxysomes align themselves at equal distances from each other along the longitudinal axis of the cell. This distribution means that each daughter of a dividing cell receives its fair share of carboxysomes and can photosynthesize soon after forming.
The amount of a protein called McdA oscillates inside bacteria, leading to regions that contain high levels of McdA and regions that contain low levels, and in 2010 researchers established a link between these oscillations and the positioning of carboxysomes within bacteria (Savage et al., 2010). However, it was not known what causes the McdA oscillations, or how these dynamics determine the arrangement of the carboxysomes. Now, in eLife, Daniel Ducat of Michigan State University, Anthony Vecchiarelli of the University of Michigan and co-workers – including Joshua MacCready as first author – report the molecular mechanism behind these processes in S. elongatus (MacCready et al., 2018).
Firstly, MacCready et al. showed that McdA oscillations take place on the nucleoid, the region within a bacterium that is occupied by DNA. Carboxysomes also localize at this position. The researchers then discovered a small protein that is able to interact directly with McdA and also with some of the proteins that make up the carboxysome shell. This protein, which MacCready et al. called McdB, thus acts as a bridge to connect the carboxysomes with McdA at the nucleoid.
But what causes the McdA oscillations? McdA binds to the nucleoid when bound to ATP, a molecule that releases energy when it is hydrolyzed. McdA is able to hydrolyze ATP highly efficiently, and this activity is further enhanced by McdB. Experiments in vitro and in vivo show that by promoting the ability of McdA to hydrolyze ATP, McdB helps McdA to detach from the nucleoid. This creates regions on the nucleoid that are depleted of McdA. Because McdB tends to localize at high concentrations of McdA, the carboxysomes move toward those regions of the nucleoid that are rich in McdA. The end result is that the carboxysomes become evenly spaced along the nucleoid. The McdA oscillations emerge from the presence of multiple McdB-containing carboxysomes, which cause McdA to repeatedly dissociate from and then re-associate with the nucleoid.
MacCready et al. performed an elegant experiment that explains and confirms the predictions of this model. Using different gene expression systems, they were able to produce cells that contained one, two or more carboxysomes. The nucleoid, carboxysomes and McdA inside these cells were fluorescently labeled to enable their behavior to be tracked using a microscope.
In cells with one carboxysome, the organelle localizes at the only McdA-depleted region of the nucleoid (Figure 1). In cells with two carboxysomes, the more central carboxysome moves away from the other one and toward the highest concentration of McdA. When they are sufficiently far apart, McdA reassembles on the McdA-depleted region of the nucleoid, and the more central carboxysome slightly moves back. In cells with multiple carboxysomes, the movements of the carboxysomes and the resulting McdA oscillations cause the organelles to space themselves equidistantly.
Figure 1
The proteins McdA and McdB interact to position carboxysomes in bacterial cells.
Left: Synechococcus elongatus cells bearing one or multiple carboxysomes (green pentagons). McdB proteins on the surface of the carboxysomes create a gradient of McdA (pink) that oscillates across the surface of the nucleoid. Carboxysomes move to the highest concentration of McdA, but McdB causes McdA to dissociate more easily from the nucleoid surface. In cells containing one carboxysome (top), the carboxysome sits at the McdA-depleted region of the nucleoid. In cells containing two or more carboxysomes, the carboxysomes move apart from each other until they end up equally spaced across the nucleoid. Right: Schematic diagram of a carboxysome (adapted from http://2014.igem.org/Team:Bielefeld-CeBiTec/Project/CO2-fixation/Carboxysome; CC BY 3.0). Carboxysomes contain the key enzymes for photosynthesis – RuBisCO and carbonic anhydrase. McdB proteins (light green diamonds) on the surface of carboxysomes allow the carboxysomes to interact with McdA on the surface of nucleoids.
MacCready et al. observe that this model fits a Brownian ratchet model (in which random motion can be used to move a cargo in one direction). A similar model has been proposed for the ParA–ParB segregation system that partitions chromosomes and plasmids (Vecchiarelli et al., 2014; Hu et al., 2017). Indeed, McdA is a ParA-like protein.
It has long been known that the cytoskeleton and the cell wall are the main organizers of the contents of bacterial cells. However, it has become clear that the nucleoid also serves as a scaffold for assembling large complexes and organelles (Thompson et al., 2006; Henry and Crosson, 2013; Moine et al., 2017; MacCready et al., 2018). When these complexes and organelles have to occupy specific positions in a cell, how does the cell ensure that they are inherited equally by both daughter cells after division? The latest results from MacCready et al. on carboxysomes add another example to the list of structures that ParA–ParB-like systems can segregate during cell division (for other examples, see Thompson et al., 2006; Alvarado et al., 2017). The presence of a reliable segregation system is essential for the emergence of bacterial populations in which all the cells perform the same function.
References
-
Chromosome replication and segregation govern the biogenesis and inheritance of inorganic polyphosphate granulesMolecular Biology of the Cell 24:3177–3186.https://doi.org/10.1091/mbc.e13-04-0182
-
Cellular targeting and segregation of bacterial chemosensory systemsFEMS Microbiology Reviews 42:462–476.https://doi.org/10.1093/femsre/fuy015
-
Cell biology of prokaryotic organellesCold Spring Harbor Perspectives in Biology 2:a000422.https://doi.org/10.1101/cshperspect.a000422
Article and author information
Author details
Publication history
- Version of Record published: January 10, 2019 (version 1)
- Version of Record updated: February 12, 2019 (version 2)
Copyright
© 2019, Mauriello
This article is distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use and redistribution provided that the original author and source are credited.
Metrics
-
- 2,345
- views
-
- 214
- downloads
-
- 6
- citations
Views, downloads and citations are aggregated across all versions of this paper published by eLife.
Download links
A two-part list of links to download the article, or parts of the article, in various formats.
Downloads (link to download the article as PDF)
Open citations (links to open the citations from this article in various online reference manager services)
Cite this article (links to download the citations from this article in formats compatible with various reference manager tools)
Carboxysomes: How bacteria arrange their organelles
eLife 8:e43777.
https://doi.org/10.7554/eLife.43777
Further reading
-
- Medicine
- Microbiology and Infectious Disease
Background:
End-stage renal disease (ESRD) patients experience immune compromise characterized by complex alterations of both innate and adaptive immunity, and results in higher susceptibility to infection and lower response to vaccination. This immune compromise, coupled with greater risk of exposure to infectious disease at hemodialysis (HD) centers, underscores the need for examination of the immune response to the COVID-19 mRNA-based vaccines.
Methods:
The immune response to the COVID-19 BNT162b2 mRNA vaccine was assessed in 20 HD patients and cohort-matched controls. RNA sequencing of peripheral blood mononuclear cells was performed longitudinally before and after each vaccination dose for a total of six time points per subject. Anti-spike antibody levels were quantified prior to the first vaccination dose (V1D0) and 7 d after the second dose (V2D7) using anti-spike IgG titers and antibody neutralization assays. Anti-spike IgG titers were additionally quantified 6 mo after initial vaccination. Clinical history and lab values in HD patients were obtained to identify predictors of vaccination response.
Results:
Transcriptomic analyses demonstrated differing time courses of immune responses, with prolonged myeloid cell activity in HD at 1 wk after the first vaccination dose. HD also demonstrated decreased metabolic activity and decreased antigen presentation compared to controls after the second vaccination dose. Anti-spike IgG titers and neutralizing function were substantially elevated in both controls and HD at V2D7, with a small but significant reduction in titers in HD groups (p<0.05). Anti-spike IgG remained elevated above baseline at 6 mo in both subject groups. Anti-spike IgG titers at V2D7 were highly predictive of 6-month titer levels. Transcriptomic biomarkers after the second vaccination dose and clinical biomarkers including ferritin levels were found to be predictive of antibody development.
Conclusions:
Overall, we demonstrate differing time courses of immune responses to the BTN162b2 mRNA COVID-19 vaccination in maintenance HD subjects comparable to healthy controls and identify transcriptomic and clinical predictors of anti-spike IgG titers in HD. Analyzing vaccination as an in vivo perturbation, our results warrant further characterization of the immune dysregulation of ESRD.
Funding:
F30HD102093, F30HL151182, T32HL144909, R01HL138628. This research has been funded by the University of Illinois at Chicago Center for Clinical and Translational Science (CCTS) award UL1TR002003.
-
- Microbiology and Infectious Disease
- Structural Biology and Molecular Biophysics
African trypanosomes replicate within infected mammals where they are exposed to the complement system. This system centres around complement C3, which is present in a soluble form in serum but becomes covalently deposited onto the surfaces of pathogens after proteolytic cleavage to C3b. Membrane-associated C3b triggers different complement-mediated effectors which promote pathogen clearance. To counter complement-mediated clearance, African trypanosomes have a cell surface receptor, ISG65, which binds to C3b and which decreases the rate of trypanosome clearance in an infection model. However, the mechanism by which ISG65 reduces C3b function has not been determined. We reveal through cryogenic electron microscopy that ISG65 has two distinct binding sites for C3b, only one of which is available in C3 and C3d. We show that ISG65 does not block the formation of C3b or the function of the C3 convertase which catalyses the surface deposition of C3b. However, we show that ISG65 forms a specific conjugate with C3b, perhaps acting as a decoy. ISG65 also occludes the binding sites for complement receptors 2 and 3, which may disrupt recruitment of immune cells, including B cells, phagocytes, and granulocytes. This suggests that ISG65 protects trypanosomes by combining multiple approaches to dampen the complement cascade.
