Intrinsically disordered CsoS2 acts as a general molecular thread for α-carboxysome shell assembly

Assembly mechanism of the β-carboxysome shell mediated by the chaperone CcmS

Carboxysomes are self-assembled bacterial microcompartments (BMCs) that encapsulate the enzymes RuBisCO and carbonic anhydrase into a proteinaceous shell, enhancing the efficiency of photosynthetic carbon fixation. The chaperone CcmS was reported to participate in the assembly of β-carboxysomes; however, the underlying molecular mechanism remains elusive. We report the crystal structure of CcmS from Synechocystis sp. PCC 6803, revealing a monomer of α/β fold. Moreover, its complex structures with two types of BMC hexamers, CcmK1 homohexamer and CcmK1-CcmK2 heterohexamer, reveal a same pattern of CcmS binding to the featured C-terminal segment of CcmK1. Upon binding to CcmS, this C-terminal segment of CcmK1 is folded into an amphipathic α-helix protruding outward that might function as a hinge to crosslink adjacent BMC-H hexamers, thereby facilitating concerted and precise assembly of the β-carboxysome shell. Deletion of the ccmS gene or the 8-residue C-terminal coding region of ccmK1 resulted in the formation of aberrant and fewer carboxysomes, suppressed photosynthetic capacity in Synechocystis sp. PCC 6803. These findings enable us to propose a putative model for the chaperone-assisted assembly of β-carboxysome shell and provide clues for the design and engineering of efficient carbon fixation machinery.

Chaotrope-Based Approach for Rapid In Vitro Assembly and Loading of Bacterial Microcompartment Shells

Bacterial microcompartments (BMCs) are proteinaceous organelles that self-assemble into selectively permeable shells that encapsulate enzymatic cargo. BMCs enhance catalytic pathways by reducing crosstalk among metabolites, preventing harmful intermediates from leaking into the cytosol and increasing reaction efficiency via enzyme colocalization. The intrinsic properties of BMCs make them attractive for biotechnological engineering. However, in vivo expression methods for shell synthesis have significant drawbacks that limit the potential design space for these nanocompartments. Here, we describe the development of an efficient and rapid method for the in vitro assembly of BMC shells from their protein building blocks. Our method enables large-scale construction of BMC shells by utilizing urea as a chaotropic agent to control self-assembly and provides an approach for encapsulation of both biotic and abiotic cargo under a broad range of reaction conditions. We demonstrate an enhanced level of control over the assembly of BMC shells in vitro and expand the design parameter space for engineering BMC systems with specialized and enhanced catalytic properties.

Engineering Rubisco condensation in chloroplasts to manipulate plant photosynthesis

Although Rubisco is the most abundant enzyme globally, it is inefficient for carbon fixation because of its low turnover rate and limited ability to distinguish CO2 and O2, especially under high O2 conditions. To address these limitations, phytoplankton, including cyanobacteria and algae, have evolved CO2-concentrating mechanisms (CCM) that involve compartmentalizing Rubisco within specific structures, such as carboxysomes in cyanobacteria or pyrenoids in algae. Engineering plant chloroplasts to establish similar structures for compartmentalizing Rubisco has attracted increasing interest for improving photosynthesis and carbon assimilation in crop plants. Here, we present a method to effectively induce the condensation of endogenous Rubisco within tobacco (Nicotiana tabacum) chloroplasts by genetically fusing superfolder green fluorescent protein (sfGFP) to the tobacco Rubisco large subunit (RbcL). By leveraging the intrinsic oligomerization feature of sfGFP, we successfully created pyrenoid-like Rubisco condensates that display dynamic, liquid-like properties within chloroplasts without affecting Rubisco assembly and catalytic function. The transgenic tobacco plants demonstrated comparable autotrophic growth rates and full life cycles in ambient air relative to the wild-type plants. Our study offers a promising strategy for modulating endogenous Rubisco assembly and spatial organization in plant chloroplasts via phase separation, which provides the foundation for generating synthetic organelle-like structures for carbon fixation, such as carboxysomes and pyrenoids, to optimize photosynthetic efficiency.

Engineering CO2-fixing modules in Escherichia coli via efficient assembly of cyanobacterial Rubisco and carboxysomes

Rubisco (ribulose-1,5-bisphosphate carboxylase/oxygenase) is the central enzyme for conversion of atmospheric CO2 into organic molecules, playing a crucial role in the global carbon cycle. In cyanobacteria and some chemoautotrophs, Rubisco complexes, together with carbonic anhydrase, are enclosed within specific proteinaceous microcompartments known as carboxysomes. The polyhedral carboxysome shell ensures the dense packaging of Rubisco and creates a high-CO2 internal environment to facilitate CO2 fixation. Rubisco and carboxysomes have been popular targets for bioengineering, with the intent of enhancing plant photosynthesis, crop yields, and biofuel production. However, efficient generation of Form 1B Rubisco and cyanobacterial β-carboxysomes in heterologous systems remains a challenge. Here, we developed genetic systems to efficiently engineer functional cyanobacterial Form 1B Rubisco in Escherichia coli by incorporating Rubisco assembly factor Raf1 and modulating the RbcL/S stoichiometry. We then reconstituted catalytically active β-carboxysomes in E. coli with cognate Form 1B Rubisco by fine-tuning the expression levels of individual β-carboxysome components. In addition, we investigated the mechanism of Rubisco encapsulation into carboxysomes by constructing hybrid carboxysomes; this was achieved by creating a chimeric encapsulation peptide incorporating small sub-unit-like domains, which enabled the encapsulation of Form 1B Rubisco into α-carboxysome shells. Our study provides insights into the assembly mechanisms of plant-like Form 1B Rubisco and the principles of its encapsulation in both β-carboxysomes and hybrid carboxysomes, highlighting the inherent modularity of carboxysome structures. These findings lay the framework for rational design and repurposing of CO2-fixing modules in bioengineering applications, e.g., crop engineering, biocatalyst production, and molecule delivery.

Carboxysomes: The next frontier in biotechnology and sustainable solutions

Some bacteria possess microcompartments that function as protein-based organelles. Bacterial microcompartments (BMCs) sequester enzymes to optimize metabolic reactions. Several BMCs have been characterized to date, including carboxysomes and metabolosomes. Genomic analysis has identified novel BMCs and their loci, often including genes for signature enzymes critical to their function, but further characterization is needed to confirm their roles. Among the various BMCs, carboxysomes, which are found in cyanobacteria and some chemoautotrophic bacteria, and are most extensively investigated. These self-assembling polyhedral proteinaceous BMCs are essential for carbon fixation. Carboxysomes encapsulate the enzymes RuBisCo and carbonic anhydrase, which increase the carbon fixation rate in the cell and decrease the oxygenation rate by RuBisCo. The ability of carboxysomes to concentrate carbon dioxide in crops and industrially relevant microorganisms renders them attractive targets for carbon assimilation bioengineering. Thus, carboxysome characterization is the first step toward developing carboxysome-based applications. Therefore, this review comprehensively explores carboxysome morphology, physiology, and biochemistry. It also discusses recent advances in microscopy and complementary techniques for isolating and characterizing this versatile class of prokaryotic organelles.

In-cell Structure and Variability of Pyrenoid Rubisco

Ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) is a key enzyme in the global carbon cycle, catalyzing CO2 fixation during photosynthesis. To overcome Rubisco's inherent catalytic inefficiency, many photosynthetic organisms have evolved CO2-concentrating mechanisms. Central to these mechanisms is the pyrenoid, a protein-dense organelle within the chloroplast of eukaryotic algae, which increases the local concentration of CO2 around Rubisco and thereby enhances its catalytic efficiency. Although the structure of Rubisco has been extensively studied by in vitro methods such as X-ray crystallography and single particle cryo-EM, its native structure within the pyrenoid, its dynamics, and its association with binding partners remain elusive. Here, we investigate the structure of native pyrenoid Rubisco inside the green alga Chlamydomonas reinhardtii by applying cryo-electron tomography (cryo-ET) on cryo-focused ion beam (cryo-FIB) milled cells, followed by subtomogram averaging and 3D classification. Reconstruction at sub-nanometer resolution allowed accurate modeling and determination of a closed (activated) Rubisco conformation. Comparison to other reconstructed subsets revealed local variations at the complex active site and at the large subunit dimers interface, as well as association with binding proteins. The different structural subsets distribute stochastically within the pyrenoid. Taken together, these findings offer a comprehensive description of the structure, dynamics, and functional organization of Rubisco within the pyrenoid, providing valuable insights into its critical role in CO2 fixation.

Quantitative Measurement of Molecular Permeability to a Synthetic Bacterial Microcompartment Shell System

Naturally evolved and synthetically designed forms of compartmentalization benefit encapsulated function by increasing local concentrations of substrates and protecting cargo from destabilizing environments and inhibitors. Crucial to understanding the fundamental principles of compartmentalization are experimental systems enabling the measurement of the permeability rates of small molecules. Here, we report the experimental measurement of the small-molecule permeability of a 40 nm icosahedral bacterial microcompartment shell. This was accomplished by heterologous loading of light-producing luciferase enzymes and kinetic measurement of luminescence using stopped-flow spectrophotometry. Compared to free enzyme, the luminescence signal kinetics was slower when the luciferase was encapsulated in bacterial microcompartment shells. The results indicate that substrates and products can still exchange across the shell, and modeling of the experimental data suggest that a 50× permeability rate increase occurs when shell vertices were vacant. Overall, our results suggest design considerations for the construction of heterologous bacterial microcompartment shell systems and compartmentalized function at the nanoscale.

Rubisco packaging and stoichiometric composition of the native β-carboxysome in Synechococcus elongatus PCC7942

Carboxysomes are anabolic bacterial microcompartments that play an essential role in CO2 fixation in cyanobacteria. This self-assembling proteinaceous organelle uses a polyhedral shell constructed by hundreds of shell protein paralogs to encapsulate the key CO2-fixing enzymes Rubisco and carbonic anhydrase. Deciphering the precise arrangement and structural organization of Rubisco enzymes within carboxysomes is crucial for understanding carboxysome formation and overall functionality. Here, we employed cryoelectron tomography and subtomogram averaging to delineate the 3D packaging of Rubiscos within β-carboxysomes in the freshwater cyanobacterium Synechococcus elongatus PCC7942 grown under low light. Our results revealed that Rubiscos are arranged in multiple concentric layers parallel to the shell within the β-carboxysome lumen. We also detected Rubisco binding with the scaffolding protein CcmM in β-carboxysomes, which is instrumental for Rubisco encapsulation and β-carboxysome assembly. Using Quantification conCATamer-based quantitative MS, we determined the absolute stoichiometric composition of the entire β-carboxysome. This study provides insights into the assembly principles and structural variation of β-carboxysomes, which will aid in the rational design and repurposing of carboxysome nanostructures for diverse bioengineering applications.

Molecular principles of the assembly and construction of a carboxysome shell

Intracellular compartmentalization enhances biological reactions, crucial for cellular function and survival. An example is the carboxysome, a bacterial microcompartment for CO2 fixation. The carboxysome uses a polyhedral protein shell made of hexamers, pentamers, and trimers to encapsulate Rubisco, increasing CO2 levels near Rubisco to enhance carboxylation. Despite their role in the global carbon cycle, the molecular mechanisms behind carboxysome shell assembly remain unclear. Here, we present a structural characterization of α-carboxysome shells generated from recombinant systems, which contain all shell proteins and the scaffolding protein CsoS2. Atomic-resolution cryo-electron microscopy of the shell assemblies, with a maximal size of 54 nm, unveil diverse assembly interfaces between shell proteins, detailed interactions of CsoS2 with shell proteins to drive shell assembly, and the formation of heterohexamers and heteropentamers by different shell protein paralogs, facilitating the assembly of larger empty shells. Our findings provide mechanistic insights into the construction principles of α-carboxysome shells and the role of CsoS2 in governing α-carboxysome assembly and functionality.

Atomic view of photosynthetic metabolite permeability pathways and confinement in synthetic carboxysome shells

Carboxysomes are protein microcompartments found in cyanobacteria, whose shell encapsulates rubisco at the heart of carbon fixation in the Calvin cycle. Carboxysomes are thought to locally concentrate CO2 in the shell interior to improve rubisco efficiency through selective metabolite permeability, creating a concentrated catalytic center. However, permeability coefficients have not previously been determined for these gases, or for Calvin-cycle intermediates such as bicarbonate ([Formula: see text]), 3-phosphoglycerate, or ribulose-1,5-bisphosphate. Starting from a high-resolution cryogenic electron microscopy structure of a synthetic [Formula: see text]-carboxysome shell, we perform unbiased all-atom molecular dynamics to track metabolite permeability across the shell. The synthetic carboxysome shell structure, lacking the bacterial microcompartment trimer proteins and encapsulation peptides, is found to have similar permeability coefficients for multiple metabolites, and is not selectively permeable to [Formula: see text] relative to CO2. To resolve how these comparable permeabilities can be reconciled with the clear role of the carboxysome in the CO2-concentrating mechanism in cyanobacteria, complementary atomic-resolution Brownian Dynamics simulations estimate the mean first passage time for CO2 assimilation in a crowded model carboxysome. Despite a relatively high CO2 permeability of approximately 10-2 cm/s across the carboxysome shell, the shell proteins reflect enough CO2 back toward rubisco that 2,650 CO2 molecules can be fixed by rubisco for every 1 CO2 molecule that escapes under typical conditions. The permeabilities determined from all-atom molecular simulation are key inputs into flux modeling, and the insight gained into carbon fixation can facilitate the engineering of carboxysomes and other bacterial microcompartments for multiple applications.

Uncovering the roles of the scaffolding protein CsoS2 in mediating the assembly and shape of the α-carboxysome shell

Carboxysomes are proteinaceous organelles featuring icosahedral protein shells that enclose the carbon-fixing enzymes, ribulose-1,5-bisphosphate carboxylase oxygenase (Rubisco), along with carbonic anhydrase. The intrinsically disordered scaffolding protein CsoS2 plays a vital role in the construction of α-carboxysomes through bridging the shell and cargo enzymes. The N-terminal domain of CsoS2 binds Rubisco and facilitates Rubisco packaging within the α-carboxysome, whereas the C-terminal domain of CsoS2 (CsoS2-C) anchors to the shell and promotes shell assembly. However, the role of the middle region of CsoS2 (CsoS2-M) has remained elusive. Here, we conducted in-depth examinations on the function of CsoS2-M in the assembly of the α-carboxysome shell by generating a series of recombinant shell variants in the absence of cargos. Our results reveal that CsoS2-M assists CsoS2-C in the assembly of the α-carboxysome shell and plays an important role in shaping the α-carboxysome shell through enhancing the association of shell proteins on both the facet-facet interfaces and flat shell facets. Moreover, CsoS2-M is responsible for recruiting the C-terminal truncated isoform of CsoS2, CsoS2A, into α-carboxysomes, which is crucial for Rubisco encapsulation and packaging. This study not only deepens our knowledge of how the carboxysome shell is constructed and regulated but also lays the groundwork for engineering and repurposing carboxysome-based nanostructures for diverse biotechnological purposes.

IMPORTANCE

Carboxysomes are a paradigm of organelle-like structures in cyanobacteria and many proteobacteria. These nanoscale compartments enclose Rubisco and carbonic anhydrase within an icosahedral virus-like shell to improve CO2 fixation, playing a vital role in the global carbon cycle. Understanding how the carboxysomes are formed is not only important for basic research studies but also holds promise for repurposing carboxysomes in bioengineering applications. In this study, we focuses on a specific scaffolding protein called CsoS2, which is involved in facilitating the assembly of α-type carboxysomes. By deciphering the functions of different parts of CsoS2, especially its middle region, we provide new insights into how CsoS2 drives the stepwise assembly of the carboxysome at the molecular level. This knowledge will guide the rational design and reprogramming of carboxysome nanostructures for many biotechnological applications.

A robust synthetic biology toolkit to advance carboxysome study and redesign

Carboxysomes are polyhedral protein organelles that microorganisms use to facilitate carbon dioxide assimilation. They are composed of a modular protein shell which envelops an enzymatic core mainly comprised of physically coupled Rubisco and carbonic anhydrase. While the modular construction principles of carboxysomes make them attractive targets as customizable metabolic platforms, their size and complexity can be a hinderance. In this work, we design and validate a plasmid set - the pXpressome toolkit - in which α-carboxysomes are robustly expressed and remain intact and functional after purification. We tested this toolkit by introducing mutations which influence carboxysome structure and performance. We find that deletion of vertex-capping genes results in formation of larger carboxysomes while deletion of facet forming genes produces smaller particles, suggesting that adjusting the ratio of these proteins can rationally affect morphology. Through a series of fluorescently labeled constructs, we observe this toolkit leads to more uniform expression and better cell health than previously published carboxysome expression systems. Overall, the pXpressome toolkit facilitates the study and redesign of carboxysomes with robust performance and improved phenotype uniformity. The pXpressome toolkit will support efforts to remodel carboxysomes for enhanced carbon fixation or serve as a platform for other nanoencapsulation goals.

Rubisco packaging and stoichiometric composition of a native β-carboxysome

Carboxysomes are anabolic bacterial microcompartments that play an essential role in carbon fixation in cyanobacteria. This self-assembling proteinaceous organelle encapsulates the key CO2-fixing enzymes, Rubisco and carbonic anhydrase, using a polyhedral shell constructed by hundreds of shell protein paralogs. Deciphering the precise arrangement and structural organization of Rubisco enzymes within carboxysomes is crucial for understanding the formation process and overall functionality of carboxysomes. Here, we employed cryo-electron tomography and subtomogram averaging to delineate the three-dimensional packaging of Rubiscos within β-carboxysomes in the freshwater cyanobacterium Synechococcus elongatus PCC7942 that were grown under low light. Our results revealed that Rubiscos are arranged in multiple concentric layers parallel to the shell within the β-carboxysome lumen. We also identified the binding of Rubisco with the scaffolding protein CcmM in β-carboxysomes, which is instrumental for Rubisco encapsulation and β-carboxysome assembly. Using QconCAT-based quantitative mass spectrometry, we further determined the absolute stoichiometric composition of the entire β-carboxysome. This study and recent findings on the β-carboxysome structure provide insights into the assembly principles and structural variation of β-carboxysomes, which will aid in the rational design and repurposing of carboxysome nanostructures for diverse bioengineering applications.

Unraveling Rubisco packaging within β-carboxysomes

In this issue of Structure, Kong et al. utilized cryoelectron tomography to closely examine Rubisco packaging within β-carboxysomes. They observed unique Rubisco packaging arrangements that may have important implications for carboxysome structural integrity.

Conserved and repetitive motifs in an intrinsically disordered protein drive ⍺-carboxysome assembly

All cyanobacteria and some chemoautotrophic bacteria fix CO2 into sugars using specialized proteinaceous compartments called carboxysomes. Carboxysomes enclose the enzymes Rubisco and carbonic anhydrase inside a layer of shell proteins to increase the CO2 concentration for efficient carbon fixation by Rubisco. In the ⍺-carboxysome lineage, a disordered and highly repetitive protein named CsoS2 is essential for carboxysome formation and function. Without it, the bacteria require high CO2 to grow. How does a protein predicted to be lacking structure serve as the architectural scaffold for such a vital cellular compartment? In this study, we identify key residues present in the repeats of CsoS2, VTG and Y, which are necessary for building functional ⍺-carboxysomes in vivo. These highly conserved and repetitive residues contribute to the multivalent binding interaction and phase separation behavior between CsoS2 and shell proteins. We also demonstrate 3-component reconstitution of CsoS2, Rubisco, and shell proteins into spherical condensates and show the utility of reconstitution as a biochemical tool to study carboxysome biogenesis. The precise self-assembly of thousands of proteins is crucial for carboxysome formation, and understanding this process could enable their use in alternative biological hosts or industrial processes as effective tools to fix carbon.

Bacterial microcompartments as a next-generation metabolic engineering tool: utilizing nature's solution for confining challenging catabolic pathways

Advancements in synthetic biology have facilitated the incorporation of heterologous metabolic pathways into various bacterial chassis, leading to the synthesis of targeted bioproducts. However, total output from heterologous production pathways can suffer from low flux, enzyme promiscuity, formation of toxic intermediates, or intermediate loss to competing reactions, which ultimately hinder their full potential. The self-assembling, easy-to-modify, protein-based bacterial microcompartments (BMCs) offer a sophisticated way to overcome these obstacles by acting as an autonomous catalytic module decoupled from the cell's regulatory and metabolic networks. More than a decade of fundamental research on various types of BMCs, particularly structural studies of shells and their self-assembly, the recruitment of enzymes to BMC shell scaffolds, and the involvement of ancillary proteins such as transporters, regulators, and activating enzymes in the integration of BMCs into the cell's metabolism, has significantly moved the field forward. These advances have enabled bioengineers to design synthetic multi-enzyme BMCs to promote ethanol or hydrogen production, increase cellular polyphosphate levels, and convert glycerol to propanediol or formate to pyruvate. These pioneering efforts demonstrate the enormous potential of synthetic BMCs to encapsulate non-native multi-enzyme biochemical pathways for the synthesis of high-value products.

Dynamic structural determinants in bacterial microcompartment shells

Bacterial microcompartments (BMCs) are polyhedral structures that segregate enzymatic cargo from the cytosol via encapsulation within a protein shell. Unlike other biological polyhedra, such as viral capsids and encapsulins, BMC shells can exhibit a highly advantageous structural and functional plasticity, conforming to a variety of anabolic (CO2 fixation in carboxysomes) and catabolic (nutrient assimilation in metabolosomes) roles. Consequently, understanding the subunit properties and associated protein-protein interaction processes that guide shell assembly and function is a necessary step to fully harness BMCs as modular, biotechnological nanomachines. Here, we describe the recent insights into the dynamics of structural features of the key BMC domain (Pfam00936)-containing proteins, which serve as a structural template for BMC-H and BMC-T shell building blocks.

Structure and assembly of the α-carboxysome in the marine cyanobacterium Prochlorococcus

Carboxysomes are bacterial microcompartments that encapsulate the enzymes RuBisCO and carbonic anhydrase in a proteinaceous shell to enhance the efficiency of photosynthetic carbon fixation. The self-assembly principles of the intact carboxysome remain elusive. Here we purified α-carboxysomes from Prochlorococcus and examined their intact structures using single-particle cryo-electron microscopy to solve the basic principles of their shell construction and internal RuBisCO organization. The 4.2 Å icosahedral-like shell structure reveals 24 CsoS1 hexamers on each facet and one CsoS4A pentamer at each vertex. RuBisCOs are organized into three concentric layers within the shell, consisting of 72, 32 and up to 4 RuBisCOs at the outer, middle and inner layers, respectively. We uniquely show how full-length and shorter forms of the scaffolding protein CsoS2 bind to the inner surface of the shell via repetitive motifs in the middle and C-terminal regions. Combined with previous reports, we propose a concomitant 'outside-in' assembly principle of α-carboxysomes: the inner surface of the self-assembled shell is reinforced by the middle and C-terminal motifs of the scaffolding protein, while the free N-terminal motifs cluster to recruit RuBisCO in concentric, three-layered spherical arrangements. These new insights into the coordinated assembly of α-carboxysomes may guide the rational design and repurposing of carboxysome structures for improving plant photosynthetic efficiency.

Nanoengineering Carboxysome Shells for Protein Cages with Programmable Cargo Targeting

Protein nanocages have emerged as promising candidates for enzyme immobilization and cargo delivery in biotechnology and nanotechnology. Carboxysomes are natural proteinaceous organelles in cyanobacteria and proteobacteria and have exhibited great potential in creating versatile nanocages for a wide range of applications given their intrinsic characteristics of self-assembly, cargo encapsulation, permeability, and modularity. However, how to program intact carboxysome shells with specific docking sites for tunable and efficient cargo loading is a key question in the rational design and engineering of carboxysome-based nanostructures. Here, we generate a range of synthetically engineered nanocages with site-directed cargo loading based on an α-carboxysome shell in conjunction with SpyTag/SpyCatcher and Coiled-coil protein coupling systems. The systematic analysis demonstrates that the cargo-docking sites and capacities of the carboxysome shell-based protein nanocages could be precisely modulated by selecting specific anchoring systems and shell protein domains. Our study provides insights into the encapsulation principles of the α-carboxysome and establishes a solid foundation for the bioengineering and manipulation of nanostructures capable of capturing cargos and molecules with exceptional efficiency and programmability, thereby enabling applications in catalysis, delivery, and medicine.

Modeling bacterial microcompartment architectures for enhanced cyanobacterial carbon fixation

The carboxysome is a bacterial microcompartment (BMC) which plays a central role in the cyanobacterial CO2-concentrating mechanism. These proteinaceous structures consist of an outer protein shell that partitions Rubisco and carbonic anhydrase from the rest of the cytosol, thereby providing a favorable microenvironment that enhances carbon fixation. The modular nature of carboxysomal architectures makes them attractive for a variety of biotechnological applications such as carbon capture and utilization. In silico approaches, such as molecular dynamics (MD) simulations, can support future carboxysome redesign efforts by providing new spatio-temporal insights on their structure and function beyond in vivo experimental limitations. However, specific computational studies on carboxysomes are limited. Fortunately, all BMC (including the carboxysome) are highly structurally conserved which allows for practical inferences to be made between classes. Here, we review simulations on BMC architectures which shed light on (1) permeation events through the shell and (2) assembly pathways. These models predict the biophysical properties surrounding the central pore in BMC-H shell subunits, which in turn dictate the efficiency of substrate diffusion. Meanwhile, simulations on BMC assembly demonstrate that assembly pathway is largely dictated kinetically by cargo interactions while final morphology is dependent on shell factors. Overall, these findings are contextualized within the wider experimental BMC literature and framed within the opportunities for carboxysome redesign for biomanufacturing and enhanced carbon fixation.

A widespread bacterial protein compartment sequesters and stores elemental sulfur

Subcellular compartments often serve to store nutrients or sequester labile or toxic compounds. As bacteria mostly do not possess membrane-bound organelles, they often have to rely on protein-based compartments. Encapsulins are one of the most prevalent protein-based compartmentalization strategies found in prokaryotes. Here, we show that desulfurase encapsulins can sequester and store large amounts of crystalline elemental sulfur. We determine the 1.78-angstrom cryo-EM structure of a 24-nanometer desulfurase-loaded encapsulin. Elemental sulfur crystals can be formed inside the encapsulin shell in a desulfurase-dependent manner with l-cysteine as the sulfur donor. Sulfur accumulation can be influenced by the concentration and type of sulfur source in growth medium. The selectively permeable protein shell allows the storage of redox-labile elemental sulfur by excluding cellular reducing agents, while encapsulation substantially improves desulfurase activity and stability. These findings represent an example of a protein compartment able to accumulate and store elemental sulfur.

α-Carboxysome Size Is Controlled by the Disordered Scaffold Protein CsoS2

Carboxysomes are protein microcompartments that function in the bacterial CO2 concentrating mechanism (CCM) to facilitate CO2 assimilation. To do so, carboxysomes assemble from thousands of constituent proteins into an icosahedral shell, which encapsulates the enzymes Rubisco and carbonic anhydrase to form structures typically > 100 nm and > 300 megadaltons. Although many of the protein interactions driving the assembly process have been determined, it remains unknown how size and composition are precisely controlled. Here, we show that the size of α-carboxysomes is controlled by the disordered scaffolding protein CsoS2. CsoS2 contains two classes of related peptide repeats that bind to the shell in a distinct fashion, and our data indicate that size is controlled by the relative number of these interactions. We propose an energetic and structural model wherein the two repeat classes bind at the junction of shell hexamers but differ in their preferences for the shell contact angles, and thus the local curvature. In total, this model suggests that a set of specific and repeated interactions between CsoS2 and shell proteins collectively achieve the large size and monodispersity of α-carboxysomes.