Scientists Map How Carbon-Fixing Enzymes Pack Inside Bacterial Nano-Compartments

Carboxysomes are protein-shelled nano-compartments found in certain bacteria that supercharge CO₂ fixation—they concentrate CO₂ around the enzyme Rubisco by trapping bicarbonate inside the shell and rapidly converting it to CO₂ via an internal carbonic anhydrase (CA). Despite their importance, the precise molecular structure of CA inside α-carboxysomes and the mechanism by which it is recruited and retained had remained unclear.

In this study, researchers used single-particle cryo-EM to solve the structure of CsoSCA from the model chemoautotrophic bacterium Halothiobacillus neapolitanus (HnCsoSCA) at 2.51 Å resolution. The enzyme forms a homohexamer with D3 symmetry—a trimer-of-dimers arrangement—consistent with its elution at ~420 kDa on size-exclusion chromatography. Crucially, unlike the recently characterized CsoSCA from Cyanobium (CyCsoSCA), which requires a zinc ion at its hexameric interface to stabilize the quaternary structure, HnCsoSCA achieves hexameric assembly through hydrophobic interactions at the threefold symmetry axis. Each monomer retains its own catalytic zinc ion coordinated by Cys173, His242, and Cys253, yielding six active sites per hexamer. A short loop (residues 143–154) connecting the N-terminal domain to the catalytic domain remained unresolved, suggesting conformational flexibility in this region.

To probe how HnCsoSCA is encapsulated, the team employed synthetic α-carboxysome minishells—minimal shell assemblies composed primarily of the CsoS1A hexameric shell protein. Co-expression and pull-down experiments demonstrated that HnCsoSCA physically interacts with CsoS1A and is incorporated into the minishell interior. Importantly, this encapsulation occurred independently of CsoS2, a linker protein previously thought essential for recruiting cargo enzymes. Truncation studies of HnCsoSCA suggested the interactions with CsoS1A are nonspecific (i.e., distributed across the enzyme surface rather than requiring a single short targeting peptide), distinguishing the α-carboxysome encapsulation mechanism from systems that rely on discrete encapsulation peptides.

The team further demonstrated that HnCsoSCA acts as a molecular bridge between Rubisco and the carboxysome shell facets. This bridging role positions CA optimally between the shell (where bicarbonate enters) and Rubisco (where CO₂ is fixed), creating an efficient metabolic channeling architecture. These findings revise the earlier model in which CsoSCA was thought to be a simple dimer with a shell-association that required CsoS2.

For longevity and medicine, the significance is primarily foundational but substantial: carboxysomes are now being explored as programmable bio-nano-containers for enzyme delivery, biosensing, and synthetic CO₂ sequestration. Understanding the precise encapsulation rules for CA—a key enzyme also relevant to human physiology and drug targeting—provides a molecular toolkit for engineering these structures with new cargoes for biotechnological and potentially therapeutic applications.