carboxysome shell

cellstructuremech:carboxysome_shell · Microcompartment · Multiprotein complex · Proposed

The polyhedral protein coat of alpha- and beta-carboxysomes, built from BMC-H, BMC-T, and BMC-P shell proteins that enclose RuBisCO and carbonic anhydrase while controlling metabolite traffic between the lumen and cytosol. [DOI:10.1038/nrmicro.2018.10]

Synonyms: carboxysome protein shell; alpha-carboxysome shell; beta-carboxysome shell

Part of: carboxysome (GO:0031470)

UniProt location names: UniProt Consortium, CC BY 4.0. Reference label and route sources.

Components

Component	Type	Part of / acts on	Grounding	Genes	Essential	Role
BMC-H shell hexamers (CcmK / CsoS1)	Protein	part of	Reviewed label only (reviewed name; no accession asserted)	ccmK, csoS1	ESSENTIAL	Hexameric single-BMC-domain proteins that tile the shell facets. `DOI:10.1038/nrmicro.2018.10` Kerfeld et al. 2018 review BMC-H proteins as hexameric carboxysome shell tiles built from CcmK proteins in beta-carboxysomes and CsoS1 proteins in alpha-carboxysomes. `DOI:10.1371/journal.pbio.0050144` Tsai et al. 2007 structurally analyzed the Halothiobacillus neapolitanus alpha-carboxysome shell protein CsoS1A. `DOI:10.1371/journal.pone.0043871` Rae et al. 2012 analyzed CcmK proteins in the Synechococcus elongatus PCC 7942 beta-carboxysome outer shell.
BMC-T shell pseudohexamers (CcmO, CcmP, CsoS1D)	Protein	part of	Reviewed label only (reviewed name; no accession asserted)	ccmO, ccmP, csoS1D	CONDITIONAL	Tandem-BMC-domain proteins that contribute pseudohexamers to the shell and can introduce larger gated pores. `DOI:10.1038/nrmicro.2018.10` Kerfeld et al. 2018 review BMC-T paralogues as pseudohexameric shell proteins in carboxysomes and other bacterial microcompartments. `PMID:23572529` Cai et al. 2013 structurally analyzed Synechococcus elongatus CcmP, a beta-carboxysome tandem-BMC-domain protein that forms a double-trimer pseudohexamer in the shell. `DOI:10.1128/JB.06444-11` Roberts et al. 2012 isolated Prochlorococcus alpha-carboxysomes and detected the tandem-BMC shell protein CsoS1D as a minor shell component.
BMC-P shell pentamers (CcmL / CsoS4)	Protein	part of	Reviewed label only (reviewed name; no accession asserted)	ccmL, csoS4A, csoS4B	ESSENTIAL	Pentameric shell proteins that cap the vertices of the polyhedral shell. `DOI:10.1038/nrmicro.2018.10` Kerfeld et al. 2018 review BMC-P proteins as pentameric caps at the vertices of carboxysome and other BMC shells. `DOI:10.1371/journal.pone.0043871` Rae et al. 2012 found that CcmL is part of the minimum structural determinant set for the Synechococcus elongatus PCC 7942 beta-carboxysome outer shell. `DOI:10.1371/journal.pone.0007521` Cai et al. 2009 showed that deleting both alpha-carboxysome vertex proteins CsoS4A and CsoS4B yields elongated shells and severe high-CO2-requiring phenotypes.

Organism-specific protein examples

Named proteins in which each component's role was established, with the organism the evidence comes from. The component above is the taxon-agnostic family; these are the specific entries behind it.

BMC-H shell hexamers (CcmK / CsoS1)

Accession	Protein	Gene	Organism	Entry
`UniProtKB:Q03511`	Carboxysome shell protein CcmK2	ccmK2	Synechococcus elongatus PCC 7942 (`NCBITaxon:1140`)	REVIEWED
Role and evidence (3 references) The single reviewed UniProtKB entry for gene ccmK, which UniProt lists under the primary name ccmK2, in Synechococcus elongatus PCC 7942, a taxon this record names. Identifier for the component's declared gene symbol; it is not itself evidence that the protein is part of this structure. `PMID:17675289` Long BM et al. 2007, ‘Analysis of carboxysomes from Synechococcus PCC7942 reveals multiple Rubisco complexes with carboxysomal proteins CcmM and CcaA’, J Biol Chem. ECO:0000269 experimental evidence cited by UniProt for the 'Carboxysome' localisation of Q03511. `PMID:20304968` Long BM et al. 2010, ‘Functional cyanobacterial beta-carboxysomes have an absolute requirement for both long and short forms of the CcmM protein’, Plant Physiol. ECO:0000269 experimental evidence cited by UniProt for the 'Carboxysome' localisation of Q03511. `PMID:22928045` Rae BD et al. 2012, ‘Structural determinants of the outer shell of β-carboxysomes in Synechococcus elongatus PCC 7942: roles for CcmK2, K3-K4, CcmO, and CcmL’, PLoS One. ECO:0000269 experimental evidence cited by UniProt for the 'Carboxysome' localisation of Q03511.
`UniProtKB:P45689`	Major carboxysome shell protein CsoS1A	csoS1A	Halothiobacillus neapolitanus c2 (`NCBITaxon:555778`)	REVIEWED
Role and evidence (2 references) The single reviewed UniProtKB entry for gene csoS1, which UniProt lists under the primary name csoS1A, in Halothiobacillus neapolitanus c2, a taxon this record names. Identifier for the component's declared gene symbol; it is not itself evidence that the protein is part of this structure. `PMID:18258595` Dou Z et al. 2008, ‘CO2 fixation kinetics of Halothiobacillus neapolitanus mutant carboxysomes lacking carbonic anhydrase suggest the shell acts as a diffusional barrier for CO2.’, J Biol Chem. ECO:0000269 experimental evidence cited by UniProt for the 'Carboxysome' localisation of P45689. `PMID:7934888` English RS et al. 1994, ‘Isolation and characterization of a carboxysome shell gene from Thiobacillus neapolitanus.’, Mol Microbiol. ECO:0000269 experimental evidence cited by UniProt for the 'Carboxysome' localisation of P45689.

BMC-T shell pseudohexamers (CcmO, CcmP, CsoS1D)

Accession	Protein	Gene	Organism	Entry
`UniProtKB:P46205`	Carboxysome assembly protein CcmO	ccmO	Synechococcus elongatus PCC 7942 (`NCBITaxon:1140`)	REVIEWED
Role and evidence (3 references) The single reviewed UniProtKB entry for gene ccmO in Synechococcus elongatus PCC 7942, a taxon this record names. Identifier for the component's declared gene symbol; it is not itself evidence that the protein is part of this structure. `PMID:22928045` Rae BD et al. 2012, ‘Structural determinants of the outer shell of β-carboxysomes in Synechococcus elongatus PCC 7942: roles for CcmK2, K3-K4, CcmO, and CcmL’, PLoS One. ECO:0000269 experimental evidence cited by UniProt for the 'Carboxysome' localisation of P46205. `PMID:24267892` Cameron JC et al. 2013, ‘Biogenesis of a bacterial organelle: the carboxysome assembly pathway’, Cell. ECO:0000269 experimental evidence cited by UniProt for the 'Carboxysome' localisation of P46205. `PMID:31048338` Sun Y et al. 2019, ‘Single-Organelle Quantification Reveals Stoichiometric and Structural Variability of Carboxysomes Dependent on the Environment’, Plant Cell. ECO:0000269 experimental evidence cited by UniProt for the 'Carboxysome' localisation of P46205.
`UniProtKB:Q31QW7`	Carboxysome shell protein CcmP	ccmP	Synechococcus elongatus PCC 7942 (`NCBITaxon:1140`)	REVIEWED
Role and evidence (1 reference) The single reviewed UniProtKB entry for gene ccmP in Synechococcus elongatus PCC 7942, a taxon this record names. Identifier for the component's declared gene symbol; it is not itself evidence that the protein is part of this structure. `PMID:23572529` Cai F et al. 2013, ‘The structure of CcmP, a tandem bacterial microcompartment domain protein from the β-carboxysome, forms a subcompartment within a microcompartment’, J Biol Chem. ECO:0000269 experimental evidence cited by UniProt for the 'Carboxysome' localisation of Q31QW7.
`UniProtKB:D0KZ73`	Carboxysome shell protein CsoS1D	csoS1D	Halothiobacillus neapolitanus c2 (`NCBITaxon:555778`)	REVIEWED
Role and evidence (1 reference) The single reviewed UniProtKB entry for gene csoS1D in Halothiobacillus neapolitanus c2, a taxon this record names. Identifier for the component's declared gene symbol; it is not itself evidence that the protein is part of this structure. `https://www.uniprot.org/uniprotkb/D0KZ73` UniProtKB reviewed (Swiss-Prot) entry D0KZ73, the single reviewed entry for gene csoS1D in Halothiobacillus neapolitanus c2. Establishes the accession and gene identity only; the component's membership in this structure rests on the record's own sources.

BMC-P shell pentamers (CcmL / CsoS4)

Accession	Protein	Gene	Organism	Entry
`UniProtKB:Q03512`	Carboxysome shell vertex protein CcmL	ccmL	Synechococcus elongatus PCC 7942 (`NCBITaxon:1140`)	REVIEWED
Role and evidence (2 references) The single reviewed UniProtKB entry for gene ccmL in Synechococcus elongatus PCC 7942, a taxon this record names. Identifier for the component's declared gene symbol; it is not itself evidence that the protein is part of this structure. `PMID:28616951` Faulkner M et al. 2017, ‘Direct characterization of the native structure and mechanics of cyanobacterial carboxysomes’, Nanoscale. ECO:0000269 experimental evidence cited by UniProt for the 'Carboxysome' localisation of Q03512. `PMID:31048338` Sun Y et al. 2019, ‘Single-Organelle Quantification Reveals Stoichiometric and Structural Variability of Carboxysomes Dependent on the Environment’, Plant Cell. ECO:0000269 experimental evidence cited by UniProt for the 'Carboxysome' localisation of Q03512.
`UniProtKB:O85043`	Carboxysome shell vertex protein CsoS4A	csoS4A	Halothiobacillus neapolitanus c2 (`NCBITaxon:555778`)	REVIEWED
Role and evidence (1 reference) The single reviewed UniProtKB entry for gene csoS4A in Halothiobacillus neapolitanus c2, a taxon this record names. Identifier for the component's declared gene symbol; it is not itself evidence that the protein is part of this structure. `PMID:19844578` Cai F et al. 2009, ‘The pentameric vertex proteins are necessary for the icosahedral carboxysome shell to function as a CO2 leakage barrier.’, PLoS One. ECO:0000269 experimental evidence cited by UniProt for the 'Carboxysome' localisation of O85043.
`UniProtKB:O85044`	Carboxysome shell vertex protein CsoS4B	csoS4B	Halothiobacillus neapolitanus c2 (`NCBITaxon:555778`)	REVIEWED
Role and evidence (1 reference) The single reviewed UniProtKB entry for gene csoS4B in Halothiobacillus neapolitanus c2, a taxon this record names. Identifier for the component's declared gene symbol; it is not itself evidence that the protein is part of this structure. `PMID:18292340` Tanaka S et al. 2008, ‘Atomic-level models of the bacterial carboxysome shell.’, Science. ECO:0000269 experimental evidence cited by UniProt for the 'Carboxysome' localisation of O85044.

Taxonomic distribution

NCBITaxon:2 Bacteria — VARIABLE: Carboxysome shells occur in cyanobacterial and proteobacterial carboxysomes but are absent from bacterial lineages and strains without carboxysome loci. [DOI:10.1038/nrmicro.2018.10]

Canonical examples

NCBITaxon:927 Halothiobacillus neapolitanus — Alpha-carboxysome model whose CsoS1A shell protein and isolated protein shell were structurally analyzed. [DOI:10.1371/journal.pbio.0050144]
NCBITaxon:1140 Synechococcus elongatus PCC 7942 — Beta-carboxysome model used to test the roles of CcmK2, CcmK3, CcmK4, CcmO, and CcmL in the outer shell. [DOI:10.1371/journal.pone.0043871]

Functions

selective metabolite permeability — Pores through shell facets allow small metabolites such as bicarbonate to cross the shell while restricting larger or unfavorably charged solutes.
- DOI:10.1038/nrmicro.2018.10 Kerfeld et al. 2018 review selective metabolite flux through pores in BMC-H and BMC-T carboxysome shell proteins.
CO2 retention — The shell acts as a CO2 leakage barrier, retaining carbon dioxide generated inside the lumen for fixation by encapsulated RuBisCO.
- DOI:10.1038/nrmicro.2018.10 Kerfeld et al. 2018 describe the carboxysome shell as limiting CO2 leakage from the compartment.
- DOI:10.1371/journal.pone.0007521 Cai et al. 2009 experimentally linked CsoS4 vertex caps to closed shells that act as a CO2 leakage barrier in Halothiobacillus neapolitanus.

Mechanism graphs

Shell tiles close a selective carboxysome barrier (FUNCTION)

BMC-H, BMC-T, and BMC-P paralogues build a closed polyhedral carboxysome shell. Pores in the shell allow bicarbonate entry while the shell slows CO2 escape from the lumen.

Subject	Predicate	Object	Evidence
BMC-H shell hexamers	tiles facets of	carboxysome shell	`DOI:10.1038/nrmicro.2018.10` Kerfeld et al. 2018 review BMC-H hexamers as the dominant facet-forming carboxysome shell proteins.
BMC-T shell pseudohexamers	contributes pores to	carboxysome shell	`PMID:23572529` Cai et al. 2013 resolved CcmP as a dimer of stacked trimers in the beta-carboxysome shell.
BMC-P shell pentamers	caps vertices of	carboxysome shell	`DOI:10.1038/nrmicro.2018.10` Kerfeld et al. 2018 review BMC-P proteins as pentameric vertex caps in carboxysomes and related BMCs. `DOI:10.1371/journal.pone.0007521` Cai et al. 2009 linked CsoS4 vertex pentamers to normal icosahedral shell closure.
carboxysome shell	permits passage of	bicarbonate	`DOI:10.1038/nrmicro.2018.10` Kerfeld et al. 2018 review the model that positively charged shell pores favor passage of bicarbonate and other anions.
carboxysome shell	limits loss of	carbon dioxide	`DOI:10.1038/nrmicro.2018.10` Kerfeld et al. 2018 review the shell permeability model in which carboxysome pores keep CO2 from leaking back to the cytosol. `DOI:10.1371/journal.pone.0007521` Cai et al. 2009 showed that CsoS4 vertex deletions compromise the carboxysome shell as a CO2 leakage barrier.

Discussions

CURATION_TODO (RESOLVED): Should encapsulated RuBisCO, carbonic anhydrase, CcmM or CcmN be duplicated as carboxysome shell components? The shell record is scoped to the polyhedral protein coat. Encapsulated cargo and beta-carboxysome adaptors are components of the whole carboxysome and shape shell closure, but they are not shell tiles or vertex proteins.

Evidence

DOI:10.1038/nrmicro.2018.10 Kerfeld CA, Aussignargues C, Zarzycki J, Cai F and Sutter M 2018, Bacterial microcompartments, Nature Reviews Microbiology.
DOI:10.1371/journal.pbio.0050144 Tsai Y et al. 2007, Structural Analysis of CsoS1A and the Protein Shell of the Halothiobacillus neapolitanus Carboxysome, PLoS Biology.
DOI:10.1371/journal.pone.0043871 Rae BD, Long BM, Badger MR and Price GD 2012, Structural determinants of the outer shell of beta-carboxysomes in Synechococcus elongatus PCC 7942, PLoS One.
PMID:23572529 Cai F et al. 2013, The structure of CcmP, a tandem bacterial microcompartment domain protein from the beta-carboxysome, forms a subcompartment within a microcompartment, Journal of Biological Chemistry.
DOI:10.1371/journal.pone.0007521 Cai F, Menon BB, Cannon GC, Curry KJ, Shively JM and Heinhorst S 2009, The pentameric vertex proteins are necessary for the icosahedral carboxysome shell to function as a CO2 leakage barrier, PLoS One.
DOI:10.1128/JB.06444-11 Roberts EW, Cai F, Kerfeld CA, Cannon GC and Heinhorst S 2012, Isolation and characterization of the Prochlorococcus carboxysome reveal the presence of the novel shell protein CsoS1D, Journal of Bacteriology.

Curation history

2026-09-07T00:20:21Z claude · CREATED_RECORD — Scaffolded by scripts/new_record.py
2026-09-07T00:31:45Z claude · EXPANDED_RECORD — Added shell synonymy, carboxysome parthood, BMC-H/BMC-T/BMC-P coat components, bacterial taxonomic scope, alpha- and beta-carboxysome examples, shell permeability functions, core literature evidence, a selective-barrier causal graph, and a boundary discussion excluding lumen cargo and adaptors from the shell record.
2026-09-07T00:40:25Z claude · FIX_DISCUSSION_ANCHOR — Removed the section-wide discussion anchor because discussion anchors must name an existing local id such as components#bmc_h, not a bare section.
2026-09-07T01:11:43Z claude · FIX_REVIEW_ISSUES — Resolved #221 by adding the Cai et al. 2009 primary CO2 leakage-barrier paper to BMC-P, co2_retention and graph evidence; resolved #222 by adding the Roberts et al. 2012 CsoS1D primary paper to BMC-T evidence.
2026-09-08T07:39:46Z uniprot_genes · SEED_PROTEIN_EXAMPLES — #183: added 8 protein example(s) by resolving each of the component's declared gene symbols to the single reviewed UniProtKB entry for that symbol in a taxon this record names, deduplicated by accession. Symbols resolving to more than one reviewed entry were skipped rather than chosen between.

Source: data/structures/microcompartment/carboxysome_shell.yaml