ccmL

UniProt ID: P72759
Organism: Synechocystis sp. PCC 6803 substr. Kazusa
Review Status: COMPLETE
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Gene Description

CcmL is a small (100 aa) protein of the CcmL/EutN family that forms the vertices of the beta-carboxysome shell, the proteinaceous coat of the cyanobacterial organelle that encapsulates RuBisCO and carbonic anhydrase as part of the CO2-concentrating mechanism. The Synechocystis protein crystallises as a tight homopentamer (a truncated pentagonal pyramid with a narrow, positively charged central pore) whose size and shape fit the five-fold vertices of an icosahedral shell whose facets are tiled by hexameric CcmK proteins. Genetic loss of the Synechococcus elongatus ortholog gives high-CO2-requiring cells with aberrant, elongated carboxysomes, consistent with a role in closing and shaping the shell. By analogy with the alpha-carboxysome vertex proteins (CsoS4), vertex pentamers are needed for the shell to act as a CO2 leakage barrier. CcmL also interacts with the long CcmM isoform (CcmM73), linking the shell to the internal bicarbonate dehydration complex.

Existing Annotations Review

GO Term Evidence Action Reason
GO:0015977 carbon fixation
IEA
GO_REF:0000104 		MARK AS OVER ANNOTATED
Summary: UniRule transfer of carbon fixation to CcmL. CcmL is a non-catalytic structural shell (vertex) protein; it neither catalyses the carboxylation step nor supplies anything incorporated into the product. RuBisCO performs carbon fixation, and the carboxysome shell creates the compartment in which it works more efficiently.
Reason: The process is the purpose of the organelle CcmL builds, not something CcmL does. Necessity evidence (ccmL mutants require high CO2; vertex proteins are needed for the shell to retain CO2) shows CcmL is required for efficient fixation in vivo, which is not the same as participating in the fixation step. Comparator check (QuickGO, 2026-10-19): other beta-carboxysome shell proteins, e.g. Synechocystis CcmK2 (P72761) and S. elongatus CcmL (Q03512), carry GO:0015977 only through this same GO_REF:0000104 rule; no shell protein has an experimental carbon-fixation annotation. The protein's role is already captured by the carboxysome location and structural constituent of carboxysome shell function. GO currently has no carboxysome-assembly or CO2-concentrating-mechanism process term that would be the appropriate process for CcmL (raised as a suggested question). Marked as over-annotation rather than removed because the term is not false at the level of the organelle.
Supporting Evidence:
PMID:18292340
The carboxysome is a bacterial microcompartment that functions as a simple organelle by sequestering enzymes involved in carbon fixation.
PMID:19844578
In the absence of the CsoS4 proteins the carboxysome shell loses its limited permeability to CO(2)
GO:0031469 bacterial microcompartment
IEA
GO_REF:0000002 		TOO GENERAL
Summary: InterPro2GO mapping from the EutN/CcmL family signature. Correct, but the carboxysome is the specific bacterial microcompartment for CcmL, and GO:0031470 carboxysome (is_a bacterial microcompartment) is accepted elsewhere in this review.
Reason: True but uninformative: entailed by the accepted carboxysome annotation. The family-level mapping is appropriate for EutN/PduN relatives in other microcompartments; for this beta-carboxysome protein the specific term applies.
Supporting Evidence:
file:SYNY3/ccmL/ccmL-uniprot.txt
Belongs to the CcmL/EutN family. CcmL subfamily.
GO:0031470 carboxysome
IEA
GO_REF:0000120 		ACCEPT
Summary: CcmL is a structural component of the beta-carboxysome shell. The crystal structure of the Synechocystis protein shows a pentamer fitted to the shell vertices, the S. elongatus ortholog is required for normal carboxysome shape, and CcmL interacts with the carboxysomal CcmM73 protein in Synechocystis.
Reason: Location is core to the protein's function. Supported by structure (PMID:18292340), genetics of the ortholog (PMID:8491708, PMID:22928045) and interaction with carboxysome cargo-linking protein CcmM (PMID:17993516).
Supporting Evidence:
PMID:18292340
Both proteins assemble to form pentameric structures whose size and shape are compatible with formation of vertices in an icosahedral shell.
PMID:17993516
This complex, through CcmM73, also interacts with the major bacterial microcompartment shell proteins CcmK and CcmL.
GO:0043886 structural constituent of carboxysome shell
IEA
GO_REF:0000120 		ACCEPT
Summary: Combined UniRule/InterPro (CcmL subfamily) transfer of the shell structural-constituent function. CcmL pentamers form the vertices of the shell, so the protein is a structural constituent of the shell even though it is not one of the hexameric facet proteins.
Reason: Consistent with the experimental IPI row and the structural and genetic literature. GO has no vertex-specific child term, so this is as precise as the ontology allows.
Supporting Evidence:
PMID:22928045
These data suggested a model for the facet structure of Ξ²-carboxysomes with CcmL forming the vertices
GO:0043886 structural constituent of carboxysome shell
IPI
PMID:18292340
Atomic-level models of the bacterial carboxysome shell. 		ACCEPT
Summary: The Science 2008 structure of the Synechocystis protein (PDB 2QW7) shows a tight homopentamer (IPI with itself) whose dimensions fit the vertices of an icosahedral carboxysome shell, and the authors built an atomic model of the shell combining CcmL pentamers with CcmK hexamers.
Reason: Core molecular function. The cached record is abstract-only, but the abstract directly states the pentameric vertex-compatible assembly, and later genetic (rod-shaped carboxysomes in ccmL mutants) and structural work on orthologs agree.
Supporting Evidence:
PMID:18292340
Combining these pentamers with the hexamers previously elucidated gives two plausible, preliminary atomic models for the carboxysome shell.
PMID:22928045
Mutant carboxysomes lacking the CcmL protein have a characteristic rod-like appearance which is consistent with its location at the icosahedral vertex in structural models

Core Functions

Forms homopentamers that occupy the vertices of the icosahedral beta-carboxysome shell, closing the CcmK hexamer facets; required for normal carboxysome shape and, by analogy with alpha-carboxysome vertex proteins, for the shell's barrier to CO2 leakage. Links to the internal bicarbonate dehydration complex via CcmM73.

Molecular Function:
structural constituent of carboxysome shell
Cellular Locations:
carboxysome
Supporting Evidence:
  • PMID:18292340
    Both proteins assemble to form pentameric structures whose size and shape are compatible with formation of vertices in an icosahedral shell.
  • PMID:22928045
    These data suggested a model for the facet structure of Ξ²-carboxysomes with CcmL forming the vertices
  • PMID:8491708
    The PVU mutant has a drug marker inserted into the ccmL gene, and it possesses abnormal rod-shaped carboxysomes.

References

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Suggested Questions for Experts

Q: GO has no process term for carboxysome (or bacterial microcompartment) assembly, nor for the CO2-concentrating mechanism. Should such a term be created, so that shell proteins like CcmL can carry an informative process instead of the inherited GO:0015977 carbon fixation?

Q: Are all twelve vertices of the beta-carboxysome occupied by CcmL pentamers in vivo, or are some left open or filled by other proteins, and does vertex occupancy vary with carboxysome size?

Q: Does the narrow, positively charged CcmL pore conduct any metabolite (e.g. bicarbonate), or is CcmL purely structural?

Suggested Experiments

Experiment: Delete ccmL in Synechocystis PCC 6803 and measure growth at air versus high CO2, carboxysome morphology (cryo-ET) and the CO2 permeability of purified carboxysomes.

Hypothesis: As for alpha-carboxysome CsoS4 mutants, Synechocystis carboxysomes lacking CcmL are misshapen and leak CO2.

Experiment: Quantify CcmL copy number per carboxysome by quantitative proteomics or single-particle fluorescence of a functional tagged allele, alongside cryo-EM of intact beta-carboxysomes.

Hypothesis: CcmL is present at about 60 copies (12 pentamers) per carboxysome, one per vertex.

Experiment: Mutate pore-lining residues of CcmL and test carboxysome function and solute flux in reconstituted shells.

Hypothesis: The CcmL pore does not make a measurable contribution to substrate flux.

External Prediction Reviews

These computational predictions are reviewed separately from the GOA annotation set used for this review. The assessments below are from this project and do not constitute official GO annotations or endorsement by GO/UniProt. They are not included in the existing annotation review above.

NCBI PGAP External predictions

View prediction review YAML Β· ccmL-pgap-predictions-review.yaml Β· Review status: COMPLETE

NCBI PGAP names WP_010871404.1 "beta-carboxysome shell vertex protein CcmL" from the NCBIfam equivalog HMM NF053795.1, which carries three GO terms: carboxysome, structural constituent of carboxysome shell, and carbon fixation. All three are also in GOA (from UniRule/InterPro IEA, plus an IPI for the shell-constituent function). The product name is accurate and more informative than the GO terms, since it records the vertex (pentamer) role; GO has no vertex-specific child of structural constituent of carboxysome shell, so the shell-constituent term is as precise as the ontology allows. Carboxysome and structural constituent of carboxysome shell are correct and agree with the main review (CNN). Carbon fixation is the same pathway-membership over-annotation that the main review marks as over-annotated: CcmL is a non-catalytic shell protein and RuBisCO performs the fixation step. These GO terms appear on the current WP_ protein record (updated 14-APR-2026) but not on the January 2026 genome annotation (GCF_000009725.1-RS_2026_01_07), so the genome record lags the WP_ records. PGAP did not pass on bacterial microcompartment from the secondary domain HMM NF015289, which would have been redundant with carboxysome.

Source documents: genes/SYNY3/ccmL/ccmL-pgap-prediction-evidence/WP_010871404.1.gp Β· genes/SYNY3/ccmL/ccmL-pgap-prediction-evidence/pgap_models.yaml Β· publications/PMID_18292340.md Β· publications/PMID_22928045.md

Review score: 2 = concordant with evidence; 1 = uncertain; 0 = discordant with evidence. This is an assessment score, not a model probability.

GO:0031470 carboxysome GO_CC
CNN β€” Correct but not novel Review score: 2/2
Prediction method: NCBI PGAP Β· Version: GCF_000009725.1-RS_2026_01_07 (live); GO from WP_ record Β· PMID:33270901
Review rationale: Correct. CcmL forms the vertices of the beta-carboxysome shell (structure of this protein, PMID:18292340). The same term is in GOA (GO_REF:0000120 IEA) and is ACCEPTed in the main review, so the prediction is correct but not novel. Not on the January 2026 genome record; present on the newer WP_ record.
Supporting Evidence:
GO:0043886 structural constituent of carboxysome shell GO_MF
CNN β€” Correct but not novel Review score: 2/2
Prediction method: NCBI PGAP Β· Version: GCF_000009725.1-RS_2026_01_07 (live); GO from WP_ record Β· PMID:33270901
Review rationale: Correct. Although CcmL is a pentameric vertex protein rather than one of the hexameric CcmK facet proteins, the vertices are part of the shell, and GO has no vertex-specific child term, so this is the most specific applicable MF. The PGAP product name ("shell vertex protein") supplies the vertex detail. Also in GOA (IEA and IPI from PMID:18292340) and ACCEPTed in the main review: correct but not novel. Not on the January 2026 genome record.
Supporting Evidence:
GO:0015977 carbon fixation GO_BP
NPI β€” Nonparalog incorrect Review score: 0/2
Prediction method: NCBI PGAP Β· Version: GCF_000009725.1-RS_2026_01_07 (live); GO from WP_ record Β· PMID:33270901
COMPLEX ACTIVITY TRANSFER
Review rationale: Over-annotation. Carbon fixation is performed by RuBisCO, the cargo of the carboxysome; CcmL is a non-catalytic shell vertex protein that builds the compartment. Its requirement for efficient fixation in vivo is necessity, not participation in the fixation step, so the organelle's process has been transferred to a structural subunit. The same term is in GOA via UniRule (GO_REF:0000104) and is MARK_AS_OVER_ANNOTATED in the main review; the comparator check found no experimental carbon-fixation annotation on any beta-carboxysome shell protein. Not on the January 2026 genome record. GO lacks a carboxysome-assembly or CO2-concentrating-mechanism process term that would be the appropriate process for this family model.
Supporting Evidence:

πŸ“š Additional Documentation

Notes

(ccmL-notes.md)

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