title: The structure, kinetics and interactions of the β-carboxysomal β-carbonic anhydrase, CcaA.
abstract: CcaA is a β-carbonic anhydrase (CA) that is a component of the carboxysomes of a subset of β-cyanobacteria. This protein, which has a characteristic C-terminal extension of unknown function, is recruited to the carboxysome via interactions with CcmM, which is itself a γ-CA homolog with enzymatic activity in many, but not all cyanobacteria. We have determined the structure of CcaA from Synechocystis sp. PCC 6803 at 1.45 Å. In contrast with the dimer-of-dimers organization of most bacterial β-CAs, or the loose dimer-of-dimers-of-dimers organization found in the plant enzymes, CcaA shows a well-packed trimer-of-dimers organization. The proximal part of the characteristic C-terminal extension is ordered by binding at a site that passes through the two-fold symmetry axis shared with an adjacent dimer; as a result, only one of a pair of converging termini can be ordered at any given time. Docking in Rosetta failed to find well-packed solutions, indicating that formation of the CcaA/CcmM complex probably requires significant backbone movements in at least one of the binding partners. Surface plasmon resonance experiments showed that CcaA forms a complex with CcmM with sub-picomolar affinity, with contributions from residues in CcmM's αA helix and CcaA's C-terminal tail. Catalytic characterization showed CcaA to be among the least active β-CAs characterized to date, with activity comparable with the γ-CA, CcmM, it either complements or replaces. Intriguingly, the C-terminal tail appears to partly inhibit activity, possibly indicating a role in minimizing the activity of unencapsulated enzyme.
authors.affiliation: Department of Molecular and Cellular Biology, University of Guelph, Guelph, Ontario, Canada N1G2W1.
authors.forename: Leah D
authors.initials: LD
authors.lastname: McGurn
authors.affiliation: Department of Cell and Systems Biology, University of Toronto, Mississauga, Ontario, Canada L5L1C6.
authors.forename: Maryam
authors.initials: M
authors.lastname: Moazami-Goudarzi
authors.affiliation: Department of Molecular and Cellular Biology, University of Guelph, Guelph, Ontario, Canada N1G2W1.
authors.forename: Sean A
authors.initials: SA
authors.lastname: White
authors.affiliation: Department of Biology, University of Toronto, Mississauga, Ontario, Canada L5L1C6.
authors.forename: Tannu
authors.initials: T
authors.lastname: Suwal
authors.affiliation: Department of Biology, University of Toronto, Mississauga, Ontario, Canada L5L1C6.
authors.forename: Beant
authors.initials: B
authors.lastname: Brar
authors.affiliation: Department of Biology, University of Toronto, Mississauga, Ontario, Canada L5L1C6.
authors.forename: Jason Q
authors.initials: JQ
authors.lastname: Tang
authors.affiliation: Department of Cell and Systems Biology, University of Toronto, Mississauga, Ontario, Canada L5L1C6.|Department of Biology, University of Toronto, Mississauga, Ontario, Canada L5L1C6.
authors.forename: George S
authors.initials: GS
authors.lastname: Espie
authors.affiliation: Department of Molecular and Cellular Biology, University of Guelph, Guelph, Ontario, Canada N1G2W1.
authors.forename: Matthew S
authors.initials: MS
authors.lastname: Kimber
mesh_terms: D000595:Amino Acid Sequence; D001426:Bacterial Proteins / Q000737:chemistry* / Q000378:metabolism*; D002256:Carbonic Anhydrases / Q000737:chemistry* / Q000378:metabolism*; D018360:Crystallography, X-Ray; D007700:Kinetics; D008969:Molecular Sequence Data; D055503:Protein Multimerization; D020349:Surface Plasmon Resonance; D046940:Synechococcus / Q000201:enzymology*
chemical_list: D001426:Bacterial Proteins; D002256:Carbonic Anhydrases
keywords: bacterial microcompartments; carbon dioxide-concentrating mechanism; carbonic anhydrase; carboxysomes; cyanobacteria; photosynthesis