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 ... More
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.
