Functions as a scaffold protein for the assembly of beta-carboxysomes, initiates carboxysome assembly by coalescing RuBisCO (ribulose bisphosphate carboxylase, rbcL-rbcS) (Probable). Produced as a full-length and a shorter form; both forms are required for correct carboxysome assembly and growth (By similarity). A carbonic anhydrase, catalyzes the reversible hydration of carbon dioxide. Essential to photosynthetic carbon dioxide fixation, supplies CO(2) to ribulose bisphosphate carboxylase (RuBisCO) in the carboxysome. Active when the disulfide bond (194-200) is oxidized, suggesting the interior of the carboxysome is oxidizing. Beta-carboxysome assembly initiates when soluble RuBisCO is condensed into a liquid matrix in a pre-carboxysome by the RbcS-like domains of probably both forms of CcmM (Probable). CcmN interacts with the N-terminus of full length CcmM, and then recruits the shell proteins (CcmK) via CcmN's encapsulation peptide. Shell formation requires both CcmK proteins and CcmO. CcmL caps the otherwise elongated carboxysome. Once fully encapsulated carboxysomes are formed, they migrate within the cell probably via interactions with the cytoskeleton (By similarity). H(+) + hydrogencarbonate = CO2 + H2O Binds 3 zinc per trimer. Carbonic anhydrase (CA) activity is probably under redox control to remain inactive in the cytoplasm. CA activity of full-length protein inhibited by ethoxyzolamide, and for both full-length and CcmM 1-209 construct by reducing agents. The N-terminal structure crystallizes as a homotrimer; zinc is bound between adjacent monomers (PubMed:20133749). The C-terminal RbcS-like domains bind to holo-RuBisCO (PubMed:30591587). Full length protein interacts with CcmN (By similarity). This cyanobacterium makes beta-type carboxysomes. The N-terminus (residues 1-164) forms a trimer with a partially blocked channel forms at the trimer interface (PubMed:20133749). The C-terminus has 4 domains that resemble the small subunit of RuBisCO (rbcS) joined by flexible linkers; the first one (CcmMS1) has been crystallized and folds much like RbcS. CcmMS1 binds to holo-RuBisCO (RbcL(8)-RbcS(8)), a CcmMS1-CcmMS2 constructs binds about 10-fold better, while the entire C-terminus with all 4 CcmMS domains binds with an intermediate affinity. CcmMS1 binds to a different region of RbcL than does RbcS, up to 5 CcmMS1 domains can bind to the intact holo enzyme (PubMed:30591587). Belongs to the gamma-class carbonic anhydrase family.